114Chapter 7: Clinical findings that are subtle

were brisk. This observation of a discrepancy in the saccadic velocity between large and small amplitude refixation movements, though subtle, was the key to the correct diagnosis in this case, suggesting neuromuscular junction disease. The patient subsequently had a positive Tensilon (edrophonium chloride) test (Figure 7.9B) and elevated acetylcholine receptor antibody titers, confirming a diagnosis of myasthenia gravis. This patient’s ocular motor disturbance would therefore be classified as a “pseudoINO”, i.e. myasthenia mimicking a supranuclear abnormality.

Discussion: Electro-oculographic studies have demonstrated that saccadic velocity in myasthenia is not only preserved but is often supra-normal,

due to an increased firing rate in the paramedian pontine reticular formation (PPRF) designed to overcome the neuromuscular junction blockade. This observation, however, does not apply to all saccadic movements. During eye movement recordings, small movements made by a myasthenic muscle do demonstrate this preservation or even augmentation of saccadic velocity. During large amplitude movements, however, an affected eye muscle may exhibit a loss of velocity during the course of a single excursion, termed “intra-saccadic fatigue”. In some cases, careful bedside examination can detect this change in velocity during the course of an eye movement. The patient is asked to quickly refixate from extreme lateral gaze to a target on the opposite side. In a normal saccade the velocity is rapid and uniform. In the presence of intra-saccadic fatigue, the eye movement appears to have two phases: an initial brisk burst of movement followed by a slower continuation toward the intended target. Another way to demonstrate this phenomenon is to compare velocities of small vs. large amplitude saccades, as described in the case example.

Slowing of medial rectus saccades is the most sensitive and invariable sign of an internuclear ophthalmoplegia (INO), demonstrable even in the absence of an adduction deficit. Such slowing can be demonstrated for both large and small ampli-

tude eye movements. In contrast, in an adduction deficit due to myasthenia, small amplitude medial rectus saccades exhibit a normal or supra-normal velocity. Because myasthenia can affect just one or two muscles, it often resembles a cranial nerve palsy or supranuclear gaze disturbance. The medial rectus muscle is frequently affected in myasthenia and thus the clinical picture of a unilateral or bilateral “pseudo-INO” is not uncommon.

Diagnosis: Myasthenic pseudo-INO

Tip: In a patient with an isolated adduction deficit, the presence of brisk saccadic velocity for small amplitude adduction movements distinguishes a myasthenic “pseudo-INO” from a true INO.

FURTHER READING

Neuroretinitis

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V.Purvin, Neuroretinitis. In D. Albert, J. Miller, eds., Albert and Jakobiec’s Principles and Practice of Ophthalmology,

3rd edn. Philadelphia: Elsevier, 2007, pp. 1859–63.

V. Purvin, G. Chioran, Recurrent neuroretinitis. Arch Ophthalmol, 112 (1994), 365–71.

J.B. Reed, D. K. Scales, M. T. Wong et al., Bartonella henselae neuroretinitis in cat scratch disease: diagnosis, management and sequelae, Ophthalmology, 105 (1998), 459–66.

Dural-cavernous fistula

R.J. W. De Keizer, Carotid-cavernous and orbital arteriovenous fistulas: ocular features, diagnostic and hemodynamic considerations in relation to visual impairment

and morbidity. Orbit, 22 (2003), 121–42.

E.Eggenberger, A. G. Lee, T. R. Forget Jr., R. Rosenwasser, A brutal headache and double vision. Surv Ophthalmol, 45

(2000), 147–53.

M. J. Kupersmith, Carotid cavernous fistulas. In Neurovascular Neuro-Ophthalmology. New York: Springer-Verlag, 1993, pp. 69–108.

P.M. Meyers, V. V. Halbach, C. F. Dowd et al., Dural carotid cavernous fistula: definitive endovascular management and long-term follow-up. Am J Ophthalmol, 134 (2002), 85–92.

following craniovertebral decompression. Eye, 14 (2000), 884–8.

R.D. Yee, Downbeat nystagmus: characteristics and localization of lesions. Trans Am Ophthalmol Soc, 87 (1990), 984–1032.

Downbeat nystagmus

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357–62.

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Neurology Series. New York: Oxford University Press, 2006, pp. 475–558.

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Myasthenic “pseudo-INO”

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The diagnostic approach to the patient with unexplained visual loss is to first determine the location of the lesion and then the nature of the disease process. In the process of localization, familiarity with the afferent visual pathways is crucial. Specific patterns of visual loss are associated with damage at various levels of the visual system. Anterior to the optic chiasm, visual deficits are monocular. Lesions at the optic disc typically produce visual field defects that emerge from the blindspot and often respect the horizontal meridian. Such “disc-related defects” consist of central or cecocentral scotomas (indicating a lesion of the papillomacular bundle), arcuate defects (damage to superior or inferior arcuate fibers) and radial bundle defects (extending from the blindspot to the periphery temporally). In contrast, more posteriorly along the optic nerve, visual defects tend to point to fixation and respect the vertical meridian. Because 90% of the fibers that comprise the optic nerve subserve central vision, damage to the nerve at any location often produces a defect involving this area of vision. Thus the presence of a central scotoma helps localize to the optic nerve but does not have much localizing power beyond that. Lesions at the chiasm produce bitemporal defects or a junctional scotoma. Lesions posterior to the chiasm result in homonymous defects, i.e. involving the same hemifield in each eye. The one exception to the last statement is the “monocular crescent”, a defect in the temporal periphery of the eye contralateral to a lesion of the anterior-most occipital lobe.

Formal perimetry provides a means to describe and quantify the visual field in a reproducible manner. Its localizing power derives from the exquisite topographic representation of the visual pathways. As with all psychophysical tests, however, there is a subjective element that is open to human error both on the part of the patient and the clinician. The cases in this chapter are intended to highlight some potentially confusing visual field defects.

Abnormal field and night blindness

Case: A 28-year-old carpenter suffered a broken wrist after falling off a ladder at work. Unable to return to carpentry until his wrist healed, he was assigned to work as the nightwatchman. In this new work setting, he had difficulty reading signs and seeing objects in the dim evening lighting and felt uncomfortable driving at night due to excessive glare from the headlights of oncoming cars. He requested temporary disability from work and was sent for a medical evaluation.

The patient was healthy and took no medications. Acuity was 20/25 in each eye at distance and near. An Octopus G1 automated test was interpreted as showing “constricted visual fields” in both eyes (Figure 8.1). Biomicroscopy and fundus examination were reportedly normal in both eyes.

The patient had no trouble when asked to count fingers presented in his peripheral visual field at a distance of one meter, and glanced in the appropriate direction whenever a small target was presented briefly in the far periphery. In addition, he

maneuvered about the office without difficulty, easily noticing objects in his path. Based on the inconsistency between the apparent severity of his visual field defect and his relative ease in performing visually guided activities, functional visual loss was suspected. He was reassured that his vision would improve with time, but six months later he reported no such improvement. His Octopus visual fields were unchanged and this time a Goldmann visual field was obtained (Figure 8.2).

How would you describe this patient’s visual field defect? What diagnoses should be considered?

Goldmann perimetry reveals that this patient has dense bilateral ring scotomas as well as diffuse loss of sensitivity. A ring scotoma is a doughnut-shaped or annular defect surrounding fixation. It may be partial (as in this patient’s left eye) or complete, and indicates photoreceptor dysfunction. Specific etiologies that should be considered in a patient with ring scotomas include: hereditary retinal dystro-

phies, paraneoplastic retinopathy, vitamin A deficiency, certain toxic states and idiopathic inflammation of the outer retina (AZOOR) (see Chapter 1, Twinkling scotoma). Further inquiry revealed that he had noticed poor night vision for several years but had not been sufficiently bothered by it to seek medical attention. There was no family history of visual loss. A full-field ERG with dark adaptometry revealed absent rod function and severe depression of cone function, consistent with a disorder of photoreceptors. Fundus examination was repeated and this time a zone of retinal and pigment epithelial atrophy in the mid-periphery with scattered, small clumps of pigment was appreciated in both eyes (Figure 8.3). A diagnosis of retinitis pigmentosa was made and the patient was appropriately counseled. He eventually transferred to the computer programming department of his company.

Discussion: A ring scotoma in the setting of night blindness and photophobia is the typical presentation of retinitis pigmentosa. This is a heterogeneous group of inherited retinopathies that diffusely