Figure 1.14 Fundus photograph of a patient with MEWDS, showing the characteristic lesions in the mid-peripheral retina.

case. The peri-papillary retina is the target of inflammation, and the focal area of disturbed photoreceptor function is best demonstrated with multi-focal rather than full-field ERG. In the acute stage, the fundus is normal save for mild optic nerve swelling in some patients. Multiple evanescent white dot syndrome (MEWDS), a closely related syndrome, is characterized in its acute stage by small whitish lesions in the posterior pole (Figure 1.14), and may also be associated with a persistently enlarged blindspot.

The etiology of AIBSE is unknown. The disorder usually affects healthy young women, and evaluation discloses no evidence of an underlying systemic disease. There is no established treatment but the natural history usually includes spontaneous improvement of the scotoma, although photopsias may persist.

As in the above case, prominent photopsias in patients with AIBSE may be mistaken for the visual disturbance of migraine. Because this disorder presents as acute onset of a monocular scotoma, it may also suggest a diagnosis of demyelinating optic neuritis. The absence of pain and the continuous nature of the scintillations are important distinguishing features.

Diagnosis: Acute idiopathic blindspot enlargement

Tip: Continuous photopsias indicate a disorder of the outer retina.

Sudden monocular visual loss with normal fundus

Case: This 70-year-old gentleman experienced sudden, painless visual loss in the left eye while watching television. At the onset of his visual loss he saw an arc of yellow lights briefly passing across the superior visual field in the left eye. He was generally healthy except for prostate cancer for which he was taking diethylstilbesterol. Examination in a local emergency room within six hours after onset of visual loss showed 20/20 acuity in the right eye (OD) and count fingers vision in the left eye (OS), but revealed no basis for his acute visual loss. Based on the normal fundus appearance, he was thought to have a retrobulbar optic neuropathy, but an MRI of brain and orbits was completely normal.

What other mechanism of visual loss would you consider? Are there any historical features that are helpful here?

A compressive or infiltrative optic nerve lesion should be in the differential, however the very abrupt onset of visual loss in this case would be unusual and his MRI was unrevealing. Optic neuritis is a possibility, but the patient’s age would be highly atypical for demyelinating disease. Acute painless monocular visual loss in a patient over the age of 50 years is most often ischemic in origin, and that should be the main consideration in this case. The question is whether this ischemia affects the optic nerve or the retina. In the large majority of cases, ischemic optic neuropathy affects the most anterior portion of the nerve so that disc edema is seen acutely, termed anterior ischemic optic neuropathy (AION). In this patient, however, the disc was normal. Ischemia of the retrobulbar segment of the optic nerve, termed posterior ischemic optic neuropathy (PION), is rare. When PION does occur,

18Chapter 1: Ocular disease or neurologic disease?

it is generally in the setting of severe prolonged hypotension, often accompanied by blood loss (e.g. peri-operative, cardiac arrest or massive hemorrhage) or due to giant cell arteritis. A diagnosis of PION in any other setting should be suspect. In addition, the presence of photopsias heralding the onset of visual loss would be highly unusual in either form of ischemic optic neuropathy but sometimes accompanies a retinal ischemic event. Retinal artery occlusion is therefore the leading diagnosis in this case.

Why might a retinal stroke not have been apparent on examination?

Retinal whitening due to arterial occlusion may take up to 24 hours to develop. Other fundus features related to lack of flow in the retinal circulation, such as ghost vessels and segmentation of the blood column (termed “boxcarring”) in retinal arteries may be seen hyperacutely, but if flow has been restored in the occluded vessel the fundus may have a completely normal appearance during this interval.

Based on the above considerations, the patient was treated presumptively for central retinal artery occlusion (CRAO) including ocular massage, anterior chamber paracentesis, oxygen/carbon dioxide breathing and acetazolamide. Ophthalmic examination three days later showed no change of visual acuity in the left eye. There was a large central scotoma and small relative afferent pupillary defect (RAPD). Dilated fundus examination now showed retinal whitening (edema) of the posterior pole with a macular cherry-red spot, confirming a diagnosis of CRAO (Figure 1.15). Ancillary investigation revealed no embolic source, and testing for vasculitis and coagulation disorders was similarly unrevealing. His CRAO was attributed to a hypercoaguable state secondary to treatment with diethystilbesterol.

Discussion: Retinal artery occlusion typically produces acute, painless, monocular visual loss. Unlike individuals with ischemic optic neuropathy, who often note visual loss upon awakening, patients

Figure 1.15 Fundus photograph of the above 70-year-old man with acute painless visual loss in his left eye three days previously. There is diffuse retinal whitening and a cherry-red spot in the macula, indicating central retinal artery occlusion.

with retinal artery occlusion can often describe just what they were doing when visual loss occurred. Permanent visual loss due to retinal artery occlusion is sometimes preceded by episodes of transient monocular visual loss, usually lasting for less than five minutes, often with an altitudinal pattern described as a “curtain” descending over vision. In contrast, such episodes of preceding transient monocular visual loss are distinctly uncommon in patients with non-arteritic AION.

The fundus appearance in retinal artery occlusion depends on the timing of the examination. In the hyperacute phase there may be obvious attenuation and segmentation of the blood column within retinal arteries, and the responsible embolus may be visible. If the embolic material has moved on through the retinal circulation, however, and flow has already been restored, the retina may have a completely normal appearance. This phase may last up to 24 hours. As edema develops, areas of retinal whitening appear. This may take the form of cottonwool spots (indicating small nerve fiber infarcts) or of patchy or diffuse retinal whitening. The distribution of retinal edema varies depending on whether the central or a branch retinal artery was involved (Figure 1.16). In cases of central retinal artery