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A 73 year old man was evaluated for the sudden onset of binocular horizontal diplopia which was worse in left gaze and which began 1 day before initial examination. He also complained of a dull headache over his left brow. He had a medical history of hip and knee surgery and was taking no medications. He was a 50 pack a year smoker but had no other history of vascular disease, including hypertension and diabetes mellitus. He had no previous history of strabismus or eye muscle surgery. His referring ophthalmologist was concerned about giant cell arteritis (GCA) and ordered a Westergren erythrocyte sedimentation rate test, which was 15 mm in the first hour.
Additional history revealed that he had no jaw claudication, scalp tenderness, or other symptoms of GCA. Visual acuity was 20/25 in both eyes and his colour vision and confrontation visual fields were normal. His pupils were equal in size and briskly reactive without a relative afferent pupillary defect. A left abduction deficit was noted (fig 1) and, with alternate cover testing, there was a 10 prism dioptre esotropia in primary position and at near, which increased to 20 prism dioptres on left gaze and decreased to 2 prism dioptres in right gaze. He had slowed saccades of the left lateral rectus muscle. There was no evidence of ptosis or ocular motor synkinesis. The remainder of his cranial nerve and dilated fundus examination were normal. Magnetic resonance imaging (MRI) (fig 2) and magnetic resonance angiography (MRA) (fig 3) of the brain revealed a lateral course of the left cavernous carotid artery consistent with dolichoectasia.
Follow up examination 1 month later revealed no history of variability of the diplopia and no change in the ocular misalignment; however, over the next 2 months the patient reported a gradual improvement in symptoms. He returned 3 months after the initial onset of symptoms and his abduction deficit had resolved. There was no evidence of an ocular misalignment with alternate cover testing. Repeat MRI/MRA showed no change in the calibre or position of the left cavernous carotid artery. He has reported no new symptoms with 1 month of additional follow up.
Dolichoectasia, or pathological enlargement, of the intracranial arteries is a finding rarely seen with neuroimaging or arteriography. Arteriosclerosis, with thinning of the media and defects in the internal elastic laminae of the vessel walls, is thought to predispose to progressive enlargement of the vessel lumen.1 Ectasia of the intracranial arteries is believed to cause symptoms because of compression of adjacent structures and/or ischaemia secondary to intraluminal thrombus formation and blockade of perforating vessels along the length of the dolichoectatic vessel.1
Dolichoectasia of the cavernous carotid artery has been suggested as an infrequent cause of sixth nerve paresis. One in 23 patients with carotid ectasia (in a series of approximately 40 000 patients undergoing carotid arteriography) was found to have an acute sixth nerve palsy with “good recovery,” although the clinical course was not specified.2 Ipsilateral dolichoectasia was noted in a 59 year old man with seven episodes of sixth nerve paresis, each lasting between 2–5 weeks.3 The authors did not provide an explanation for the mechanism of recurrence. A single patient with bilateral sixth nerve paresis was reported to have bilateral carotid dolichoectasia as the underlying cause.4 However, in the discussion the causal relation of the dolichoectasia, presumably from compression of the carotid artery, was called into question. In addition, dolichoectasia of the cavernous carotid artery has been noted in patients without ocular motor deficits.1
This patient’s left sixth nerve paresis spontaneously resolved 3 months after the initial onset of symptoms. Despite the presence of ipsilateral cavernous carotid dolichoectasia, his clinical course is most consistent with that of a vasculopathic sixth nerve paresis. Whether the dolichoectasia was causative or an incidental finding is not clear in this patient. Arterial dissection in a previously ectatic vessel has been suggested as an explanation for the acute onset of symptoms in patients with dolichoectasia5; however, no evidence of arterial dissection was seen in this patient’s MRI/MRA. Ischaemia of the vaso vasorum of the sixth nerve, perhaps because of intraluminal thrombus formation, may have resulted in a vasculopathic sixth nerve palsy, but there was no evidence of thrombus formation on the MRI/MRA.
Because the causative mechanism in patients with persistent sixth nerve paresis from presumed dolichoectasia is not certain treatment guidelines are not clear. Monocular occlusion and prism therapy may provide temporary or long lasting relief of diplopia. Neurosugrical intervention to relieve mechanical compression between the cavernous carotid artery is a difficult, potentially life threatening, procedure. Extraocular muscle surgery may correct the ocular misalignment, without treating the underlying mechanical compression, with uncertain long term benefit. Spontaneous resolution of the left sixth nerve palsy in this patient with ipsilateral carotid dolichoectasia suggests that a period of careful observation should precede plans for surgical correction of the ocular misalignment.
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