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The importance of internal carotid artery occlusion is emphasized by the numerous cases recently reported in the literature (4, 6, 8, 11, 16), but, whether partial or complete, it is diagnosed with difficulty, since the clinical picture may resemble such intracranial lesions as cerebral tumour, cerebral haemorrhage, peripheral cerebral artery occlusion, or subdural haematoma. The clinical findings will depend chiefly on the rapidity of the occlusion and on the collateral circulation but may be modified by other factors such as anaemia, hypotension, cardiac failure, or arrhythmias. Palpation for internal carotid artery pulsation in the neck or pharynx is often misleading. Insufficiency may be suggested by homolateral retinal blanching and syncope on compression of the contralateral side, but the diagnosis must always be presumptive in the absence of carotid arteriography. It is for the radiologist, therefore, to perform the definitive examination, by which the clinical impression is either confirmed or excluded. While previously this condition was of only academic interest, advances in peripheral vascular surgery and in long-term anticoagulant therapy have made accurate radiological evaluation essential. It is the purpose of this paper to show that errors in arteriographic technique may result in appearances simulating internal carotid artery occlusion. Few published reports stress the importance of such errors (2, 7, 9). The following cases from the National Hospital for Nervous Diseases, London, illustrate some of the difficulties that may arise. Subintimal Injection Case I: I. M., a 60-year-old white woman, was admitted with a history and physical findings suggesting a suprasellar lesion compressing the optic chiasm. A right carotid arteriogram showed only faint filling of the internal carotid artery in the neck, while some of the contrast medium lay subintimally at the site of injection (Fig. 1, A). An anteroposterior film of the neck (Fig. 1, B) during a repeat injection showed complete obstruction of both internal and external carotid arteries, with reflux into the vertebral system. A fine curvilinear negative shadow, representing the stripped intima, was noted just distal to the needle tip. Another examination, five days later, was normal, with no evidence of obstruction (Fig. 1, C). Case II: A. M., a 38-year-old white woman with epilepsy of late onset, exhibited features suggesting a focus in the right temporal lobe. Physical examination was negative, apart from possible intellectual deterioration. Electroencephalographic tracings indicated a right temporal abnormality. A right carotid arteriogram showed complete obstruction to the flow of contrast substance, 1.5 cm. distal to the needle point, with some of the medium lying subintimally at the point of injection (Fig. 2, A) and reflux into the right vertebral and basilar arteries via the subclavian artery.