Overview and definitions

For a patient with vertical strabismus, one should first determine whether the deviation is comitant or incomitant. If the latter, one next determines if the problem is paretic, restrictive, or a manifestation of primary oblique muscle dysfunction. Finally, one determines if the deviation is dissociated (e.g. does not appear to follow Hering’s Law with respect to the vertical component).

It is best to describe the deviation as it is actually manifested. Thus, if a patient fixes with the left eye and has a restrictive right hypotropia, you should call it a right hypotropia rather than using the old convention of describing it in terms of a hypertopia − in this example a left hypertropia. If the patient freely alternates fixation, you should then use the old convention. There has also been confusion about the terminology for describing dissociated vertical divergence (DVD). Terminology should be descriptive by addressing three issues; indicate if the deviation is:

1. Constant or intermittent.

2. Latent of manifest (e.g. a phoria or tropia).

3. Dissociated or not dissociated.

The most common presentation for DVD is for one eye to have a hypertropia that is intermittently manifest and for the other to have a hypertropia that is latent (only present in the dissociated state, e.g. under cover). Appropriate terminology for describing such a patient would be intermittent manifest DVD in one eye and latent DVD in the other. An alternative would be an intermittent dissociated hypertropia in one eye and a dissociated hyperphoria in the other.

Physiology

The cyclovertical muscles have a triple function that includes a vertical, torsional, and, to a lesser degree, horizontal action. With head tilt right or left, there is a small partial compensatory torsional rotation of each eye, which corrects for about 5−10% of the head tilt.¹ This comes from stimulation of the intorters (superior oblique and superior rectus) in the eye on the side to which the head is tilted, and the extorters (inferior oblique and inferior rectus) in the fellow eye, and forms the basis for the Bielschowksy Head Tilt Test and the Parks’ 3-Step Test.² Although the vertical rectus muscles have their main vertical action in adduction, they are still the primary elevators and depressors across the horizontal gaze fields. Oblique muscles have a relatively weak vertical action. If the superior rectus is detached from the globe, the inferior oblique alone cannot elevate the eye above the midline. The superior oblique, however, has a stronger vertical action than the inferior oblique.

Patient evaluation

History

Before measuring the deviation, observe whether the patient has a spontaneous compensatory head posture (CHP) with visual effort. Although there are many causes for CHPs, I find they mostly serve to place the eyes in a gaze field in which the deviation is less, or to damp nystagmus (see Chapters 87, 88, and 89). The presence of facial asymmetry typically identifies the CHP as dating back to early childhood. There is a shortening of the midface between the horizontal canthus laterally and the corner of the child’s mouth on the side to which the head is habitually tilted³ (Fig. 79.1). Measure the deviation with the head posture using the prism and alternative cover test, and then in the head erect position. Although a diagnosis can often be made by just measuring a patient in seven fields of gaze (primary, up, down, right and left) and on head tilt right and left, an optimum treatment plan often requires additional measurements in the four oblique fields. Ductions and versions should be assessed with particular attention to the presence of overelevation or overdepression in the oblique fields. If there is overelevation of the adducting eye on side gaze, do a cover test in that gaze field to ascertain if the vertical deviation is a manifestation of DVD or true inferior oblique overaction.

Fig. 79.1 Facial asymmetry. There is shortening of the left side of the face in this boy with a long-standing head tilt to the left for control of a right superior oblique palsy.

Determine if the deviation is dissociated or not. With a non-dissociated deviation, a hypotropia is present in the contralateral eye when fixation is with the higher eye. Unless a secondary deviation is present due to paresis or restriction, the hypotropia in one eye will be of equal size to the hypertropia of the other eye. With DVD, the hypotropia of the fellow eye is either smaller or absent when fixation is with the eye with the DVD (see section on Dissociated vertical divergence).

The Parks’ 3-Step Test will tell, in theory, which of the eight cyclovertical muscles is paretic; however, in practice it works best for confirming the diagnosis of unilateral superior oblique palsy.^2,⁴ Even in cases of isolated superior or inferior rectus palsy, it may be misleading. Most importantly, it does not tell whether a vertical strabismus is due to a palsy of one of the cyclovertical muscle, but is based on the assumption that it is. Box 79.1 lists some of the common situations in which the 3-Step Test leads to an incorrect diagnosis (see Video 79.1). The man in this video developed vertical diplopia immediately after closed head trauma from a motor vehicle accident. He had a left hypertropia which increased in right gaze and on left head tilt, thus meeting the 3-Step Test criteria for a left superior oblique palsy. In fact, he had an orbital floor fracture in his right eye. He consistently fixed with his right eye due to mild amblyopia in his left eye. His diplopia resolved when the fracture was repaired

Box 79.1

Situations in which the Parks’ 3-step test may be misleading⁴

DVD

Multiple muscle involvement

Bilateral fourth nerve palsy

Multiple other muscle weakness

Pulley heterotopia

Superior rectus overaction/contracture

Inferior rectus restriction

Superior rectus palsy

Inferior rectus palsy

Skew deviation

Prior extraocular muscle surgery

You should test for the presence of torsion both objectively and subjectively. The latter is accomplished with the double Maddox rod test using two red lenses, because the use of one red and one white lens frequently leads to a localization artifact; if the eye behind the white lens is the one that is torted, the patient will frequently perceive the torsion as being in the eye behind the red lens.⁵ However, even when I use two red lenses, I sometimes find that the double Maddox rod test just tells me the total amount of torsion between the two eyes, and may be misleading with respect to localizing to the specific eye affected. Objective torsion is determined with the indirect ophthalmoscope and may be more useful in determining which eye actually is torted.⁶ Normally the position of the fovea should be level with the lower one-third of the optic disk (Fig. 79.2). The absence of subjective torsion in the presence of objective fundus torsion usually means the deviation is long-standing and sensory adaptations have occurred. If a patient describes torsion, determine if the patient can fuse when the vertical and horizontal deviations are offset with prisms. If the patient can fuse comfortably despite the presence of torsion, it may be possible to ignore the torsion when designing a surgical strategy. If they cannot fuse, there may be a central disruption of fusion. Testing on the synoptophore, which can offset the torsion and may determine whether you can expect fusion if the strabismus is successfully treated.