Acute Isolated Sixth Nerve Palsy Caused by Unruptured Intradural Saccular Aneurysm

Abstract

Background:

Unruptured intracranial aneurysms that cause isolated sixth nerve palsies are usually located extradurally in the cavernous sinus or intradurally as part of fusiform enlargement of the parent artery. Intradural saccular aneurysms of the vertebrobasilar circulation are a rare but life-threatening cause of isolated sixth nerve palsy. We provide documentation of 2 such cases.

Methods:

Case records of the Neuro-ophthalmology Clinics at the University of Michigan.

Results:

A 56-year-old woman with an arteriosclerotic risk profile reported new diplopia and had an isolated left sixth nerve palsy caused by an unruptured saccular aneurysm of the anterior inferior cerebellar artery. It was successfully coiled, and the sixth nerve palsy markedly improved. A 62-year-old woman with an arteriosclerotic risk profile and new diplopia had an isolated right sixth nerve palsy caused by a large unruptured vertebral artery aneurysm. It was coiled and later stented and recoiled. These procedures isolated the aneurysm and relieved the sixth nerve palsy.

Conclusions:

Isolated sixth nerve palsies in these 2 older adult patients with arteriosclerotic risk profiles were caused by unruptured intradural vertebrobasilar aneurysms. This report adds documentation to a rare phenomenon that must be taken into consideration in deciding whether brain imaging is needed in isolated sixth nerve palsy.

When intracranial aneurysms cause sixth nerve palsy, they typically are fusiform and arise from the intracavernous carotid artery or the vertebrobasilar system (1,2). Because intracavernous aneurysms lie extradurally, rupture causes a carotid-cavernous fistula, which is not life-threatening. Fusiform dolichoectatic aneurysms of the vertebral and basilar arteries, which are caused by arteriosclerotic changes in the inner layers of the vessel, lie intradurally and rarely rupture, instead causing damage mostly by compression or ischemia of adjacent structures (3).

Conversely, rupture of intradural saccular aneurysms causes subarachnoid hemorrhage and a high rate of morbidity. Such saccular aneurysms rarely produce an isolated sixth nerve palsy. We are aware of only 2 reported cases of isolated sixth nerve palsy from unruptured vertebrobasilar aneurysms. Jindal et al (4) described a patient with an aneurysm presumed to have originated from the anterior inferior cerebellar artery (AICA) by MRI, but confirmatory digital angiography was not documented. Er et al (5) reported a case of an isolated sixth nerve palsy attributed to an unruptured and thrombosed AICA aneurysm.

We describe 2 patients who had sixth nerve palsy as the only clinical manifestation of unruptured intradural saccular aneurysms of the vertebrobasilar circulation, one involving the AICA and the other involving the vertebral artery (VA).

CASE 1

A 56-year-old woman with systemic hypertension developed sudden diplopia and headache. A diagnosis of left sixth nerve palsy was initially attributed to migraine because the patient described the pain as episodic and she had a remote history of migraine. Eight days later, she returned with worsening headaches. Brain MRI revealed an enhancing posterior fossa lesion with mass effect and underlying vasogenic edema of the brainstem (Fig. 1A, B). Digital subtraction angiography confirmed a saccular aneurysm projecting superiorly and posteriorly from the midportion of the left AICA (Fig. 1C). Brain MRI performed 3 years earlier for chronic episodic headaches had revealed no evidence of an aneurysm.

FIG. 1.

Case 1. A. Postcontrast T1 axial MRI shows a heterogeneously enhancing mass (arrow) arising from the expected location of the left anterior inferior cerebellar artery (AICA). B. Axial fluid-attenuated inversion recovery image reveals that the lesion has mass effect with edema within the adjacent brainstem (arrow). C. Digital subtraction angiogram confirms the presence of an aneurysm of the left AICA (arrow). D. Axial FIESTA (fast imaging employing steady-state acquisition) scan demonstrates the proximity of the coiled aneurysm (arrow) to the emergence of the left sixth nerve (arrowhead) at the pontomedullary junction.

The AICA aneurysm was successfully coiled. Postoperatively, the patient had a complete left abduction deficit but no additional abnormalities. A follow-up MRI acquired using FIESTA (fast imaging employing steady-state acquisition) sequence demonstrated proximity of the coiled aneurysm to the emerging sixth nerve at the pontomedullary junction (Fig. 1D). Thirteen months after treatment, she had regained full abduction of the left eye.

CASE 2

A 62-year-old woman with hypercholesterolemia and coronary artery disease developed intermittent horizontal diplopia that eventually became constant. Examination disclosed an abduction deficit of the left eye with esotropia of 30PD in primary gaze position. The left sixth nerve palsy initially was attributed to microvascular ischemia. However, because of an ensuing report of intermittent diplopia, a brain MRI was performed 43 days later. It revealed a lesion that was confirmed by digital angiography to be a large right VA aneurysm crossing the midline and compressing the left ventral paramedian brainstem (Fig. 2).

FIG. 2.

Case 2. A. Postcontrast T1 coronal scan shows a heterogeneously enhancing mass (arrow) arising from the right vertebral artery. B. T2 axial MRI shows that the lesion (arrow) projects medially from the vertebral artery across the midline with mass effect on the underlying left pontomedullary junction. C. Digital subtraction angiogram delineates a saccular aneurysm of the distal right vertebral artery (arrow).

The VA aneurysm was successfully coiled. Six months later, esotropia in primary gaze had improved to 6 PD but imaging demonstrated aneurysmal filling owing to coil compression. The patient underwent stenting and further coil embolization. Two years later, cerebral angiography showed no aneurysmal filling and a widely patent stent in the parent vessel. At the last follow-up, the patient only experienced diplopia in far left gaze.

DISCUSSION

Each of our 2 patients had a sixth nerve palsy owing to compression by an unruptured saccular aneurysm of the vertebrobasilar circulation. Yet both had arteriosclerotic profiles to initially suggest a microvascular ischemic cause. In Case 1, brain imaging that revealed the aneurysm was prompted by worsening headache; in Case 2, brain imaging was triggered by the slightly atypical history of intermittent diplopia.

We are reporting rare phenomena. Aneurysms of the posterior circulation make up only 8%–12% of all intracranial aneurysms. AICA aneurysms constitute a small fraction of posterior circulation aneurysms (6). Most of them are located at the basilar-AICA junction, slightly medial and rostral to the exit of the sixth nerve at the pontomedullary junction (7). The aneurysm in our Case 1, like the aneurysm previously reported to have caused an isolated sixth nerve palsy (5), was located somewhat more distally than the typical AICA aneurysm. In that unusual position, it could compress the sixth nerve where it emerges from the brainstem. This relationship was visualized on the post-coil MRI FIESTA sequence.

In our Case 2, the aneurysm seemed to arise from the VA, proximal to its junction with the basilar artery and rostral to the origin of the posterior inferior cerebellar artery, where the majority of VA aneurysms arise (8). Unruptured VA aneurysms are not reported to cause sixth nerve palsy by direct compression, probably because most are too caudal to reach the sixth nerve (8,9). Among 40 patients with such aneurysms, none caused a sixth nerve palsy (10), but rather caused dysesthesias, ataxia, and nystagmus, related to their mass effect on adjacent cervicomedullary structures that are caudal to the exit of the sixth nerve (11). An unruptured VA aneurysm would have to be very large to compress the sixth nerve, as was documented in a previously reported case (12). In that patient, as in ours, the lack of other neurologic manifestations is unexplained.

In reports of patients with isolated sixth nerve palsy, aneurysm has not appeared as a cause. Most occur in adults with vasculopathic profiles. The palsy resolves spontaneously, and brain imaging is not performed (13). In a prospective study that included 62 patients with isolated sixth nerve palsy who underwent prompt brain imaging (14), 50 (80%) had no other pertinent abnormalities and were presumed to have had microvascular ischemia of the extra-axial portion of the nerve. Ten (17%) patients had strong historical evidence for a nonischemic cause of sixth nerve palsy. Only 2 patients (3%) had unexpected nonischemic causes: one petroclival meningioma and one cavernous sinus B-cell lymphoma. A delayed diagnosis would not have adversely affected their medical outcome. In a review of sixth nerve palsy (2), aneurysm was a rare cause, and always located on the extradural intracavernous portion of the carotid artery.