Chapter 79 Vertical strabismus
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:
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.
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.
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.
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
Situations in which the Parks’ 3-step test may be misleading4
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.
Fig. 79.2 Normal relationship of fovea to disk with respect to torsion. The fovea is normally positioned level with the lower third of the optic disk. The borders of normal lie between the two black lines in the figure. This photograph indicates an absence of objective torsion but is near the border for intorsion.
2. Is incomitance greater vertically or horizontally? If it is greater vertically, consider a restriction or palsy of a vertical rectus. If the largest difference is horizontal, consider an oblique problem.
You should choose a treatment plan that will give the maximum correction in the field of gaze in which the deviation is greatest. Thus, you need to pay attention to the pattern of incomitance and the presence of torsion. Keep in mind that the primary position and downgaze (for reading) are the two most important fields of gaze and should not be sacrificed for the benefit of eccentric gazes. Oblique muscle surgery will give much more correction in adduction than abduction, but with vertical rectus surgery the difference in correction between abduction and adduction is less dramatic. Also, oblique muscle surgery causes more torsional change than rectus muscle surgery. An exception is with vertical rectus restrictions in which releasing the restriction may correct substantial torsion.
In general, surgery on the inferior rectus or superior oblique is least “forgiving.” Inferior rectus recessions of 5 mm or more may cause a lag of that eye in downgaze unless there was a hypotropia that increased in downgaze prior to surgery. Also, large recessions of the inferior rectus may cause lower eyelid retraction postoperatively, minimized by advancement of the capsulopalpebral head at surgery.7 Large resections of the inferior rectus muscle may cause narrowing of the palpebral fissure, and large recessions of the inferior rectus using a suspension technique (hangback adjustable suture) have a higher incidence of muscle slippage or non-adherance. This is probably caused by the shorter arc of contact of that muscle, which can cause the muscle to lose apposition with the globe in downgaze after surgery if the muscle is not fixed to the sclera.8 Slippage can be prevented by using the semi-adjustable suture technique,9 or a non-absorbable suture.
Some patients may appear to have a hypertropia when in fact they do not. These include cases of orbital dystopia, anterior segment anomalies, or vertical angle kappa (Fig. 79.3). This latter may be due to a displaced fovea secondary to retinopathy of prematurity (see Chapter 43) or other causes of retinal dragging.
Fig. 79.3 Pseudohypotropia. (A) Patient with what appears to be a right hypotropia. (B) Cover test shows he is actually fixating with the eye when it appears to be infraducted. (C) His right fovea is dragged inferiorly due to traction secondary to retinopathy of prematurity, resulting in a large vertical angle kappa.