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Determining the cause of transient monocular visual loss can be tricky. Following simple steps may help sort out the diagnosis.
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.
"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."
"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.