visual loss should suggest the possibility of neuroretinitis or other macular disturbance. The age of the patient may also be helpful, in that childhood optic neuritis is uncommon. When neuroretinitis is suspected, a repeat evaluation including dilated fundus examination one to two weeks later can confirm the diagnosis.

Diagnosis: Neuroretinitis due to cat scratch disease

Tip: A fully developed macular star is readily recognized; in the very early and very late stages the fundus findings are more subtle. The presence of a macular star at any stage is inconsistent with a diagnosis of demyelinating optic neuritis.

Chronic “pink eye”

Case: This 75-year-old, active grandmother noticed mild redness of her right eye (Figure 7.4). Thinking she had caught an eye infection from one of her grandchildren (who had been treated for conjunctivitis several weeks previously) she did not seek medical advice immediately. The redness did not improve over the next three months, so she consulted her eye doctor who found normal visual function, ocular motility and fundus appearance. Warm compresses and observational management were recommended.

Figure 7.4 External appearance of a 75-year-old, active grandmother with mild conjunctival injection of the right eye.

Her red eye persisted and over the next month she developed new right periorbital pain and intermittent horizontal diplopia. A second examination revealed no objective abnormalities and she was given a mild analgesic for her headache and a return appointment in four months.

This patient had an additional non-ocular symptom which she did not volunteer because she didn’t think it was relevant to her eye problem, yet this symptom was an important clue to the correct diagnosis. What question should be asked?

This patient should be questioned about pulsatile tinnitus. Once queried, she admitted to “hearing her heartbeat” on the right side of her head for the past few months, suggesting the presence of a carotid-cavernous sinus fistula as the cause of her red eye. Four months later, examination of the right eye revealed more prominent dilation and tortuosity of conjunctival and episcleral vessels, mild conjunctival edema, 3 mm of right-sided proptosis, and a mild right abduction deficit (Figure 7.5A and B). Intraocular pressure was elevated in the right eye (25 mmHg OD vs. 18 mmHg OS) and funduscopy revealed mild tortuosity of the retinal veins. An orbital CT showed dilation of the right superior ophthalmic vein and mild diffuse enlargement of the extraocular muscles, consistent with orbital venous congestion (Figure 7.5C).

Discussion: A carotid-cavernous sinus fistula represents an abnormal communication between the carotid system and the cavernous sinus, introducing arterial blood into the venous space and thus leading to an increase in venous pressure. A “direct” or “high flow” fistula occurs when the communication is between the internal carotid artery and the cavernous sinus, usually the consequence of trauma. Due to the high-flow shunting of arterial blood, clinical manifestations of a direct fistula are sudden and dramatic. In contrast, a dural fistula occurs when the defect involves a meningeal branch of the external or internal carotid artery, resulting in a

low-flow communication with the cavernous sinus. Such low-flow fistulas usually arise spontaneously, typically in postmenopausal women and during pregnancy, and the associated signs and symptoms are generally milder.

The drainage pattern of the additional blood volume from the cavernous sinus dictates the clinical presentation of a dural fistula. In most cases, the shunted blood flows anteriorly through the superior and inferior ophthalmic veins, causing a variety of signs and symptoms related to orbital venous congestion. Occasionally, drainage is directed posteriorly through the superior or inferior petrosal sinuses. In these cases, an isolated cranial nerve palsy (usually sixth nerve, occasionally fourth) may be the only clinical manifestation. Due to the absence of orbital congestion, such cases are sometimes referred to as a “white-eyed shunt” and pose more of a diagnostic challenge. Uncommonly, signs and symptoms are bilateral or even contralateral to a unilateral fistula due to prominent intercavernous venous connections. Regardless of the direction of drainage, most patients have some degree of ipsilateral pain, although the severity is quite variable. While helpful when present, a bruit (subjective or objective) is reported in only 25% of cases.

Examination typically shows signs of orbital congestion including proptosis, chemosis, lid edema and conjunctival injection. Abnormal ocular motor

motility may be due to cranial nerve palsy secondary to pressure and/or ischemia within the cavernous sinus or to extraocular muscle dysfunction caused by muscle swelling. Increased orbital venous pressure, also causes an increase in episcleral venous pressure, resulting in elevated intraocular pressure and congestion of surface vessels. In its full-fledged form, the episcleral vessels are dilated and tortuous with a classic “corkscrew” appearance characterized by looping in a radial pattern from the corneal limbus, an appearance that is virtually diagnostic (Figure 7.6). Milder forms may be more difficult to distinguish from anterior segment inflammatory conditions (conjunctivitis, episcleritis and scleritis). Posterior segment changes of venous congestion include retinal venous stasis or obstruction, choroidal folds and effusion and disc swelling. Visual loss, when it occurs, is multifactorial, with elements of glaucomatous damage to the disc, compression of the intracranial optic nerve by a dilated cavernous sinus and ischemia of the optic nerve and/or retina.

In most cases, orbital signs are prominent and thus the main differential diagnoses are thyroid orbitopathy, orbital inflammatory disease (cellulitis or pseudotumor) and cavernous sinus thrombosis. There is some overlap in the radiographic appearance in these conditions and thus a diagnosis cannot be made solely on the basis of scan findings. MRI and CT do not directly demonstrate a dural

Figure 7.6 Close-up view of the superior conjunctiva in a different patient with a dural-cavernous fistula, showing characteristic “corkscrew vessels”.

A

fistula but instead reveal signs related to increased orbital venous drainage, providing indirect radiographic support for the diagnosis. Enlargement of the extraocular muscles is common and may be mistakenly attributed to Graves’ disease. Associated enlargement of the superior ophthalmic vein is an extremely helpful finding, typically present in fistulas but not in thyroid eye disease. This same radiographic appearance, however, may also be seen in cavernous sinus thrombosis. In some cases, careful inspection will show relative fullness of the cavernous sinus on the side of the fistula and an MRA may show subtle hypervascularity (Figure 7.7). Catheter angiography is the only way to conclusively demonstrate a duralor carotid-cavernous fistula,

B C