Look for clues that indicate cause of transient visual loss

Ann Arbor, MI-Transient monocular visual loss can result from systemic disease. Jonathan Trobe, MD, described simple steps for sorting the causes of transient monocular visual loss:

• Abrupt onset. Visual loss can be described as transient only when the onset is abrupt. Recognizing this automatically allows the clinician to remove from the differential diagnosis conditions in which vision fluctuates, which might occur with ocular surface diseases, corneal endothelial dysfunction, fluctuating blood sugar, and demyelination.

“Everyone [experiences] fluctuating vision, [so] the clinician should work up patients whose vision comes and goes,” said Dr. Trobe, professor of ophthalmology and neurology, Kellogg Eye Center, University of Michigan, Ann Arbor.

• Monocular versus binocular. A distinction should be made between monocular and binocular transient visual loss. Making this distinction is an extremely difficult task, Dr. Trobe said, because patients have usually not tested their vision by covering one eye at a time.

Four clues suggesting transient visual loss are an inability to read during the visual episode; appreciation that the visual loss was restricted to a hemifield; presence of a slowly expanding scotoma or scintillation; and accompanying neurologic symptoms that suggest brain hemisphere dysfunction.

• Ischemia. Monocular transient visual loss always represents ischemia to the eye.

“Conventional wisdom [says] the cervical carotid [artery] is the source of a particle that finds its way into the retinal circulation,” Dr. Trobe said. “Much less likely is a source from the aortic arch or heart. The heart and aortic arch can give rise to particles, but they rarely cause transient visual loss.”

Interestingly, many patients with monocular transient visual loss have a normal carotid artery seen with imaging. Therefore, visual loss may occur because of reduced ocular perfusion caused by vasospasm in the eye or systemic hypotension.

“That is more complicated because it is difficult to fix,” he said. “Rare causes include arteritis papilledema and a hyperviscous/hypercoagulable state.”

The clues to the presence of non-embolic hypoperfusion transient visual loss are onset related to posture, bright light exposure, heavy exercise, or eating. Other clues are occurrence upon awakening after a night’s sleep, the presence of low cardiac output, cardiac arrhythmia, recent increase in blood pressure medication, dehydration, and symptoms of presyncope, Dr. Trobe said.

As a cause of transient monocular visual loss, vasospasm is often invoked but rarely demonstrated. He provided the example of a 57-year-old woman who experienced a 40-minute episode of transient visual loss in one eye. The episode occurred when the patient was in the ophthalmic office and photographed with the normal carotids.

“As the episode of visual loss began, the vasospasm started,” Dr. Trobe said. “When the vision returned to normal, everything in the eye returned to normal.”

• Binocular transient visual loss. Binocular transient visual loss has three mechanisms: migraine, ischemia, and seizure. The most common is migraine, regardless of the presence of scintillations or patient age.

“Ischemia and seizures also have to be considered, but with much reduced frequency,” he said.

Dr. Trobe explained that although a diagnosis may be difficult, it is possible by following the Lawton-Smith adage: “The sum of the incompletes. For example, each feature alone might not lead to a diagnosis of migraine, but when many characteristic features are added together, the diagnosis can be made,” he said.

The signs to look for in migraine are scintillations, which occur in about 80% of patients; fortification in about 20%; march in 20%; attacks that last 20 to 30 minutes and are restricted to one side; and attacks that switch from one hemifield to another in consecutive attacks.

With occipital ischemia, the episodes are shorter than those in migraine but variable, the attacks are not restricted to a hemifield, scintillations may or may not be present, and other concurrent neurologic symptoms may exist. It is always difficult to determine whether an episode of occipital ischemia is the result of vertebrobasilar embolism or low flow.

With seizures, an occipital epileptogenic lesion should be visible on magnetic resonance imaging or computed tomography, and there usually is a homon-ymous hemianopia on visual field testing. Scintillations typically are stationary, usually colored, and of any duration; the attacks do not switch sides.

• Plaque. In monocular transient visual loss, the eye examination usually is normal, but an intraluminal retinal artery plaque may be seen, he said.

“The ophthalmologist should always examine patients with transient monocular visual loss because of the possible presence of a Hollenhorst plaque,” Dr. Trobe said. “If [plaque is] found, the ophthalmologist has reason to believe that the plaque might have caused the transient visual loss.”

He also pointed out that another culprit could be the presence of venous stasis retinopathy, which represents a chronic perfusion problem in the eye, indicating perhaps carotid occlusion or distal stenosis.

“The chances of endarterectomy being helpful in such a case are virtually zero,” he stated.

Papilledema and malignant hypertension with swings in blood pressure and autoregulation in the retina and brain are other possibilities.

• Workup. The workup for transient visual loss is based on age, risk factors for arteriosclerosis, and symptoms. In non-elderly patients, the diagnosis is probably vasospasm, but the differential diagnosis must include paroxysmal hypertension, cardiac arrhythmia, premature atherosclerosis, carotid dissection/dysplasia, and a hypercoagulable state. If the workup is negative, physicians should consider the “triple threat”: being a woman, being on birth control pills, and smoking. In elderly patients, the first rule is to eliminate from consideration giant cell arteritis. After ruling out that, the patient should be referred to an internist. In cases of recent or crescendo transient visual loss, the patient should be referred immediately.

• Stroke occurrence. In transient visual loss, if a stroke is to happen, it will happen soon. Stroke is rare following transient visual loss. An article published in 2001 in the New England Journal of Medicine indicated that the 2-year risk of ipsilateral hemispheric stroke in patients with transient monocular visual loss was 9% following endarterectomy and 7.2% in those managed without endarterectomy.

By comparison, patients with hemispheric transient ischemic attack had 2-year stroke risks of 9% with endarterectomy and 25% without it.

• Carotid endarterectomy. Based on those facts, carotid endarterectomy probably is not beneficial for most patients with transient monocular visual loss alone.

“There have to be numerous risk factors in order to benefit from carotid endarterectomy. Most patients with transient monocular visual loss do not fulfill those criteria,” Dr. Trobe explained. “In fact, in the New England Journal of Medicine study, only 30% of patients had enough risk factors to benefit from carotid endarterectomy. The perioperative risk of stroke or death in that study was 3.6%, but in the real world, the risk of stroke or death in patients undergoing endarterectomy ranges from 6% to 22%.”OT