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Editor—Posterior ischaemic optic neuropathy (PION), which is a diagnosis of exclusion, has been associated with a variety of vascular diseases such as giant cell arteritis, systemic lupus erythematosus, atherosclerosis, polyarteritis nodosa, hypotension, and rarely acute occlusion of the internal carotid artery.1 We present a patient with monocular PION resulting from ipsilateral internal carotid artery dissection.
A 55-year-old man presented to our neuro-ophthalmology unit with blindness in the right eye, which he had noticed on awakening 3 days ago. His past medical history showed a 5 year history of hypertension and 3 year history of diabetes mellitus with poor control. He had a 1 month history of right frontal headache occasionally associated with blurred vision in the right eye. On examination, visual acuity was no light perception in the right eye, 20/20 in the left eye, and a right afferent pupillary defect was present. Extraocular motility was full in each eye. The slit-lamp examination was unremarkable. Dilated fundus examination showed a depigmentation patch in the pupillomacular bundle in the right eye, normal retinal vessels, and healthy appearing optic discs on both eyes. Fluorescein angiogram examination showed normal circulation time of retinal vessels in both eyes (Fig 1). The Humphrey visual field examination of the left eye was normal. Orbit computed tomography did not show a swollen optic nerve or any abnormality. Right carotid angiography revealed a string sign suggestive of internal carotid artery dissection 3 cm above the bifurcation (Fig 2). Anticoagulant treatment was started but was unsuccessful. Evidence of retinal ischaemia was not seen in the follow up period although by 6 weeks the optic disc had been pale. One month after the episode, he developed proptosis, ocular motility limitation, and conjunctival congeston in the right eye. There was a subjective bruit in the right retrobulbar area.
Repeated carotid angiography showed a dural-cavernous arteriovenous shunt in the right cavernous sinus, which was caused by external carotid arteries of both side. Neuroradiological intervention with embolisation was done. Four weeks later, the ocular symptoms disappeared but he is still blind in the right eye.
The reported ophthalmic manifestations of carotid dissection include postganglionic oculosympathetic paralysis,2 3 transient monocular blindness,2 3 homonymous visual field defect,3 ocular ischaemic syndrome,4 6th nerve palsy,5 and transient cilio-retinal artery occlusion.6 Internal carotid artery dissection with subsequent extension of the dissection, or attendant thrombus across into the ipsilateral ophthalmic artery or embolism explains the patients with central retinal artery occlusion reported by Newmanet al.7 The only case with PION caused by traumatic carotid artery dissection was reported by Rivkin et al. They postulated that the occlusion of internal carotid perforators to the intracranial portion of the optic nerve with preservation of the circulation subserving the anterior optic nerve and retina explained the mechanism of monocular sudden blindness with normal looking fundi in their patient.8
As our patient had a normal ocular fluorescein angiogram, normal extraocular motility, and normal optic nerve shown by computed tomography, we believed the circulation of the ophthalmic artery was normal in the initial stage of vision loss. The lesion site should be more proximal to branches of the internal carotid artery. Unfortunately, we cannot prove this by the cerebral arteriogram.
The optic nerve head receives its blood supply from the central retinal artery and the posterior ciliary artery, both arteries are branches of the ophthalmic artery. The intraorbital and intracanalicular portions of the optic nerve also receive their blood supply from the ophthalmic artery via pial capillaries.9 The intracranial portion of the optic nerve, however, derives most of its blood supply from anterior cerebral, anterior communicating branches, and internal carotid artery perforators.10 Infarction of this portion of the optic nerve may cause monocular blindness yet preserve the ophthalmic, retinal, and ciliary circulation, as shown in our patient. We are not sure whether the subsequent development of ipsilateral dural-cavernous fistula is related to the occlusion of internal carotid perforators to the intracranial portion of the optic nerve or just a coincident episode.
PION causes no particular changes in the optic disc at the initial stage. It may be accompanied by retrobulbar pain, making it hard to differentiate from retrobulbar optic neuritis. Isayama et al had reported 14 cases of PION and proposed the detail criteria in clinical diagnosis of PION.11 Most of their cases were over 50 years of age and always associated with systemic disease such as hypertension, diabetes mellitus, hyperlipidaemia, and hypotension, etc. Demonstration of abnormal haemodynamics in the posterior portion of the optic nerve by carotid angiography and fluorescein fundus angiography may help us establish the diagnosis.
We think our patient provides another example of spontaneous internal carotid artery dissection presenting as isolated PION. As the visual field in the left eye and fluorescein angiography in both eyes were normal, the probable mechanism was occlusion of the ipsilateral internal carotid perforators to the intracranial portion of the optic nerve with preservation of the circulation of ophthalmic artery subsurving the anterior optic nerve and retina.
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