Strabismus (Squint)

CHAPTER 17 Strabismus (Squint)


Visual Axis

It is the line passing from the point of fixation to the fovea. It passes through the nodal point of the eye. In normal binocular vision, two visual axes intersect at the point of fixation.

Anatomical (Pupillary) Axis

It is the line passing from the posterior pole through the center of the cornea. As fovea is temporal to the posterior pole of the eye, the visual axis does not correspond to the anatomical axis of the eye (Fig. 17.1).

Fig. 17.1 Illustration depicting visual and anatomical axes.

Angle Kappa

It is the angle between the visual and anatomical axes of the eye. It is usually about 5 degrees.

Normally, fovea is temporal to the posterior pole of the eye. A light thrown onto the cornea will therefore cause a reflex just nasal to the center of cornea. This is termed a positive angle kappa. A large positive angle kappa may produce the appearance of an outward turn of the eye and simulate an exotropia. When the fovea is situated nasal to the posterior pole (high myopia and ectopic fovea), corneal reflex is situated temporal to the center of cornea. It is termed negative angle kappa and may produce the appearance of an inward eye turn and simulate an esotropia. So, an abnormal angle kappa is a common cause of pseudostrabismus.


It is the difference in refraction between two eyes.

Primary Deviation

When the normal eye fixates, the deviation of squinting eye is called primary deviation.

Secondary Deviation

By covering the normal eye, the deviated eye is forced to fixate. When uncovered, the deviation shown by the normal eye is called secondary deviation.


A condition of diminished visual form sense, which is not associated with any structural abnormality or disease of the media, fundus, or visual pathways; it is not overcome by correction of the refractive error.

Eccentric Fixation

A uniocular condition in which some part of the retina other than the fovea is used for fixation.


The mental ability to blend two similar images (one formed on each retina) and perceive them as one.

Fusional Reserve (Range of Fusion) (OP9.3)

It is the range of convergence, divergence, supravergence, and infravergence through which fusion can be maintained. The horizontal range of fusion can be expressed as the sum of the range of convergence and divergence. The vertical range of fusion can be expressed as the sum of the range of supravergence and infravergence. There is also a torsional range of fusion.


A condition in which the image of an object formed upon the retina is not perceived, but is mentally ignored or neglected, either partially or completely. It is one of the means whereby double vision is overcome.


It is the perfect or normal ocular alignment in the absence of any stimulus for fusion. It is uncommon, and slight phoria is present in most normal individuals.

Heterophoria (OP9.2)

It implies latent strabismus. It is the tendency of the eyes to deviate when the fusion is blocked.

Heterotropia (OP9.2)

It implies manifest strabismus. It is the misalignment of the visual axes of two eyes.

Heterophoria and Heterotropia

Heterophoria (Latent Strabismus)

It implies latent strabismus. It is the tendency of the eyes to deviate when the fusion is blocked. It may be:

Compensated heterophoria: When heterophoria is compensated by the fusional reflex. So, deviation is kept latent by fusion reflex.

Decompensated heterophoria: When the fusional amplitudes are insufficient to maintain the alignment, heterophoria may present clinically with associated symptoms such as esophoria, exophoria, hyperphoria, hypophoria, incyclophoria, and excyclophoria.


It is the tendency of eye to converge/turn inwards relative to other. It may be due to:

Convergence excess

Esophoria for near


Esophoria for distance

Divergence weakness

Esophoria for distance


Esophoria for near


It is the tendency of eye to diverge/turn outwards relative to other. It may be due to:

Convergence weakness

Exophoria for near


Exophoria for distance

Divergence excess

Exophoria for distance


Exophoria for near


It is the tendency of the eye to deviate upwards. Hypophoria is the tendency of the eye to deviate downwards. Vertical phoria are caused by abnormal ocular motility and expressed in terms of the relative position of the two eyes.

However, in practice, the term right or left hyperphoria is used depending on the eye which is up relative to other.

Cyclophoria (Torsional Deviation)

It is the tendency of eye to rotate (around anteroposterior axis) relative to the other. It may be:

Incyclophoria (nasal rotation of 12 o’ clock meridian of cornea).

Excyclophoria (temporal rotation of 12 o’ clock meridian of cornea).

When fusional amplitude is insufficient, it may cause asthenopic symptoms (OP9.3) consisting of:

Eye strain.


Pain in the eyes.

Feeling of heaviness.

Diplopia (momentary).

Running of letters and blurring.

The symptoms are less severe or disappear when patients do not use their eyes in close work.

Vertical deviations are especially likely to cause symptoms, since the vertical fusional amplitudes are limited.

Smaller degrees of heterophoria cause little symptoms than the larger degrees (more than 10°).

The symptoms are maximum in case of cyclophoria.

Heterotropia (Manifest Strabismus)

When the visual axes do not intersect at the point of fixation, heterotropia results. The image falls upon the fovea of one eye but not on the fovea of the other eye. It may be:

Esotropia (convergent strabismus): It is the inward deviation.

Exotropia (divergent strabismus): It is the outward deviation.

Hypertropia: It is the upward displacement of eye.

Hypotropia: It is the downward displacement of eye.

Incyclotropia/excyclotropia (torsional deviation).


Heterotropia may be classified in several different ways as depicted in Flowchart 17.1.

Flowchart 17.1 Classification of heterotropia.

Based on the type of deviation:











Based on age of onset:



Based on constancy:



Based on involvement of the eye:

Uniocular (deviation in one eye only).

Alternating (deviation in either eye).

Based on the variation of deviation with gaze:





A & V patterns.


Paralytic strabismus.

Restrictive strabismus.

In comitant strabismus:

Efferent pathways are normal and, thus, retain their coordination.

Either the afferent path is defective (usually due to poor visual acuity) or the central mechanism mediating fixation and fusional reflexes is underdeveloped.

In incomitant strabismus, the efferent pathway is defective.

Pseudostrabismus (Apparent Squint)

In pseudostrabismus, eyes seem to have a squint in the absence of any deviation. Visual axes are, in fact, parallel.

1.Pseudoesotropia (Fig. 17.2a): It is the false impression of a convergent strabismus and is associated with:

Broad or prominent epicanthal folds which cover the normally visible nasal aspect of the globe.

Negative angle kappa.

2.Pseudoexotropia (Fig. 17.2b): It is the false impression of a divergent strabismus and is associated with:

Hypertelorism: It is the condition of wide separation of two eyes.

Large positive angle kappa.

Fig. 17.2 (a) Pseudoesotropia. (b) Pseudoexotropia. Source: Pseudostrabismus. In: Agarwal A, Jacob S, ed. Color atlas of ophthalmology. The quick-reference manual for diagnosis and treatment. 2nd edition. Thieme; 2009.

Comitant Strabismus (OP9.2)

Comitant strabismus may be:







Convergent (esotropia)


divergent (exotropia)


hypertropia (vertical deviation)

In comitant strabismus, primary deviation is equal to secondary deviation and ocular deviation is equal in all directions of gaze.


The following factors are contributory in the etiology of comitant strabismus:


Defective vision in one eye: It may result due to anisometropia or opacities in the media (corneal opacity or congenital cataract). It causes the affected eye to lose fixation and subsequently deviate.

Muscular imbalance: The congenital or developmental defects of extraocular muscles (EOMs) may result in muscular imbalance. Initially, the fusional amplitudes maintain the alignment and heterophoria may precede the comitant strabismus.

Imbalance between accommodation and convergence: In hypermetropia, the children accommodate continuously to see clearly, even for distance. It stimulates the excessive convergence, resulting in inward deviation. On the other hand, in high myopia, lack of accommodative impulse results in outward deviation.

Defects in central mechanisms mediating fixation and fusional reflexes can lead to comitant strabismus.

Comitant Convergent Squint (Esotropia)

It is more common in hypermetropes and usually starts in childhood. It may be unilateral (monocular) or alternating. Esotropia could be accommodative or non-accommodative.

Accommodative Esotropia

Owing to the relatively late development of ciliary muscle, this type of esotropia rarely occurs before the age of 2 to 2½ years. To focus a near target, eyes accommodate and simultaneously converge to fixate bifoveally on the target. Accommodation and convergence have a fairly constant relationship to each other called AC/A ratio (accommodative convergence/accommodation). AC/A ratio is the amount of convergence in prism dioptres per dioptre (D) change in accommodation. Normal value of AC/A ratio is 3–5 prism dioptres. This means that 1D of accommodation is associated with 3 to 5 prism dioptre of accommodative convergence.

Accommodative esotropia is subdivided into three types:

Refractive accommodative esotropia– normal AC/A ratio.

Non-refractive accommodative esotropia– high AC/A ratio.

Mixed (partially accommodative) esotropia.

Refractive Accommodative Esotropia

It is associated with uncorrected hypermetropia (usually between +2 and +7D). Excessive accommodation occurs due to uncorrected hypermetropia which, in turn, leads to excessive convergence. This manifests as convergent strabismus (esotropia). In this:

AC/A ratio is normal.

Ocular deviation at near ≥ at distance fixation.

Asthenopic symptoms (OP9.3), intermittent diplopia (as evolution of esotropia is gradual), or closure of one eye when doing close work commonly occur during development of the disease.


Full correction of hypermetropia is done after cycloplegic refraction. Deviation is present if glasses are not worn (Fig. 17.3).

Fig. 17.3 Refractive accommodative esotropia without (a) and with glasses (b).

Nonrefractive Accommodative Esotropia

It results from abnormal synkinesis between accommodation and accommodative convergence. The effort to accommodate elicits an abnormally high accommodative convergence response, that is, AC/A ratio is high. This occurs independent of refractive error, although hypermetropia generally coexists. If the eyes are straight for distance and esotropic for near fixation, it means AC/A ratio is higher than normal.


Eyes are straightened through bifocals. Bifocals are prescribed to relieve accommodation (and thereby accommodative convergence), thus allowing the ocular alignment at near. Surgery should only be considered if spectacles do not fully correct the deviation (Fig. 17.4).

Fig. 17.4 (a, b) Non-refractive accommodative esotropia with accommodative convergence excess (high AC/A ratio). Source: Accommodative esotropia. In: Lang G, ed. Ophthalmology. A pocket textbook atlas. 3rd edition. Thieme; 2015.

Partially Accommodative Esotropia

Majority of patients have esotropia that is partially accommodative and partially non-accommodative.


Full hypermetropic correction should be prescribed. Bifocals are useless since deviation is only reduced and residual esotropia persists. Surgery may be needed for the correction of residual esotropia. Only the non-accommodative component of strabismus should be corrected surgically.

Non-accommodative Esotropia

It is not associated with accommodation; it includes:

Essential infantile esotropia (early onset esotropia).

Acquired (late onset esotropia).

Secondary esotropia–sensory deprivation esotropia and consecutive esotropia.

Essential Infantile Esotropia (Early Onset or Congenital Esotropia)

Onset: Infantile esotropia is an esodeviation with an onset before 6 months of age (esotropia with an onset after 6 months of age is referred to as early acquired esotropia) (Fig. 17.5).

Fig. 17.5 (a–c) Infantile esotropia. Source: Infantile esotropia. In: Agarwal A, Jacob S, ed. Color atlas of ophthalmology. The quick-reference manual for diagnosis and treatment. 2nd edition. Thieme; 2009.

Etiology: The prefix essential is added to emphasize its obscure etiology, that is, it is an idiopathic condition.

Key features: Infantile esotropia is usually characterized by:

Esotropia >30 prism dioptres (2 prism dioptre ≈ 1 degree deviation), that is, angle of deviation is fairly large.

Angle of deviation is usually quite stable.

Deviation at near = deviation at distance fixation which indicates a normal AC/A ratio.

There is alternate fixation in primary gaze and cross-fixation in side gaze. In cross-fixation, a child uses the left eye in right gaze and vice-versa, that is, there is no binocular vision.


Surgery is done to achieve binocular vision. Ideally, eyes should be surgically aligned by the age of 12 to 18 months. Amblyopia is treated pre-operatively.

Acquired Non Accommodative Esotropia

Onset: It occurs after 6 months of age. The onset of acquired esodeviations occur at a later age and is, often, insidious. Therefore, binocular vision has existed before the onset of acquired esotropia. Hence, prognosis for restoration of binocular vision is better than those with infantile esotropia, provided treatment is started without delay. It may be basic, near, or distance esotropia (Table 17.1).

Table 17.1 Types of acquired nonaccommodative esotropia



Basic esotropia

Near deviation = distance deviation

It is treated by elimination of amblyopia followed by surgery.

Near esotropia (convergence excess)

Near deviation > distance deviation

AC/A ratio is normal or abnormally low

It is treated by surgery

Distance esotropia (divergence insufficiency)

Distance deviation > near deviation

It is treated by prisms or surgery

Abbreviation: AC/A,accommodative-convergence over accommodation.

Secondary Esotropia

It includes sensory deprivation esotropia and consecutive esotropia.

Sensory deprivation esotropia: Decrease in visual acuity in one eye is due to the following:



Corneal opacity.

Unilateral cataract.

Macular lesion.

Optic atrophy.

Decrease in visual acuity abolishes fusion, resulting in strabismus.

Consecutive esotropia: It is iatrogenic and occurs after surgical overcorrection of an exodeviation (Flowchart 17.2).

Flowchart 17.2 Outline of esotropia. Abbreviations: AC/A, accommodative-convergence over accommodation; ND, near deviation; DD, distant deviation.

Comitant Divergent Squint (Exotropia)

Exotropia is outward deviation of one eye when the other eye takes fixation. Exotropia often manifests at a later age and is most common in myopes. When one eye loses most of its vision, the sound eye fixates and the eye with poor vision takes up the position of rest which is usually that of divergence.

Exotropia can be intermittent or constant and unilateral or alternating (Flowchart 17.3).

Flowchart 17.3 Classification of exotropia.

It may be:

Distance exotropia is of two types– divergence excess and simulated divergence excess.

Basic exodeviation (nonspecific exotropia).

Near exotropia (convergence insufficiency type).

Primary Exotropia

Onset of majority of exodeviations occurs shortly after birth. It may be constant or intermittent. Constant (early onset) exotropia may occur as a result of decompensated intermittent exotropia. It often presents at birth, has large angle of deviation, and is frequently associated with neurological anomalies. Its treatment is mainly surgical.

Intermittent exotropia is more common and occurs at approximately 2 years of age with exophoria which breaks down to exotropia under conditions of fatigue or ill health. Amblyopia is uncommon in intermittent exotropia. Its subtypes are explained in Table 17.2.

Table 17.2 Types of intermittent exotropia



Distance exotropia

(divergence excess)

Deviation for distance > deviation for near

Simulated divergence excess is associated with high AC/A ratio. Initially, deviation for distance > deviation for near, as high AC/A ratio controls exodeviation for near. Deviations for near and distance are similar when near angle is remeasured with the patient looking through + 3D lenses or after occlusion of one eye for a time period.

+ 3D lens → accommodation relaxed → no accommodative convergence → near deviation and distant deviation are equal

Basic exotropia

(non-specific exotropia)

Deviation for distance = deviation for near

Near exotropia

(convergence insufficiency)

Deviation for near > deviation for distance

Abbreviation: AC/A, accommodative-convergence over accommodation.

Secondary Exotropia

It may be:

Sensory exotropia as a result of primary sensory deficit.

Consecutive exotropia arising iatrogenically after surgical overcorrection.

Sensory deprivation exotropia results from monocular visual impairment on account of acquired lesions. Common causes include:



Opacities of media.

Optic atrophy.

Unilateral aphakia.

Exotropia tends to occur in older children or adults.
Esotropia tends to occur in infancy, but this is not invariable.

Consecutive exotropia results from surgical overcorrection of esotropia. Deviation is unilateral and constant.

Hypertropia (Vertical Deviation)

These may occur alone or in combination with horizontal deviation. The term right or left hypertropia is used depending on the eye which is up related to the other, for example, when right eye is fixing and left eye is higher than the right, it is called left hypertropia.

A- and V- Patterns

A horizontal deviation may increase or decrease with the eyes in upward or downward gaze, giving characteristic “A” and “V” patterns. Therefore, assessment is made by measuring horizontal deviations in primary position, upgaze and downgaze. They can occur in both comitant and incomitant deviations.

An “A” pattern is considered significant if the difference between upgaze and downgaze is ≥10∆ (Fig. 17.6a). It could be either esotropia or exotropia. “A” pattern esotropia increases in upgaze and decreases in downgaze (Fig. 17.6b), while “A” pattern exotropia decreases in upgaze and increases in downgaze (Fig. 17.6c).

Fig. 17.6 (a) “A” pattern. (b) “A” pattern esotropia increases in upgaze. (c) “A” pattern exotropia increases in downgaze.

A “V” pattern is considered significant if the difference between upgaze and downgaze is ≥15∆. (Fig. 17.7a). It could be either esotropia or exotropia. “V” pattern esotropia decreases in upgaze and increases in downgaze (Fig. 17.7b), while “V” pattern exotropia increases in upgaze and decreases in downgaze (Fig. 17.7c).

Fig. 17.7 (a) “V” pattern. (b) “V” pattern esotropia increases in down gaze. (c) “V” pattern exotropia increases in upgaze.


Dysfunctions of oblique and rectus muscles contribute to A and V pattern deviations.

“A” pattern develops following paresis or under action of inferior muscles (inferior oblique [IO] and inferior rectus [IR] muscles).

“V” pattern develops following paresis or under action of superior muscles (superior oblique [SO] and superior rectus [SR] muscles).

Clinical Features

Common complaints in patients with “A” and “V” patterns are asthenopia, diplopia, and anomalous head posture. In A exotropia and V esotropia, deviation increases in down gaze, leading to depression of chin and visual discomfort during reading or near work.

In V exotropia, deviation increases in upgaze, resulting in elevation of chin and no interference with binocular vision in down gaze. It is best tolerated by most patients.

In A esotropia, deviation decreases in downgaze, resulting in elevation of chin. There is difficulty with reading if the patient is binocular.


“A” and “V” patterns are treated surgically. Unilateral or bilateral surgical procedures for horizontal strabismus combined with weakening or strengthening procedures on the oblique muscles have proved effective in most patients with “A” and “V” patterns (Table 17.3).

Table 17.3 Surgical treatment for correction of “A” and “V” patterns

“A” and “V” pattern


A esotropia

The cases without oblique muscle dysfunction are treated by bilateral medial rectus recession with upward shifting of insertions.

Patients with oblique muscle dysfunction are treated by bilateral weakening of superior oblique muscle (SO posterior tenotomy).

A exotropia

The cases without oblique muscle dysfunction are treated by bilateral lateral rectus recession with downward shifting of insertions.

Patients with oblique muscle dysfunction are treated by bilateral weakening of superior oblique muscle (SO posterior tenotomy).

V esotropia

The cases without oblique muscle dysfunction are treated by bilateral medial rectus recession with downward shifting of insertions.

When oblique muscle dysfunction is present, the treatment involves inferior oblique weakening.

V exotropia

The cases without oblique muscle dysfunction are treated by bilateral lateral rectus recession with upward shifting of insertions.

When oblique muscle dysfunction is present, the treatment involves inferior oblique weakening.

Abbreviation: SO, superior oblique.

1.When there is no associated over action of oblique muscle:

Medial recti are recessed in esotropia and transposed toward apex of “A” or “V” (upward shifting in “A” pattern and downward shifting in “V” pattern).

Lateral recti are recessed in exotropia and transposed toward broad end of “A” or “V” (downward shifting in “A” pattern and upward shifting in “V” pattern).

2.When there is overaction of oblique muscle:

Weakening of overacting SO muscle is done for “A” pattern.

Weakening of overacting IO muscle is done for “V” pattern.

Incomitant Strabismus

Incomitant strabismus is characterized by variation in the angle of deviation in different directions of gaze. It may be paralytic strabismus or restrictive strabismus.


The restriction of eye movements may be caused by neurogenic or mechanical factors.

Neurogenic Cause

Ocular movements can be affected by supranuclear, nuclear, or infranuclear lesions.

Lesions in supranuclear pathways produce conjugate gaze palsies which affect both eyes equally. They maintain their relative coordination despite the abnormal movements, so diplopia is absent.

In nuclear and infranuclear lesions the relative coordination of the eyes is disturbed and diplopia appears. These lesions result in paralytic strabismus.

Mechanical Cause

The restriction of eye movement may be due to mechanical factors such as:

Muscle fibrosis (ocular myositis and Grave’s disease).

Space occupying lesions in the orbit.

Blowout fracture of orbit.

Paralytic Strabismus (OP9.2)

Paralytic strabismus is caused by paresis or paralysis of one or more EOMs. It is usually sudden in onset and is associated with diplopia and variation in the amount of deviation in different directions of gaze.


Nuclear lesions in midbrain.

Vascular cause: It includes hemorrhagic or thrombotic lesions of midbrain and may be associated with diabetes, hypertension, and arteriosclerosis.

Inflammation: It includes encephalitis and meningitis.

Toxins: It includes diphtheria, lead poisoning, and botulism.

Neoplasms: Brain tumors can produce ocular muscle palsies.

Infranuclear lesions.

The ocular motor nerves (III, IV, and VI) may be involved in various pathological conditions as they traverse the cranial and orbital cavities. Therefore, these nerves may be involved at the level of fascicle, subarachnoid space, cavernous sinus, and orbit. The lesions involving the nerves may be inflammatory, vascular, pressure by tumors or aneurysm, and trauma. In raised intracranial pressure, the most frequently involved nerve is abducens (VI) nerve.

Different Types of Ocular Paralysis

There are six types of ocular paralysis, namely, IIIrd nerve palsy, total ophthalmoplegia, external ophthalmoplegia, internal ophthalmoplegia, double elevator palsy, and isolated muscle palsy– IVth and VIth nerve palsy.

IIIrd Nerve Palsy

Paralysis of IIIrd nerve may be complete, but often incomplete, and individual muscles may be selectively affected. Clinical features of complete IIIrd nerve palsy are listed in Table 17.4.

Table 17.4 Clinical features of third nerve palsy

III rd nerve palsy

Clinical features

Paralysis of LPS muscle

Ptosis: It prevents diplopia. On raising the lid, there is crossed diplopia.

Unopposed action of lateral rectus and superior oblique muscles

Eyeball rotates outwards (divergent squint) and slightly downwards.

Restriction of ocular movements in all directions except outwards.

Intorsion of eye ball (because of intact SO muscle).

Paralysis of sphincter pupillae muscle

Pupil is fixed and dilated.

Paralysis of ciliary muscle

Loss of accommodation.

Loss of tone of paralyzed muscles

Slight proptosis.

Abbreviations: LPS, levator palpabrae superioris muscle; SO, superior oblique.

Total Ophthalmoplegia

It involves both extrinsic and intrinsic muscles of the eyeball. It results due to combined paralysis of IIIrd, IVth, and VIth nerves. It may be unilateral (in cavernous sinus thrombosis or lesions in superior orbital fissure) or bilateral (in lesions of the brain stem due to vascular or inflammatory causes). Clinical features include:


No movement of eyeball in any direction.

Fixed and dilated pupil.

Total loss of accommodation.

Slight proptosis.

External Ophthalmoplegia

It is due to paralysis of the extrinsic muscle of the eyeball (intraocular muscles are spared). It includes paralysis of six extraocular muscles and levator palpabrae superioris (LPS) muscle due to nuclear lesions sparing the Edinger–Westphal nucleus which supplies the intrinsic muscle. In external ophthalmoplegia, pupillary reactions and accommodation are normal.

Internal Ophthalmoplegia

It only involves intrinsic muscles (sphincter pupillae and ciliary muscle).

Double Elevator Palsy (Monocular Elevator Deficit)

It is characterized by paresis of superior rectus and inferior oblique muscle in the involved eye. Clinical features include:

Inability to elevate one eye.

Chin elevation to obtain fusion in downgaze.

Treatment involves the following:

Base-up prism over the involved eye.

Surgery must be considered if the chin elevation is required to maintain fusion.

Isolated Muscle Palsy—IVth and VIth Nerve Palsy

Trauma is the most common cause of isolated IVth nerve palsy, while VIth nerve palsy commonly occurs in raised intracranial tension due to its long intracranial course.

Clinical Features

The signs and symptoms of incomitant strabismus are as follows:

Deviation of eye.


Limitation of ocular movements.

False projection (false orientation).

Abnormal head posture.



In incomitant strabismus, the affected eye gets deviated. The magnitude of deviation (angle of deviation) depends on the degree of paralysis and the direction in which the patient is looking. In incomitant strabismus:

Secondary deviation (fixation with paretic eye) is always greater than primary deviation (fixation with normal eye). This can be explained by Hering’s law.

Angle of deviation is more if paralysis is severe.

The eye deviates in the direction opposite to the field of action of paralyzed muscle due to unopposed action of ipsilateral antagonist of paralytic muscle, for example, the eye turns outward in medial rectus (MR) paralyses due to unopposed action of lateral rectus muscle.

Angle of deviation is more if the patient is looking in the direction in which the paralyzed muscle acts, for example, in left lateral rectus (LR) muscle palsy, the deviation is more obvious on laevoversion (left gaze).

Clinical Application of Hering’s Law in Paretic Squint

In case of paretic squint, amount of innervation flowing to both eyes is always determined by the fixating eye. Therefore, angle of deviation depends on which eye is used for fixation, for example, in case of right LR palsy, right eye is deviated inward due to unopposed action of right MR.

When left normal eye is used for fixation: When normal eye fixates, the deviation shown by the paralyzed eye is called primary deviation.

When paretic eye (right eye) is used for fixation: As right eye is deviated inward, excessive innervation is required to abduct the right eye. According to Hering’s law, an equal amount of innervation flows to the normal MR of the left eye; hence, excessive adduction of the left eye takes place. This deviation of normal eye is known as secondary deviation. In paretic (incomitant) squint, secondary deviation is more than primary deviation.


Binocular diplopia occurs in the field of action of paralyzed muscle. The image seen by the squinting eye is the false image, while that seen by the sound eye is the true image. The false image is less distinct than the true image because true image falls upon the fovea. The diplopia may be homonymous (uncrossed) or heteronymous (crossed) (Fig. 17.8), for example:

Fig. 17.8 (a) Right esotropia (object projects to nasal retina resulting in homonymous diplopia). (b) Right exotropia (object projects to temporal retina resulting in heteronymous diplopia). Abbreviations: LLR, left inferior rectus; LMR, left medial rectus; RLR, right lateral rectus; RMR, right medial rectus.

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Nov 20, 2022 | Posted by in OPHTHALMOLOGY | Comments Off on Strabismus (Squint)
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