CASE 6: TRANSIENT MONOCULAR BLINDNESS
For the past 6 weeks, a 52-year-old man has been having spells of transient monocular visual loss. Every few days, it seems as if a curtain descends over his right eye, with total loss of vision for several minutes. He has verified that only the right eye is affected by covering first one eye and then the other during the attacks. Within an hour of a typical episode, examination reveals a small yellowish refractile body at a bifurcation of an inferior retinal arteriole, obstructing distal flow. A soft, high-pitched, continuous, systolic murmur extending into diastole is present over the right anterior neck at the level of the thyroid cartilage. Physical and neurological examinations are otherwise normal.
Treated with aspirin and clopidogrel, he has one more attack 2 days later and none thereafter. Doppler ultrasonography reveals severe (90%) stenosis of the right internal carotid artery just above its origin from the common carotid artery. This finding is confirmed by carotid angiography, which further defines an ulceration on the surface of the atherosclerotic plaque. Neurosurgical consultation is obtained.
Neuroanatomy explains the location of this patient's symptoms, and vascular anatomy explains the pathophysiology. Unilateral visual loss means that the lesion involves either the optic nerve or the eye itself (see Figure 3–2). The refractile body observed on fundoscopy is within an arteriolar branch of the central retinal artery, which is a branch of the ophthalmic artery. The central retinal artery and the optic nerve enter the orbit through the optic foramen. The ophthalmic artery, in turn, is the first major intracranial branch of the internal carotid artery. This patient is thus having transient ischemic attacks of his retina (transient monocular blindness, amaurosis fugax) secondary to emboli arising from thrombi on an ulcerated atherosclerotic plaque in his right internal carotid artery.
Because the internal carotid artery bifurcates into the middle and anterior cerebral arteries, patients with transient retinal ischemia often have cerebral symptoms as well, particularly hemiparesis contralateral to the affected eye.
By definition, transient ischemic attacks last less than 24 hours; in fact, most last a few minutes rather than hours. Their primary significance is prognostic, for such patients are at increased risk for both major stroke and myocardial infarction. Clinical trials have demonstrated that agents that decrease platelet aggregation (notably, aspirin and clopidogrel) reduce both the frequency of attacks and the likelihood of cerebral infarction. (Ischemia means reduced blood supply, with or without symptoms; infarction means tissue death as a consequence of ischemia.) In patients who meet specific medical, neurological, and radiological criteria, the treatment of choice is revascularization with either carotid endarterectomy—surgical removal of the atherosclerotic plaque—or carotid angioplasty and stenting.
C. 2016. Visual impairment. Handbook Clin Neurol 136:887–903.