Figure 8-2 A, Double Maddox rod test for excyclotorsion. A red Maddox rod (left) is placed in front of the right eye and a white Maddox rod in front of the left. A patient with vertical diplopia sees the red line below the white line, indicating a right hypertropia. B, With cyclotorsion, the 2 lines do not appear parallel. The red Maddox rod is then rotated (large arrow) until the 2 lines appear parallel. The degree of rotation required to make the lines appear parallel (in this case about 12°)
quantitates the amount of excyclotorsion. (Used with permission from Kline LB, Bajandas FJ. Neuro-Ophthalmology Review Manual. Rev. 5th ed. Thorofare, NJ: Slack; 2004. Originally modified from Van Noorden GK. Atlas of Strabismus. 4th ed. St Louis: Mosb y; 1983.)
A qualitative method for detecting relative cyclotropia uses a metal pointer or other horizontal line. A base-down prism is placed over 1 eye to dissociate the pointer such that 2 vertically displaced lines are visible. The patient is asked if both lines are parallel or if they converge to 1 side. A fourth nerve palsy is typically associated with convergence of the lines toward the side of the palsy.
Clues to the presence of ocular deviation may be provided by a consistent head tilt or head turn on examination. Evidence of chronicity may exist in old photographs (eg, a driver’s license photograph).
Monocular Diplopia
Patients may describe optical aberrations as distorted or double vision. Monocular diplopia usually results from abnormalities of the refractive media (eg, uncorrected astigmatism; corneal irregularities, including keratoconus; tear film abnormalities; and cataract) and typically improves with use of a pinhole. Less commonly, monocular diplopia arises from retinal pathology (eg, maculopathy with retinal distortion by fluid, hemorrhage, or fibrosis); cerebral diplopia or polyopia is extremely rare. In contrast to monocular diplopia, binocular diplopia can be relieved by closing either eye, because the diplopia results from misalignment of the visual axes. Occasionally, monocular and binocular causes of diplopia may both be present in a patient. The demonstration of monocular diplopia alone effectively obviates the need for a neurologic workup to explain the cause of the diplopia.
Differentiating Paretic From Restrictive Etiologies of
Diplopia
Restriction of eye movements should be strongly suspected in patients with proptosis or enophthalmos, or a history of orbital trauma or eye surgery. Thyroid eye disease and orbital trauma are the most common causes of restrictive strabismus; patients with these conditions typically have associated orbital signs and symptoms. Patients may have both neural and restrictive components, especially after trauma.
Paretic and restrictive syndromes may be distinguished by assessing saccadic speed; paretic conditions reduce the saccadic velocity, whereas restrictive conditions do not. If this method does not provide an answer, then the forced duction test (Fig 8-3) may be performed. A restrictive process produces a mechanical limitation of the range of eye movements that can often be felt by an examiner when forceps or a cotton swab is used to advance the limited eye movement. Because chronic neural
lesions may also, rarely, cause mechanical limitation by gradual shortening of the unopposed antagonist muscle, a “tight” muscle may give a false-positive result.
Figure 8-3 Forced duction testing. Before the eye is grasped with forceps, topical proparacaine drops are applied to the eye and held over the limbal region with a cotton tip for 1–2 minutes. This patient has a left esotropia and limited abduction. The conjunctiva is grasped with toothed forceps and the globe passively rotated in the direction of limited abduction to assess for restriction of the eye movement. (Used with permission from Yanoff M, Duker JS, eds. Ophthalmology. 2nd ed.
St Louis: Mosb y; 2004:569, Fig 70-12.)
Ocular restriction, particularly from thyroid eye disease, can also be judged by measuring intraocular pressure in primary position and in eccentric gaze. An increase of ≥5 mm Hg of intraocular pressure in upgaze raises the possibility that this eye movement is being mechanically restricted by a “tight” inferior rectus muscle.
Comitant and Incomitant Deviations
Comitant misalignment is characteristically found in patients with congenital or early-onset strabismus. Patients with this condition typically do not report diplopia because of suppression, an adaptation that reduces the responsiveness of the visual neurons in the occipital cortex to the input from 1 eye (see BCSC Section 6, Pediatric Ophthalmology and Strabismus, Chapter 6, Fig 6-5). Patients with a history of childhood strabismus may experience diplopia later in life if their ocular misalignment changes. For example, horizontal diplopia may develop in the fifth decade of life in patients with a long-standing exophoria when accommodation and convergence capacities wane.
Conversely, an incomitant deviation may become comitant with the passage of time. This “spread of comitance” is related to a gradual recalibration of the innervation to yoke muscles of each eye. This apparent violation of Hering’s law, which is probably mediated at a cerebellar level, adjusts the neural input signal gain to individual extraocular muscles. Spread of comitance may occur with either a restrictive or paretic incomitant deviation and is especially likely with a fourth nerve palsy.
Incomitant strabismus is most frequently acquired and usually causes diplopia (Fig 8-4). If the deviation is very small, fusional amplitudes may eliminate the diplopia. Relatively small misalignments may produce blurred vision rather than an obvious perception of 2 images. Patients with subnormal visual acuity may not recognize diplopia or may have difficulty providing details of
how the visual percept changes in various positions of gaze. Congenital incomitant deviations, such as those caused by overaction of the inferior oblique muscles, typically do not produce diplopia, even when the strabismus is quite obvious. The clinician generally cannot distinguish with confidence by gross observation alone whether subnormal ductions are secondary to a neural or restrictive process.
Figure 8-4 Left sixth cranial nerve palsy. A, In right gaze, the eyes are aligned. B, In straight-ahead gaze, the left eye is inwardly deviated. C, In left gaze, the left eye does not abduct, causing a marked misalignment of the eyes. (From Trob e JD.
The Physician’s Guide to Eye Care. 3rd ed. San Francisco: American Academy of Ophthalmology; 2006:118. Image courtesy of W. K. Kellogg Eye Center, University of Michigan.)
Localization
Because many eye movement disorders have a neural basis, the clinician should attempt to localize the potential lesion that could produce the patient’s eye movement disorder. If local orbital disease or neuromuscular junction problems are not likely, then neural localization will dictate patient evaluation, including imaging modality and differential diagnosis. It is useful to conceptualize the anatomical pathway of the cranial nerve or nerves that are assumed to be involved (Fig 8-5). Adopting such a “wiring diagram” approach takes into account the supranuclear, internuclear, nuclear, and fascicular pathways within the brainstem, which then traverse the subarachnoid space, cavernous sinus, superior orbital fissure, and orbit, ending in the neuromuscular junctions of the extraocular muscles. In general, a lesion involving the cranial nerve nucleus or fascicle will cause neurologic signs with deficits other than ophthalmoparesis (discussed later in the chapter). Despite the fact that modern neuroimaging has demonstrated that central lesions can produce a clinically isolated (ie, no other clinically evident problems can be detected) sixth or third nerve palsy, such isolated cranial neuropathies are the exception and not the rule.