Arterial-cavernous sinus fistula
Encephalotrigeminal angiomatosis (Sturge-Weber syndrome)
Retinal angiomatosis (von Hippel disease)
Ataxia telangiectasia (Louis-Bar syndrome)
Tuberous sclerosis (Bourneville syndrome)
The topic of neurovascular diseases is a broad one, and a number of the disorders discussed in earlier chapters belong in this category. The goal of this chapter is not to be all-inclusive, but rather to discuss the mechanisms of vascular disease that commonly affect the visual system. Inclusion of the phakomatoses in this chapter is appropriate because many of these diseases have neurovascular manifestations.
The vascular supply of the brain consists of the carotid system anteriorly and the vertebrobasilar system posteriorly. The circle of Willis is an anastomotic complex (of variable completeness) formed by the junction of the two systems (Figure 14–1). Common ischemic manifestations of cerebrovascular disease include transient neurological deficits and stroke. Seizures can also occur as a result of ischemia.
The vascular supply to the brain consists of the carotid arteries anteriorly and the vertebrobasilar system posteriorly (see also Figure 5–11). (Reproduced, with permission, from Waxman SG: Clinical Neuroanatomy, 26th ed. New York: McGraw-Hill; 2010, Figure 12–1.)
A transient ischemic attack (TIA) is an episode of transient neurological dysfunction that lasts less than 24 hours. Most TIAs last less than 20 minutes. The 24-hour time limit in the definition of a TIA is an arbitrary boundary; neurological episodes may last longer than 24 hours and still be reversible. Stroke occurs when ischemic events are of sufficient magnitude or duration to cause infarction. Vascular disease can cause a TIA or stroke in the carotid or the vertebrobasilar distribution (Table 14–1), producing signs and symptoms that help to localize the ischemic area of the brain and implicate specific arteries that supply the affected territory.
MECHANISMS OF CEREBROVASCULAR ISCHEMIA
LARGE VESSEL STENOSIS
SMALL VESSEL ATHEROTHROMBOSIS
TIAs or completed strokes in the distribution of the carotid system frequently involve the eye (Table 14–2).
MANIFESTATIONS OF ISCHEMIA IN THE CAROTID DISTRIBUTION
Causes of carotid insufficiency include emboli from the internal carotid arteries or more distant sources, stenosis of the internal carotid artery, internal carotid artery dissection, and thrombotic or vasculitic occlusion of the smaller terminal vessels in the carotid distribution (see Table 14–1).
Emboli to areas supplied by the carotid arteries may arise from carotid artery atheromas or from a more proximal source, such as the aortic arch or heart. Valvular heart disease, atrial myxoma, cardiac arrhythmia, patent foramen ovale with right-to-left shunt, carotid dissection, and antiphospholipid antibody syndrome are all potential sources of emboli in the carotid distribution.
Critical stenosis or occlusion of the internal carotid artery can cause neurological events from hypoperfusion. A significant reduction in distal flow generally requires 50% to 90% stenosis of the carotid artery. The redistribution of perfusion at the circle of Willis (see Figure 14–1) and other cerebral anastomotic connections may preserve function even with complete carotid occlusion, but TIAs or stroke can occur with even small fluctuations in blood pressure in this tenuous situation.
Carotid dissection occurs when a break in the intimal lining of the carotid artery allows intraluminal blood to dissect into the arterial wall. The dissection can narrow the lumen, occlude branches in the area of the dissection, or produce emboli. Carotid dissection was discussed in Chapter 13. Arterial dissection can also occur in the basilar and vertebral arteries.
Thrombosis of the smaller distal supply arteries to the brain or eye can cause transient or permanent ischemic events. Damage to the small vessels from systemic hypertension (lipohyalinosis), vasculitis, and hypercoagulable states can lead to local occlusion from thrombosis (see Table 14–1). Subcortical lacunar infarcts and anterior ischemic optic neuropathy result from these thrombotic mechanisms rather than from emboli.
TIAs or stroke related to carotid disease manifest as ipsilateral monocular visual loss and/or contralateral hemiparesis, clumsiness, numbness, paresthesias, and aphasia (see Table 14–2).
Transient monocular visual loss (TMVL, also called amaurosis fugax) is a TIA involving the retinal circulation. Visual loss can occur as a result of emboli to the central retinal artery or its retinal branches, or when hypoperfusion results from high-grade carotid stenosis. Patients often describe visual loss as a curtain or shade that is pulled over the vision from above or below, or as a more generalized dark cloud obscuring vision. The visual loss usually reaches its greatest extent before 30 seconds, and usually resolves in less than 5 minutes (range: 2–30 min). Other causes of transient visual loss are listed in Table 1–3.
Emboli to the central retinal artery and its branches can often be seen and identified with the ophthalmoscope and include cholesterol (Hollenhorst plaque), fibrin, or platelet material originating from carotid atheromatous ulcerations, and less commonly calcific emboli from cardiac valves or fat emboli from long bone fracture.
Hollenhorst plaques are cholesterol crystals that appear similar to mica flakes on ophthalmoscopy. They appear as shiny, refractile orange-yellow crystals slightly larger than the blood column, lodged at bifurcations in the retinal arterial tree (Figures 14–2 and 14–3). Because they are flat, they do not always occlude the vessel and may be asymptomatic.
Branch retinal artery occlusion.
A 62-year-old woman reported a sudden change in the vision of her right eye. (A) An embolus is seen, lodged at a bifurcation in the inferior vascular arcade. Retinal edema can be seen in the distribution of the occluded retinal artery. (B) Humphrey automated perimetry shows a superior altitudinal defect (mimicking an optic nerve–related visual field defect).
Fibrin-platelet emboli form a dull, white material that fills the lumen and occludes the arterioles, causing transient visual loss or retinal infarct. This toothpaste-like material can occasionally be seen in long strands, slowly snaking through the retinal arterial tree (see Figure 14–2). Hollenhorst plaques and fibrin-platelet emboli typically originate from atheromatous disease of the carotid arteries or the aortic arch.
Calcific emboli are dull-white discrete emboli that suggest a cardiac source, such as a diseased valve. Fat emboli can occur in patients with long bone and flat bone fractures or with pancreatitis. Other retinal embolic sources include septic vegetations from bacterial endocarditis, cardiac myxoma, and amniotic fluid.
Transient monocular visual loss in patients <40 years old is unlikely to be related to atherosclerotic disease. Potential causes include cardiac abnormalities (right-to-left shunts, valvular disease), retinal migraine (see Chapter 13), or hypercoagulable states (see Table 4–4).
Central retinal artery occlusion (CRAO) produces sudden, profound, painless visual loss. Acutely, the retinal arteries are narrowed, and the infarcted retina is pale and edematous. Sludging and “box car” formation (separation of red blood cell clumps with intervals of plasma) may be evident in the retinal vessels. Because no inner retinal layers are present over the foveola, this area does not swell and the normal color of the intact choroidal circulation creates the characteristic foveolar “cherry red spot” surrounded by pale retina (see Figure 6–4). If there is a portion of the retina supplied by a cilioretinal vessel (arising from the choroidal circulation), it will be spared (Figure 14–4). An embolus can be seen in the central retinal artery at the disc in 10% to 20% of patients. Retinal edema generally dissipates over several days, and over time the disc develops a mild diffuse pallor along with global nerve fiber layer dropout, and the narrowed arteries may display sheathing.
Central retinal artery occlusion with cilioretinal artery sparing.
(A) Retinal ischemia and edema are present throughout the fundus except for an area of the macula (enclosed by dotted line) supplied by a cilioretinal artery (arrow). (B) The visual field shows sparing of a portion of the visual field corresponding to the distribution of the cilioretinal artery and unaffected retina.
The most common mechanism of a CRAO is atheroembolism. The emboli usually arise from a carotid source, but the heart, aortic arch, or other distal sites can be sources of emboli. Giant cell arteritis is a possible cause in patients older than 50 years and must be an urgent consideration (directed history, erythrocyte sedimentation rate, and C-reactive protein) because failure to diagnose this condition early can result in bilateral blindness. In addition, localized atheromatous stenosis, vasospasm, hypoperfusion, and hypercoagulable states can cause a CRAO.
The prognosis for visual recovery following an embolic central retinal artery occlusion is poor. Studies demonstrate irreversible retinal damage after 60 to 100 minutes of occlusion. Therapies aimed at lowering the intraocular pressure (IOP) (glaucoma medications, paracentesis) in the acute setting to increase the perfusion gradient have a sound theoretical basis but are rarely effective. Other modalities (usually equally disappointing) include ocular massage to dislodge emboli and carbogen therapy.
Branch retinal artery occlusion (BRAO) results when emboli lodge in the more distal parts of the retinal arterial tree, resulting in a focal retinal ischemic event with a corresponding focal visual field defect (see Figure 14–3). Similar to CRAO, emboli are the most common cause, but retinal vasculitis and ophthalmic migraine are other possible mechanisms.
Ophthalmic artery occlusion may be difficult to differentiate from a CRAO. However, a “cherry red spot” is not present because the choroidal circulation is also affected. This entity is far more likely than CRAO to cause NLP (no light perception) or LP (light perception) vision.
Ocular ischemic syndrome is produced when chronic hypoperfusion of the eye occurs. This condition is usually the result of vascular occlusive disease of the carotid artery or aortic arch, where few collateral arterial anastomoses are present. Unlike embolic events, the signs and symptoms are slow in onset (Table 14–3) and may be confused with other ocular disorders.
OCULAR ISCHEMIC SYNDROME
Venous stasis retinopathy (VSR) is a manifestation of carotid occlusive disease characterized by dot and blot hemorrhages and venous engorgement. The relationship between retinal venous changes and carotid insufficiency is unclear, but the cause is most likely related to ischemia at the microvascular level. The retinal findings may be confused with diabetic retinopathy or central retinal vein occlusion.
Curiously, carotid occlusive disease seems to have a protective effect with regard to the development of hypertensive and diabetic retinopathy: Retinal disease is less severe on the side ipsilateral to severe carotid stenosis. Long-standing unilateral carotid occlusive disease can also “protect” the cornea from deposits of serum lipids, hence a unilateral corneal arcus suggests carotid stenosis on the side without the arcus.
Patients with poor ocular perfusion may describe transient visual loss following exposure to a bright light, similar to a persistent afterimage following flash photography. This symptom likely results from an inability of the retina to quickly recover metabolically because of ischemia. These patients would be expected to have a markedly abnormal photo-stress test (discussed in Chapter 2).
Carotid stenosis often produces an audible bruit, but this finding lacks sensitivity and specificity. Patients with total or near-total occlusion do not have a bruit (because there is little or no flow), and patients with bruits may have little or no carotid occlusion.
Carotid duplex ultrasound permits a noninvasive assessment of the extracranial carotid arteries. In skilled hands, this method is reasonably sensitive and specific. Transcranial Doppler uses ultrasound methods through specific acoustic windows in the skull to evaluate intracranial arteries but is less precise than carotid duplex studies. Magnetic resonance angiography (MRA) (see Figure 1–13) allows visualization of both extracranial and intracranial cerebral blood flow without injection of contrast material (though contrasted studies provide the best resolution). Computed tomography angiography (CTA) combines intravascular contrast injection with 3-D imaging, providing exquisite views of the vasculature and surrounding anatomy (see Figure 1–14).