Principles

The lateral and medial orbital walls are at an angle of 45° with each other. The orbital axis therefore forms an angle of 22.5° with both lateral and medial walls, though for the sake of simplicity this angle is usually regarded as being 23° ( Fig. 18.2A ). When the eye is looking straight ahead at a fixed point on the horizon with the head erect (primary position of gaze), the visual axis forms an angle of 23° with the orbital axis ( Fig. 18.2B ); the actions of the extraocular muscles depend on the position of the globe at the time of muscle contraction ( Figs 18.2C and D ).

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  The primary action of a muscle is its major effect when the eye is in the primary position.

  
  
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  Subsidiary actions are the additional effects; these depend on the position of the eye.

  
  
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  The Listing plane is an imaginary coronal plane passing through the centre of rotation of the globe. The globe rotates on the axes of Fick, which intersect in the Listing plane ( Fig. 18.3 ).

  
  
  
  
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    The globe rotates left and right on the vertical Z axis.

    
    
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    The globe moves up and down on the horizontal X axis.

    
    
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    Torsional movements (wheel rotations) occur on the Y (sagittal) axis which traverses the globe from front to back (similar to the anatomical axis of the eye).

    
    
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    Intorsion occurs when the superior limbus rotates nasally, and extorsion on temporal rotation.

  
  

  
  

  
  

  

  
  

  
  

  The Listing plane and axes of Fick

  
  

Anatomy of the extraocular muscles

Horizontal recti

When the eye is in the primary position, the horizontal recti are purely horizontal movers on the vertical Z axis and have only primary actions.

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  Medial rectus originates at the annulus of Zinn at the orbital apex and inserts 5.5 mm behind the nasal limbus. Its sole action in the primary position is adduction.

  
  
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  Lateral rectus originates at the annulus of Zinn and inserts 6.9 mm behind the temporal limbus. Its sole action in the primary position is abduction.

Vertical recti

The vertical recti run in line with the orbital axis and are inserted in front of the equator. They therefore form an angle of 23° with the visual axis (see Fig. 18.2C ).

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  Superior rectus originates from the upper part of the annulus of Zinn and inserts 7.7 mm behind the superior limbus.

  
  
  
  
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    The primary action is elevation ( Fig. 18.4A ); secondary actions are adduction and intorsion.

    
    

    
    

    
    

    

    
    

    
    

    Actions of the right superior rectus muscle

    
    
    
    
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    When the globe is abducted 23°, the visual and orbital axes coincide. In this position it has no subsidiary actions and can act only as an elevator ( Fig. 18.4B ). This is therefore the optimal position of the globe for testing the function of the superior rectus muscle.

    
    
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    If the globe were adducted 67°, the angle between the visual and orbital axes would be 90°. In this position the superior rectus could only act as an intortor ( Fig. 18.4C ).

  
  
  
  
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  Inferior rectus originates at the lower part of the annulus of Zinn and inserts 6.5 mm behind the inferior limbus.

  
  
  
  
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    The primary action is depression; secondary actions are adduction and extorsion.

    
    
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    When the globe is abducted 23°, the inferior rectus acts purely as a depressor. As for superior rectus, this is the optimal position of the globe for testing the function of the inferior rectus muscle.

    
    
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    If the globe were adducted 67°, the inferior rectus could act only as an extortor.

  
  

Spiral of Tillaux

The spiral of Tillaux ( Fig. 18.5 ) is an imaginary line joining the insertions of the four recti and is an important anatomical landmark when performing surgery. The insertions are located progressively further away from the limbus in a spiral pattern; the medial rectus insertion is closest (5.5 mm) followed by the inferior rectus (6.5 mm), lateral rectus (6.9 mm) and superior rectus (7.7 mm).

Spiral of Tillaux. IR = inferior rectus; LR = lateral rectus; MR = medial rectus; SR = superior rectus

Oblique muscles

The obliques are inserted behind the equator and form an angle of 51° with the visual axis (see Fig. 18.2D ).

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  Superior oblique originates superomedial to the optic foramen. It passes forwards through the trochlea at the angle between the superior and medial walls and is then reflected backwards and laterally to insert in the posterior upper temporal quadrant of the globe ( Fig. 18.6 ).

  
  
  
  
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    The primary action is intorsion ( Fig. 18.7A ); secondary actions are depression and abduction.

    
    

    
    

    
    

    

    
    

    
    

    Actions of the right superior oblique muscle

    
    
    
    
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    The anterior fibres of the superior oblique tendon are primarily responsible for intorsion and the posterior fibres for depression, allowing separate surgical manipulation of these two actions (see below).

    
    
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    When the globe is adducted 51°, the visual axis coincides with the line of pull of the muscle. In this position it can act only as a depressor ( Fig. 18.7B ). This is, therefore, the best position of the globe for testing the action of the superior oblique muscle. Thus, although the superior oblique has an abducting action in primary position, the main effect of superior oblique weakness is seen as failure of depression in adduction.

    
    
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    When the eye is abducted 39°, the visual axis and the superior oblique make an angle of 90° with each other. In this position the superior oblique can cause only intorsion ( Fig. 18.7C ).

  
  

  
  

  
  

  

  
  

  
  

  Insertion of the superior oblique (SO) tendon; SR = superior rectus

  
  
  
  
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  Inferior oblique originates from a small depression just behind the orbital rim lateral to the lacrimal sac. It passes backwards and laterally to insert in the posterior lower temporal quadrant of the globe close to the macula.

  
  
  
  
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    The primary action is extorsion; secondary actions are elevation and abduction.

    
    
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    When the globe is adducted 51°, the inferior oblique acts as an elevator only.

    
    
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    When the eye is abducted 39°, its main action is extorsion.

  
  

Muscle pulleys

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  The four rectus muscles pass through condensations of connective tissue and smooth muscle just posterior to the equator. These condensations act as pulleys and minimize upward and downward movements of the bellies of the medial and lateral rectus muscles during upgaze and downgaze, and horizontal movements of the superior and inferior rectus bellies in left and right gaze.

  
  
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  Pulleys are the effective origins of the rectus muscles and play an important role in the coordination of eye movements by reducing the effect of horizontal movements on vertical muscle actions and vice versa.

  
  
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  Displacement of the pulleys is a cause of abnormalities of eye movements such as ‘V’ and ‘A’ patterns (see below).

Innervation

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  Lateral rectus. Sixth cranial nerve (abducent nerve – abducting muscle).

  
  
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  Superior oblique. Fourth cranial nerve (trochlear nerve – muscle associated with the trochlea).

  
  
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  Other muscles together with the levator muscle of the upper lid and the ciliary and sphincter pupillae muscles are supplied by the third (oculomotor) nerve.

Ductions

Ductions are monocular movements around the axes of Fick. They consist of adduction, abduction, elevation, depression, intorsion and extorsion. They are tested by occluding the fellow eye and asking the patient to follow a target in each direction of gaze.

Versions

Versions ( Fig. 18.8 , top) are binocular, simultaneous, conjugate movements (conjugate – in the same direction, so that the angle between the eyes remains constant).

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  Dextroversion and laevoversion (gaze right and gaze left), elevation (upgaze) and depression (downgaze). These four movements bring the globe into the secondary positions of gaze by rotation around either the vertical (Z) or the horizontal (X) axes of Fick.

  
  
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  Dextroelevation and dextrodepression (gaze up and right; gaze down and right) and laevoelevation and laevodepression (gaze up and left; gaze down and left). These four oblique movements bring the eyes into the tertiary positions of gaze by rotation around oblique axes lying in the Listing plane, equivalent to simultaneous movement about both the horizontal and vertical axes.

  
  
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  Torsional movements to maintain upright images occur on tilting of the head; these are known as the righting reflexes. On head tilt to the right the superior limbi of the two eyes rotate to the left, causing intorsion of the right globe and extorsion of the left (laevocycloversion).

Binocular movements

Vergences

Vergences ( Fig. 18.8 , bottom) are binocular, simultaneous, dis­jugate movements (disjugate – in opposite directions, so that the angle between the eyes changes; also termed disjunctive). Convergence is simultaneous adduction (inward turning); divergence is outwards movement from a convergent position. Convergence may be voluntary or reflex; reflex convergence has four components:

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  Tonic convergence, which implies inherent innervational tone to the medial recti.

  
  
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  Proximal convergence is induced by psychological awareness of a near object.

  
  
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  Fusional convergence is an optomotor reflex that maintains binocular single vision (BSV) by ensuring that similar images are projected onto corresponding retinal areas of each eye. It is initiated by bitemporal retinal image disparity.

  
  
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  Accommodative convergence is induced by the act of accommodation as part of the synkinetic-near reflex.

  
  
  
  
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    Each dioptre of accommodation is accompanied by a constant increment in accommodative convergence, giving the ‘accommodative convergence to accommodation’ (AC/A) ratio.

    
    
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    This is the amount of convergence in prism dioptres (Δ) per dioptre (D) change in accommodation.

    
    
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    The normal value is 3–5 Δ. This means that 1 D of accommodation is associated with 3–5 Δ of accommodative convergence. Abnormalities of the AC/A ratio play an important role in the aetiology of strabismus.

    
    
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    Changes in accommodation, convergence and pupil size which occur in concert with a change in the distance of viewing are known as the ‘near triad’.

  
  

Positions of gaze

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  Six cardinal positions of gaze are identified in which one muscle in each eye is principally responsible for moving the eye into that position as follows:

  
  
  
  
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    Dextroversion (right lateral rectus and left medial rectus).

    
    
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    Laevoversion (left lateral rectus and right medial rectus).

    
    
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    Dextroelevation (right superior rectus and left inferior oblique).

    
    
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    Laevoelevation (left superior rectus and right inferior oblique).

    
    
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    Dextrodepression (right inferior rectus and left superior oblique).

    
    
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    Laevodepression (left inferior rectus and right superior oblique).

  
  
  
  
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  Nine diagnostic positions of gaze are those in which deviations are measured. They consist of the six cardinal positions, the primary position, elevation and depression ( Fig. 18.9 ).

  
  

  
  

  
  

  

  
  

  
  

  Diagnostic positions of gaze. IO = inferior oblique; IR = inferior rectus; LR = lateral rectus; MR = medial rectus; SO = superior oblique; SR = superior rectus

  
  

Laws of ocular motility

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  Agonist–antagonist pairs are muscles of the same eye that move the eye in opposite directions. The agonist is the primary muscle moving the eye in a given direction. The antagonist acts in the opposite direction to the agonist. For example, the right lateral rectus is the antagonist to the right medial rectus.

  
  
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  Synergists are muscles of the same eye that move the eye in the same direction. For example, the right superior rectus and right inferior oblique act synergistically in elevation.

  
  
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  Yoke muscles (contralateral synergists) are pairs of muscles, one in each eye, that produce conjugate ocular movements. For example, the yoke muscle of the left superior oblique is the right inferior rectus.

  
  
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  The Sherrington law of reciprocal innervation ( Fig. 18.10 ) states that increased innervation to an extraocular muscle (e.g. right medial rectus) is accompanied by a reciprocal decrease in innervation to its antagonist (e.g. right lateral rectus). This means that when the medial rectus contracts the lateral rectus automatically relaxes and vice versa. The Sherrington law applies to both versions and vergences.

  
  

  
  

  
  

  

  
  

  
  

  Sherrington law of reciprocal innervation

  
  
  
  
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  The Hering law of equal innervation states that during any conjugate eye movement, equal and simultaneous innervation flows to the yoke muscles ( Fig. 18.11 ).

  
  
  
  
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    In the case of a paretic squint, the amount of innervation to both eyes is symmetrical, and always determined by the fixating eye, so that the angle of deviation will vary according to which eye is used for fixation.

    
    
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    For example if, in the case of a left lateral rectus palsy, the right normal eye is used for fixation, there will be an inward deviation of the left eye due to the unopposed action of the antagonist of the paretic left lateral rectus (left medial rectus). The amount of misalignment of the two eyes in this situation is called the primary deviation ( Fig. 18.12 , left).

    
    

    
    

    
    

    

    
    

    
    

    Primary and secondary deviations in paretic strabismus

    
    
    
    
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    If the paretic left eye is now used for fixation, additional innervation will flow to the left lateral rectus, in order to establish this. However, according to the Hering law, an equal amount of innervation will also flow to the right medial rectus (yoke muscle). This will result in an overaction of the right medial rectus and an excessive amount of adduction of the right eye.

    
    
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    The amount of misalignment between the two eyes in this situation is called the secondary deviation (see Fig. 18.12 , right). In a paretic squint, the secondary deviation exceeds the primary deviation.

  
  

  
  

  
  

  

  
  

  
  

  Hering law of equal innervation of yoke muscles

  
  
  
  
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  Muscle sequelae are the effects of the interactions described by these laws. They are of prime importance in diagnosing ocular motility disorders and in particular in distinguishing a recently acquired from a longstanding palsy (see ‘ Clinical evaluation ’). The full pattern of changes takes a variable period to develop:

  
  
  
  
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    Primary underaction (e.g. left superior oblique).

    
    
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    Secondary overaction of the contralateral synergist or yoke muscle (right inferior rectus; Hering law).

    
    
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    Secondary overaction and later contracture of the unopposed ipsilateral antagonist (left inferior oblique; Sherrington law).

    
    
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    Secondary inhibition of the contralateral antagonist (right superior rectus; Hering and Sherrington laws).

  
  

Basic aspects

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  Normal binocular single vision (BSV) involves the simultaneous use of both eyes with bifoveal fixation, so that each eye contributes to a common single perception of the object of regard. This represents the highest form of binocular cooperation. Conditions necessary for normal BSV are:

  
  
  
  
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    Normal routing of visual pathways with overlapping visual fields.

    
    
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    Binocularly driven neurones in the visual cortex.

    
    
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    Normal retinal (retinocortical) correspondence (NRC) resulting in ‘cyclopean’ viewing.

    
    
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    Accurate neuromuscular development and coordination, so that the visual axes are directed at, and maintain fixation on, the object of regard.

    
    
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    Approximately equal image clarity and size for both eyes.

    
    
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    BSV is based on NRC, which requires first an understanding of uniocular visual direction and projection.

  
  
  
  
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  Visual direction is the projection of a given retinal element in a specific direction in subjective space.

  
  
  
  
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    The principal visual direction is the direction in external space interpreted as the line of sight. This is normally the visual direction of the fovea and is associated with a sense of direct viewing.

    
    
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    Secondary visual directions are the projecting directions of extrafoveal points with respect to the principal direction of the fovea, associated with indirect (eccentric) viewing.

  
  
  
  
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  Projection is the subjective interpretation of the position of an object in space on the basis of stimulated retinal elements.

  
  
  
  
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    If a red object stimulates the right fovea (F), and a black object which lies in the nasal field stimulates a temporal retinal element (T), the red object will be interpreted by the brain as having originated from the straight-ahead position and the black object will be interpreted as having originated in the nasal field ( Fig. 18.13A ). Similarly, nasal retinal elements project into the temporal field, upper retinal elements into the lower field and vice versa.

    
    

    
    

    
    

    

    
    

    
    

    Principles of projection. F = fovea; N = nasal retinal element; T = temporal retinal element

    
    
    
    
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    With both eyes open, the red fixation object is now stimulating both foveae, which are corresponding retinal points. The black object is now not only stimulating the temporal retinal elements in the right eye but also the nasal elements of the left eye. The right eye therefore projects the object into its nasal field and the left eye projects the object into its temporal field.

    
    
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    Because both of these retinal elements are corresponding points, they will both project the object into the same position in space (the left side) and there will be no double vision.

  
  
  
  
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  Retinomotor values

  
  
  
  
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    The image of an object in the peripheral visual field falls on an extrafoveal element. To establish fixation on this object a saccadic version of accurate amplitude is required.

    
    
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    Each extrafoveal retinal element therefore has a retinomotor value proportional to its distance from the fovea, which guides the amplitude of saccadic movements required to ‘look at it’.

    
    
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    Retinomotor value, zero at the fovea, increases progressively towards the retinal periphery.

  
  
  
  
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  Corresponding points are areas on each retina that share the same subjective visual direction (for example, the foveae share the primary visual direction).

  
  
  
  
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    Points on the nasal retina of one eye have corresponding points on the temporal retina of the other eye and vice versa. For example, an object producing images on the right nasal retina and the left temporal retina will be projected into the right side of visual space. This is the basis of normal retinal correspondence.

    
    
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    This retinotopic organization is reflected back along the visual pathways, each eye maintaining separate images until the visual pathways converge onto binocularly responsive neurones in the primary visual cortex.

  
  
  
  
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  The horopter is an imaginary plane in external space, relative to both the observer’s eyes for a given fixation target, all points on which stimulate corresponding retinal elements and are therefore seen singly and in the same plane ( Fig. 18.13B ). This plane passes through the intersection of the visual axes and therefore includes the point of fixation in BSV.

  
  
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  The Panum fusional space (or volume) is a zone in front of and behind the horopter in which objects stimulate slightly non-corresponding retinal points (retinal disparity).

  
  
  
  
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    Objects within the limits of the fusional space are seen singly and the disparity information is used to produce a perception of binocular depth (stereopsis). Objects in front of and behind Panum space appear double.

    
    
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    This is the basis of physiological diplopia. The Panum space is shallow at fixation (6 seconds of arc) and deeper towards the periphery (30–40 seconds of arc at 15° from the fovea).

    
    
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    The retinal areas stimulated by images falling within the Panum fusional space are termed Panum fusional areas.

    
    
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    Therefore objects on the horopter are seen singly and in one plane. Objects in Panum fusional areas are seen singly and stereoscopically. Objects outside Panum fusional areas appear double.

    
    
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    Physiological diplopia is usually accompanied by physiological suppression.

  
  
  
  
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  BSV is characterized by the ability to fuse the images from the two eyes and to perceive binocular depth:

  
  
  
  
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    Sensory fusion involves the integration by the visual areas of the cerebral cortex of two similar images, one from each eye, into one image. It may be central, which integrates the image falling on the foveae, or peripheral, which integrates parts of the image falling outside the foveae. It is possible to maintain fusion with a central visual deficit in one eye, but peripheral fusion is essential to BSV and may be affected in patients with advanced field changes in glaucoma and pituitary lesions.

    
    
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    Motor fusion involves the maintenance of motor alignment of the eyes to sustain bifoveal fixation. It is driven by retinal image disparity, which stimulates fusional vergences.

  
  
  
  
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  Fusional vergence involves disjugate eye movements to overcome retinal image disparity. Fusional convergence helps to control an exophoria whereas fusional divergence helps to control an esophoria. The fusional vergence mechanism may be decreased by fatigue or illness, converting a phoria to a tropia. The amplitude of fusional vergence mechanisms can be improved by orthoptic exercises, particularly in the case of near fusional convergence for the relief of convergence insufficiency. Amplitudes can be measured with prisms or a synoptophore. Normal values are:

  
  
  
  
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    Convergence: about 15–20 Δ for distance and 25 Δ for near.

    
    
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    Divergence: about 6–10 Δ for distance and 12–14 Δ for near.

    
    
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    Vertical: 2–3 Δ.

    
    
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    Cyclovergence: about 8°.

  
  
  
  
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  Stereopsis is the perception of depth. It arises when objects behind and in front of the point of fixation (but within Panum fusional space) stimulate horizontally disparate retinal elements simultaneously. The fusion of these disparate images results in a single visual impression perceived in depth. A solid object is seen stereoscopically (in 3D) because each eye sees a slightly different aspect of the object.

  
  
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  Sensory perceptions. At the onset of a squint two sensory perceptions arise based on the normal projection of the retinal areas stimulated; confusion and pathological diplopia may result. These require simultaneous visual perception, that is, the ability to perceive images from both eyes simultaneously. Young children readily suppress diplopia but it is persistent and usually troublesome with strabismus in older children and adults, when it arises after the sensitive period for binocularity (see below).

  
  
  
  
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    Confusion is the simultaneous appreciation of two superimposed but dissimilar images caused by stimulation of corresponding retinal points (usually the foveae) by images of different objects ( Fig. 18.14 ).

    
    

    
    

    
    

    

    
    

    
    

    Confusion

    
    
    
    
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    Pathological diplopia is the simultaneous appreciation of two images of the same object in different positions and results from images of the same object falling on non-corresponding retinal points. In esotropia the diplopia is homonymous (uncrossed – Fig. 18.15A ), in exotropia the diplopia is heteronymous (crossed – Fig. 18.15B ).

    
    

    
    

    
    

    

    
    

    
    

    Diplopia. (A) Homonymous (uncrossed) diplopia in right esotropia with normal retinal correspondence; (B) heteronymous (crossed) diplopia in right exotropia with normal retinal correspondence

    
    

  
  

Sensory adaptations to strabismus

The ocular sensory system in children has the ability to adapt to anomalous states (confusion and diplopia) by two mechanisms: suppression and abnormal retinal correspondence (ARC). These occur because of the plasticity of the developing visual system in children under the age of 6–8 years. Occasional adults who develop sudden-onset strabismus are able to ignore the second image after a time and therefore do not complain of diplopia.

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  Suppression involves active inhibition by the visual cortex of the image from one eye when both eyes are open. Stimuli for suppression include diplopia, confusion and a blurred image from one eye resulting from astigmatism/anisometropia. Clinically, suppression may be:

  
  
  
  
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    Central or peripheral . In central suppression the image from the fovea of the deviating eye is inhibited to avoid confusion. Diplopia, on the other hand, is eradicated by the process of peripheral suppression, in which the image from the peripheral retina of the deviating eye is inhibited.

    
    
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    Monocular or alternating . Suppression is monocular when the image from the dominant eye always predominates over the image from the deviating (or more ametropic) eye, so that the image from the latter is constantly suppressed. This type of suppression leads to amblyopia. When suppression alternates (switches from one eye to the other), amblyopia is less likely to develop.

    
    
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    Facultative or obligatory . Facultative suppression occurs only when the eyes are misaligned. Obligatory suppression is present at all times, irrespective of whether the eyes are deviated or straight. Examples of facultative suppression include intermittent exotropia and Duane syndrome.

  
  
  
  
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  Abnormal (anomalous) retinal correspondence (ARC) is a condition in which non-corresponding retinal elements acquire a common subjective visual direction, i.e. fusion occurs in the presence of a small angle manifest squint; the fovea of the fixating eye is paired with a non-foveal element of the deviated eye. Binocular responses in ARC are never as good as in normal bifoveal BSV. It represents a positive sensory adaptation to strabismus (as opposed to negative adaptation by suppression), which allows some anomalous binocular vision in the presence of a heterotropia. It is most frequently encountered in small angle esotropia (microtropia), but is less common in accommodative esotropia because of the variability of the angle of deviation and in large angle deviations because the separation of the images is too great.

  
  
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  Microtropia is discussed further later in this chapter.

  
  
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  Consequences of strabismus

  
  
  
  
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    The fovea of the squinting eye is suppressed to avoid confusion.

    
    
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    Diplopia will occur, since corresponding retinal elements receive different images.

    
    
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    To avoid diplopia, the patient will develop either peripheral suppression of the squinting eye or ARC.

    
    
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    If constant unilateral suppression occurs this will subsequently lead to strabismic amblyopia.

  
  

Motor adaptation to strabismus

Motor adaptation involves the adoption of a compensatory head posture (CHP) and occurs primarily in children with congenitally abnormal eye movements who use the CHP to maintain BSV. In these children loss of a CHP may indicate loss of binocular function and the need for surgical intervention. These patients may present in adult life with symptoms of decompensation, often unaware of their CHP. Acquired paretic strabismus in adults may be consciously controlled by a CHP provided the deviation is neither too large nor too variable with gaze (incomitance). The CHP eliminates diplopia and helps to centralize the binocular visual field. The patient will turn the head into the direction of the field of action of the weak muscle, so that the eyes are then automatically turned the opposite direction and as far as possible away from its field of action (i.e. the head will turn where the eye cannot).

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  A face turn will be adopted to control a purely horizontal deviation. For example, if the left lateral rectus is paralysed, diplopia will occur in left gaze; the face will be turned to the left which deviates the eyes to the right away from the field of action of the weak muscle and area of diplopia. A face turn may also be adopted in a paresis of a vertically acting muscle to avoid the side where the vertical deviation is greatest (e.g. in a right superior oblique weakness the face is turned to the left).

  
  
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  A head tilt is adopted to compensate for torsional and/or vertical diplopia. In a right superior oblique weakness, the right eye is relatively elevated and the head is tilted to the left ( Fig. 18.16 ), towards the hypotropic eye; this reduces the vertical separation of the diplopic images and permits fusion to be regained. If there is a significant torsional component preventing fusion, tilting the head in the same left direction will reduce this by invoking the righting reflexes (placing the extorted right eye in a position that requires extorsion).

  
  

  
  

  
  

  

  
  

  
  

  Compensatory head posture in a right fourth nerve palsy

  
  
  
  
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  Chin elevation or depression may be used to compensate for weakness of an elevator or depressor muscle or to minimize the horizontal deviation when an ‘A’ or ‘V’ pattern is present.

Classification

Amblyopia is the unilateral, or rarely bilateral, decrease in best corrected visual acuity (VA) caused by form vision deprivation and/or abnormal binocular interaction, for which there is no identifiable pathology of the eye or visual pathway.

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  Strabismic amblyopia results from abnormal binocular interaction where there is continued monocular suppression of the deviating eye.

  
  
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  Anisometropic amblyopia is caused by a difference in refractive error between the eyes and may result from a difference of as little as 1 dioptre. The more ametropic eye receives a blurred image, in a mild form of visual deprivation. It is frequently associated with microstrabismus and may coexist with strabismic amblyopia.

  
  
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  Stimulus deprivation amblyopia results from vision deprivation. It may be unilateral or bilateral and is typically caused by opacities in the media (e.g. cataract) or ptosis that covers the pupil.

  
  
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  Bilateral ametropic amblyopia results from high symmetrical refractive errors, usually hypermetropia.

  
  
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  Meridional amblyopia results from image blur in one meridian. It can be unilateral or bilateral and is caused by uncorrected astigmatism (usually >1 D) persisting beyond the period of emmetropization in early childhood.

Diagnosis

In the absence of an organic lesion, a difference in best corrected VA of two Snellen lines or more (or >1 log unit) is indicative of amblyopia. Visual acuity in amblyopia is usually better when reading single letters than letters in a row. This ‘crowding’ phenomenon occurs to a certain extent in normal individuals but is more marked in amblyopes and must be taken into account when testing preverbal children.

Treatment

It is essential to examine the fundi to diagnose any visible organic disease prior to commencing treatment for amblyopia. Organic disease and amblyopia may coexist and a trial of patching may still be indicated in the presence of organic disease. If acuity does not respond to treatment, investigations such as electrophysiology or imaging should be reconsidered. The sensitive period during which acuity of an amblyopic eye can be improved is usually up to 7–8 years in strabismic amblyopia and may be longer (into the teens) for anisometropic amblyopia where good binocular function is present.

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  Occlusion of the normal eye, to encourage use of the amblyopic eye, is the most effective treatment. The regimen, full-time or part-time, depends on the age of the patient and the density of amblyopia.

  
  
  
  
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    The younger the patient, the more rapid the likely improvement but the greater the risk of inducing amblyopia in the normal eye. It is therefore very important to monitor VA regularly in both eyes during treatment.

    
    
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    The better the VA at the start of occlusion, the shorter the duration required, although there is wide variation between patients.

    
    
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    If there has been no improvement after 6 months of effective occlusion, further treatment is unlikely to be fruitful.

    
    
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    Poor compliance is the single greatest barrier to improvement and must be monitored. Amblyopia treatment benefits from time spent at the outset on communication of the rationale and the difficulties involved.

  
  
  
  
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  Penalization , in which vision in the normal eye is blurred with atropine, is an alternative method. It may work best in the treatment of moderate amblyopia (6/24 or better). Patch occlusion is likely to produce a quicker response than atropine, which has conventionally been reserved for use when compliance with patch occlusion is poor. Weekend instillation may be adequate.

Testing in preverbal children

The evaluation can be separated into the qualitative assessment of visual behaviour and the quantitative assessment of visual acuity using preferential looking tests. Assessment of visual behaviour is achieved as follows:

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  Fixation and following may be assessed using bright attention-grabbing targets (a face is often best). This method indicates whether the infant is visually alert and is of particular value in a child suspected of being blind.

  
  
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  Comparison between the behaviour of the two eyes may reveal a unilateral preference. Occlusion of one eye, if strongly objected to by the child, indicates poorer acuity in the other eye. However, it is possible to have good visual attention with each eye but unequal visual acuity and all risk factors for amblyopia must be considered in the interpretation of results.

  
  
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  Fixation behaviour can be used to establish unilateral preference if a manifest squint is present.

  
  
  
  
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    Fixation is promoted in the squinting eye by occluding the dominant eye while the child fixates a target of interest (preferably incorporating a light).

    
    
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    Fixation is then graded as central or non-central and steady or unsteady (the corneal reflection can be observed).

    
    
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    The other eye is then uncovered and the ability to maintain fixation is observed.

    
    
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    If fixation immediately returns to the uncovered eye, then visual acuity is probably impaired.

    
    
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    If fixation is maintained through a blink, then visual acuity is probably good.

    
    
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    If the patient alternates fixation, then the two eyes probably have equal vision.

  
  
  
  
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  The 10 Δ test is similar and can be used regardless of whether a manifest squint is present. It involves the promotion of diplopia using a 10 Δ vertical prism. Alternation between the diplopic targets suggests equal visual acuity.

  
  
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  Rotation test is a gross qualitative test of the ability of an infant to fixate with both eyes open. The test is performed as follows:

  
  
  
  
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    The examiner holds the child facing him or her and rotates briskly through 360°.

    
    
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    If vision is normal, the eyes will deviate in the direction of rotation under the influence of the vestibulo-ocular response. The eyes flick back to the primary position to produce a rotational nystagmus.

    
    
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    When rotation stops, nystagmus is briefly observed in the opposite direction for 1–2 seconds and should then cease due to suppression of post-rotary nystagmus by fixation.

    
    
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    If vision is severely impaired, the post-rotation nystagmus does not stop as quickly when rotation ceases because the vestibulo-ocular response is not blocked by visual feedback.

  
  
  
  
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  Preferential looking tests can be used from early infancy and are based on the fact that infants prefer to look at a pattern rather than a homogeneous stimulus. The infant is exposed to a stimulus and the examiner observes the eyes for fixation movements, without themselves knowing the stimulus position.

  
  
  
  
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    Tests in common use include the Teller and Keeler acuity cards, which consist of black stripes (gratings) of varying widths, and Cardiff acuity cards ( Fig. 18.17 ), which consist of familiar pictures with variable outline width.

    
    

    
    

    
    

    

    
    

    
    

    Cardiff acuity cards

    
    
    
    
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    Low frequency (coarse) gratings or pictures with a wider outline are seen more easily than high frequency gratings or thin outline pictures, and an assessment of resolution (not recognition) visual acuity is made accordingly.

    
    
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    Since grating acuity often exceeds Snellen acuity in amblyopia, Teller cards may overestimate visual acuity. These methods may not be reliable if a proper forced-choice staircase protocol is not followed during testing, and neither method has high sensitivity to the presence of amblyopia. The results must be considered in combination with risk factors for amblyopia.

  
  
  
  
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  Pattern visual evoked potentials (VEP) give a representation of spatial acuity but are more commonly used in the diagnosis of optic neuropathy.

Testing in verbal children

The tests described below should be performed at 3–4 metres from the target, as it is easier to obtain compliance than at 6 metres, with little or no clinical detriment. It is important to note that amblyopia can only be accurately diagnosed using a crowded test requiring target recognition and that logMAR tests (logarithm of the minimal angle of resolution – see Ch. 14 ) provide the best measure against which improvement with amblyopia therapy can be assessed. These are readily available in formats suited to normal children from 2 years onwards.

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  At age 2 years most children will have sufficient language skills to undertake a picture naming test such as the crowded Kay pictures ( Fig. 18.18A ).

  
  

  
  

  
  

  

  
  

  
  

  (A) Kay pictures; (B) Keeler logMAR crowded test

  
  

  (Courtesy of E Dawson)

  
  
  
  
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  At age 3 years most children will be able to undertake the matching of letter optotypes as in the Keeler logMAR ( Fig. 18.18B ) or Sonksen crowded tests. If a crowded letter test proves too difficult it is preferable to perform the crowded Kay pictures than to use single optotype letters.

  
  
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  Older children may continue with the crowded letter tests, naming or matching them; LogMAR tests are in common usage and are preferable to Snellen for all children at risk of amblyopia.

Titmus

The Titmus test consists of a three-dimensional polarized vectograph comprising two plates in the form of a booklet viewed through polarized spectacles. On the right is a large fly, and on the left is a series of circles and animals ( Fig. 18.19 ). The test should be performed at a distance of 40 cm.

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  The fly is a test of gross stereopsis (3000 seconds), and is especially useful for young children. It should appear to stand out from the page and the child is encouraged to pick up the tip of one of its wings between finger and thumb.

  
  
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  The animals component consists of three rows of stylized animals (400–100 seconds), one of which will appear forward of the plane of reference.

  
  
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  The circles comprise a graded series measuring 800–40 seconds; one of a set of four circles should appear to stand out from the plate surface.

Titmus test

TNO

The TNO random dot test consists of seven plates of randomly distributed paired red and green dots viewed with red–green spectacles, and measures from 480 down to 15 seconds of arc at 40 cm. Within each plate the dots of one colour forming the target shape (squares, crosses etc. – Fig. 18.20 ) are displaced horizontally in relation to paired dots of the other colour so that they have a different retinal disparity to those outside the target. Control shapes are visible without the spectacles.

TNO test

Frisby

The Frisby stereotest consists of three transparent plastic plates of varying thickness. On the surface of each plate are printed four squares of small randomly distributed shapes ( Fig. 18.21 ). One of the squares contains a ‘hidden’ circle, in which the random shapes are printed on the reverse of the plate. The test does not require special spectacles because disparity (600–15 seconds) is created by the thickness of the plate; the working distance must be measured.

Frisby test

Lang

The Lang stereotest does not require special spectacles; the targets are seen alternately by each eye through the built-in cylindrical lens elements. Displacement of the dots creates disparity (1200–200 seconds) and the patient is asked to name or point to a simple shape, such as a star, on the card ( Fig. 18.22 ).

Lang test

Base-out prism

This is a simple method for detecting fusion in children. The test is performed by placing a 20 Δ base-out prism in front of one eye (the right eye in Fig. 18.23 ). This displaces the retinal image temporally with resultant diplopia.

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  There will be a shift of the right eye to the left to resume fixation (right adduction) with a corresponding shift of the left eye to the left (left abduction) in accordance with the Hering law ( Fig. 18.23B ).

  
  
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  The left eye will then make a corrective re-fixational saccade to the right (left re-adduction) ( Fig. 18.23C ).

  
  
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  On removal of the prism both eyes move to the right ( Fig. 18.23D ).

  
  
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  The left eye then makes an outward fusional movement ( Fig. 18.23E ).

  
  
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  Most children with good BSV should be able to overcome a 20 Δ prism from the age of 6 months; if not, weaker prisms (16 Δ or 12 Δ) may be tried, but the response is then more difficult to identify.

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