Bilateral abducens nerve palsy by compression from bilateral anterior inferior cerebellar artery

Abstract

Isolated abducens nerve palsy affects patients with various pathological conditions, including head trauma, brain tumor, and infection. Systemic conditions such as diabetes mellitus or hypertension are also risk factors for abducens nerve palsy and may cause ischemic neuropathy [1]. Furthermore, compression by vascular abnormalities such as dolichoectatic arteries or aneurysms may induce abducens nerve palsy at its long extracerebral segment [2–7]. However, bilateral cases of isolated abducens nerve palsy are rare. Such cases have been reported in patients with ruptured aneurysms of the anterior communicating [8] or vertebrobasilar artery [9, 10], bilateral intracavernous carotid artery aneurysms [7], or subarachnoid hemorrhage without aneurysm [9]. Here, we report a case of bilateral isolated abducens nerve palsy due to compression by the bilateral anterior inferior cerebellar artery (AICA). A 75-year-old man developed diplopia without ocular pain 6 months earlier. He had undergone surgeries for gastric ulcer 33 years earlier and lung adenocarcinoma 9 years earlier. Follow-up for lung cancer revealed no evidence of relapse or metastasis. Two months earlier, diplopia worsened without other symptoms. Neurological examination revealed left ptosis, which had previously been identified. Additionally, there was complete right, and moderate left, abducens nerve palsy, but no abnormalities in pupil size, light reflex, and vertical eye movement (Fig. 1a). Laboratory tests showed slightly elevated glycosylated hemoglobin Alc level (6.6%; normal range: 4.3–5.8%), but normal erythrocyte sedimentation rate and thyroid function. Anti-acetylcholine esterase antibody and edrophonium tests were negative. Cerebrospinal fluid analysis showed a mild increase in protein content (50 mg/dL) and normal cell count (1 lymphocyte/lL) without malignant cells. Positron emission tomography/computed tomography scan revealed no abnormalities. Brain magnetic resonance (MR) imaging, including with gadolinium enhancement, showed no brainstem or cavernous sinus lesions. MR angiography demonstrated dolichoectatic basilar and left vertebral arteries (Fig. 1b). MR 3D heavy T2-weighted imaging showed stretching of the right abducens nerve by the right AICA (Fig. 1c) and compression of the left abducens nerve at the point of its exit zone by the left AICA (Fig. 1d). The condition improved slightly over 4 months, and there was no evidence of focal or systemic recurrence of lung cancer at 25 months after the onset of diplopia. An anatomical study showed that the ventral surface of the abducens nerve was crossed by the AICA in 75% of autopsied brains and that, in the majority of cases, the AICA was in contact with the abducens nerve [11]. Another study showed the AICAs or their branches were in contact with it in all cases of 44 hemispheres, and pierced in 11.4% [12]. In this patient, the left vertebral artery and basilar arteries were dolichoectatic with a deviation to the right, and the bilateral abducens nerves were compressed by the A. Taniguchi (&) Y. Ii H. Tomimoto Department of Neurology, Mie University Graduate School of Medicine, Edobashi 2-174, Tsu, Mie 514-8507, Japan e-mail: a2005t@clin.medic.mie-u.ac.jp