Lateralized carotid artery: An unusual cause of pulsatile tinnitus.
We describe the case of a patient who had a pulsatile tinnitus that was caused by a laterally displaced internal carotid artery. Her condition was treated with the use of a hearing aid, which suppressed the tinnitus. We also review the literature on laterally displaced internal carotid arteries, and discuss their differentiation from a congenitally aberrant artery.
A laterally displaced internal carotid artery (ICA) in the middle ear is rare. In this article, we describe one such case and review what has thus far been reported about this condition.
A 59-year-old woman came to the Ear, Nose, and Throat Clinic with a 6-month history of pulsatile tinnitus of the left ear. Otoscopic examination revealed a reddish mass behind an intact drum. Her tuning fork tests were normal. Neurologic examination, including that of the lower cranial nerves, was normal. Pure-tone audiometry showed a mild bilateral sensorineural hearing loss. A 24-hour urine catecholamine test was within the age-adjusted normal range.
Computed tomography (CT) and magnetic resonance angiography (MRA) were performed. The CT (figure 1) and MRA (figure 2) scans of the temporal bone demonstrated that the extracranial portion of the left internal carotid artery (ICA) entered the petrous part of the left temporal bone and perforated the medial wall of the mesotympanum. The extracranial portion of the ICA was bent at a sharp angle before turning medially to become the horizontal portion of the ICA. Its appearance was characteristic of a laterally displaced ICA. The patient was aided with a digital behind-the-ear hearing device, which successfully suppressed the annoying tinnitus.
The ICA traverses the temporal bone in a complex route. It enters through the carotid foramen and ascends vertically in the petrous temporal bone to the level of the cochlea through the bony carotid canal. The canal then bends at a right angle, courses horizontally in an anteromedial direction, and ends at the apex of the petrous pyramid. At the genu of the horizontal and vertical portions, the canal is separated from the middle ear cavity by a relatively thin bony plate. A small branch (the caroticotympanic artery) arises from the ICA near the genu and perforates the bony plate of the carotid canal to enter the posteromedial tympanic cavity.
In rare situations, normal arterial variants--such as an aberrant carotid artery and a laterally placed carotid artery--are encountered.  Most of these variants are asymptomatic. When they are symptomatic, hearing loss and pulsatile tinnitus are the most common complaints, followed by vertigo and bleeding. The cardinal physical finding is a reddish mass behind an intact drum. The differential diagnosis includes a glomus tumor, middle ear effusion, high jugular bulb, and cholesterol granuloma. Most reported patients were initially misdiagnosed as having a glomus tumor or middle ear effusion. It was not until a myringotomy or biopsy was performed, which resulted in profuse or even uncontrolled bleeding, that this diagnosis was considered.  Because mismanagement of this condition can lead to catastrophic consequences, early recognition by radiographic study is essential before any form of surgical procedure is performed. Prior to the emergence of MRA, carotid angiography was the definitive radiologic stud y for this condition. Carotid angiography has been replaced by MRA and CT as the preferred diagnostic procedure.
The aberrant carotid artery occurs when an enlarged inferior tympanic artery anastomoses with an enlarged caroticotympanic artery as a result of a regression of the cervical ICA during embryogenesis. The aberrant artery enters the tympanic cavity via an enlarged inferior tympanic canaliculus posterior to the vertical segment of the ICA. It traverses the cochlear promontory and joins the horizontal carotid through a dehiscence in the carotid plate. 
A laterally displaced ICA in the middle ear is rare. It is formed when the lateral wall of the petrous ICA is dehiscent. The bony dehiscence is usually present at the genu between the vertical and horizontal segments of the petrous ICA, and it allows the vessel to protrude into the middle ear cavity. Inside the middle ear, the ICA can swing more posteriorly and laterally and give a false impression of an aberrant artery. The bony defect can be caused by a congenital failure of ossification or erosion by cholesteatoma [3,4] Another explanation involves the abnormal persistence of embryonic vessels, which pull the elongated and tortuous artery into the middle ear. This explanation is supported by the association of the lateralized carotid artery with a persistent stapedial artery [5,6] and with a fibrous band at the site of the artery. 
A lateralized carotid artery can be differentiated from an aberrant artery only by close inspection of the CT scan. CT will identify a laterally displaced artery penetrating the floor of the middle ear cavity anterolateral to the cochlea without passing through the enlarged inferior tympanic canaliculus.
Because most cases are asymptomatic, no treatment is warranted. Treatment should be reserved for cases when pulsatile tinnitus, hemorrhage, or cranial nerve palsy are present. Two procedures--the interposition of a Silastic sheet between the aberrant artery and the malleus and the disarticulation of the ossicular chain--have been reported to yield varying degrees of success. 
From the Division of Otorhinolaryngology, Department of Surgery (Dr. Pak and Dr. van Hasselt), and the Department of Diagnostic Radiology and Organ Imaging (Dr. Kew), Prince of Wales Hospital, The Chinese University of Hong Kong.
Reprint requests: Prof. Charles Andrew van Hasselt, Chief, Division of Otorhinolaryngology, Department of Surgery, Prince of Wales Hospital, The Chinese University of Hong Kong, 30-32 Ngan Shing St., Shatin, N.T., Hong Kong, SAR, China. Phone: +852-2632-2628; fax: +852-2646-6312; e-mail: email@example.com
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|Comment:||Lateralized carotid artery: An unusual cause of pulsatile tinnitus.|
|Author:||van Hasselt, C. Andrew|
|Publication:||Ear, Nose and Throat Journal|
|Date:||Mar 1, 2001|
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