Neuro-ophthalmic Manifestations of Chronic Basilar Artery Occlusion
Chronic basilar artery occlusion can result in either afferent or efferent visual pathway damage. In rare instances, both afferent and efferent symptoms arise from a single lesion. Afferent visual symptoms (e.g., homonymous hemianopsia, cortical blindness) are due to the posterior cerebral arteries (terminal arteries of the basilar artery) which supply the occipital lobes. While efferent visual symptoms (e.g., diplopia, nystagmus) can occur due to ischemia from the perforating arteries of the basilar trunk to the brainstem.
Posterior circulation strokes account for about 15% to 20% of all ischemic strokes but basilar artery occlusion (BAO) is only 1% to 4% of all ischemic strokes.
The basilar artery starts at the medullopontine junction and terminates at the pons-midbrain junction. The basilar artery is split into three sections: proximal, middle, and distal. The basilar artery terminates as bilateral posterior cerebral arteries. Occlusive lesions can occur anywhere along the basilar artery and frequently manifest as pons ischemia. Most of the ischemia occurs in the paramedian base, but sometimes occasionally in the paramedian tegmentum. Atherosclerotic lesions are most common in the proximal and mid-basilar arteries, but thromboembolism and dissection are more common causes of posterior circulation large vessel ischemia including BAO. Top of the basilar syndrome and propagation of thrombus can be life threatening or can result in “locked in syndrome” where patients are awake and alert (intact cortex) but have lost all voluntary motor control (except sometimes the eye movements).
Signs and Symptoms
The pons includes the sixth nerve nucleus and fascicle as well as the horizontal gaze pathways via the medial longitudinal fasciculus to the midbrain third nerve nucleus. Thus, ischemia to the efferent visual pathway of the brainstem can produce neuro-ophthalmic findings including unilateral or bilateral internuclear ophthalmoplegia (INO), One-and-a-half syndrome (a conjugate gaze palsy combined with an INO), and eight-and-a-half syndrome (One-and-a-half syndrome with seventh nerve palsy). In addition, upper and lower extremity paraesthesia as well as generalized weakness, difficulty walking, and light-headedness can also be found depending on the location of specific brainstem lesions. In the afferent visual pathway, because the posterior cerebral arteries are the terminal branches of the basilary artery, the most frequent finding in BAO is homonymous hemianopia but cortical visual loss can occur from bilateral occipital ischemia. Occasional variations, such as occlusion of the anterior/posterior choroidal arteries can lead to homonymous sectoranopia, a wedge-shaped defect located near the horizontal meridian.
As with other ischemic changes, non-contrast head CT and CT angiography (CTA) are included in the initial workup of BAO. Vascular imaging (e.g., CTA) is a more sensitive method for determining the degree of stenosis and infarction in BAO. Diffuse weighted imaging (DWI) sequences in magnetic resonance imaging (MRI) of the head can demonstrate acute ischemic infarct. CT or MRI of the neck with CTA or MRA of the neck can demonstrate the occlusion as well as more proximal disease in BAO (e.g., vertebral artery dissection). Although CTA and MRA are often diagnostic in BAO, catheter digital subtraction angiography may be necessary in some cases.
The differential diagnosis is broad and encompassing both cortical and brainstem lesions that produce either efferent or afferent symptoms, these include but are not limited to:
•Millard Gubler Syndrome
There is no consensus on the best acute management of BAO, but most are treated like large vessel occlusive disease in other locations. An acute stroke evaluation and possible treatment if within the treatment window for thrombolytic or thrombectomy treatment options are recommended in BAO. Although both intra-arterial thrombolysis (IAT) and intravenous thrombolysis (IV t-PA) have been used in BAO, one meta-analysis study of 420 patients BAO showed no difference in outcomes between patients treated with IAT or IVT. Mechanical thrombectomy has shown promising results among patients with BAO who presented 6 to 24 hours after symptom onset. Acute and complete BAO typically leads to morbidity and possible long-term disability or death in the absence of spontaneous or post-treatment vascular recanalization. Some patients however have sufficient collaterals to allow alternate routes of perfusion to critical structures in chronic BAO. After a complete stroke evaluation including embolic and cardiogenic etiologies, the decision for antiplatelet (e.g., aspirin or dual platelet therapy) or anticoagulation will likely rest with a multidisciplinary team that includes the primary care physician and stroke neurology. Control of treatable vasculopathic risk factors (e.g., hypertension, diabetes mellitus, hyperlipidemia, weight loss, and smoking cessation).
Acute and complete BAO in the past was associated with extremely high mortality (up to 90%) and high morbidity (e.g., pontine “locked-in” syndrome). However, more recent evaluation and treatment has improved the prognosis for BAO. Improved neuroimaging techniques, access to stroke centers, faster recognition of posterior stroke symptoms, and better treatment options (e.g., thrombolysis) have improved outcomes in BAO.
Although most patients with acute BAO have other neurologic findings that suggest posterior fossa ischemia, BAO can produce a wide range of neuro-ophthalmologic symptoms and signs affecting both the efferent and afferent visual pathways. These include lesions of the abducens nucleus (horizontal gaze palsy), ipsilateral or bilateral MLF (unilateral or bilateral INO), and facial nucleus/fascicular lesion (cranial nerve VII palsy). The combination of these efferent abnormalities with afferent visual pathway complaints (e.g., homonymous hemianopsia, cortical blindness) is suggestive of vertebrobasilar disease including BAO. Prompt and aggressive stroke evaluation and timely thrombolytic therapy or other emergent neuroradiological interventions can be vision, brain, or lifesaving.
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