Aberrant internal carotid artery in the middle ear: the duplication variant

Abstract

Vascular variants concerning the internal carotid artery (ICA) at the skull base level are rare. Correct workup and diagnosis in case of suspicion of such a variant are important as it mimics glomus tumours and could complicate myringotomy or middle ear surgery. We report a case of a 39-year-old woman presented with a 6-month history of right pulsatile tinnitus and aural fullness. Ear microscopy examination revealed a pale red pulsatile mass anterior to the umbo. Radiological assessment using CT and MRI/magnetic resonance angiography confirmed the diagnosis of an aberrant ICA. Interestingly, a duplication of the ICA was demonstrated, in which the enlarged inferior tympanic artery presented as the aberrant ICA, whereas a narrower collateral hypoplastic native ICA was also present. A conservative approach with regular follow-up appointments was recommended.

Background

An aberrant course of the internal carotid artery (ICA) in the middle ear is a rare but well-known non-specific otologic anomaly. It can lead to symptoms such as pulsatile tinnitus and hearing loss, or it can be asymptomatic.^1–3 Iatrogenic injury during middle ear surgery may have dramatic consequences.

Case presentation

A 39-year-old otherwise healthy woman presented at the outpatient department with a 6-month history of right pulsatile tinnitus and aural fullness. Other accompanying ear symptoms were denied. Otomicroscopic examination revealed a pale red pulsatile mass anterior to the umbo (figure 1). The rest of the head and neck examination, including the left ear, was unremarkable.

Figure 1.

Ear microscopy image of the right ear. A pale red mass anterıor to the umbo is recognised. The pulse-synchronous movements of the mass were clearly visible.

Investigations

Pure tone audiogram showed a right mild conductive hearing loss up to 30 dB, whereas hearing on the left side was normal.

High-resolution CT of the temporal bone revealed a retrotympanic mass covering the promontory with an absence of bone plate separating the ICA from the middle ear. The vertical segment of the carotid canal was hypoplastic, whereas an enlarged inferior tympanic canaliculus (Jacobson canal) was seen (figure 2).

Figure 2.

Bone window axial high-resolution CT scan of the temporal bone from caudal to cranial. (A) The native internal carotid artery (ICA) runs in the carotid canal at the inferior surface of the petrous bone (black arrow). In a more latero-posterior position and adjacent to the jugular bulb runs the inferior tympanic artery (aberrant ICA (white arrow)) through the inferior tympanic canaliculus (Jacobson canal). (B) The native ICA anastomoses to the horizontal petrous portion of the ICA (star), whereas the aberrant ICA enters the hypotympanum. (C and D) The aberrant ICA, presented as a retrotympanic mass over the promontory, merges with the horizontal petrous portion of the ICA. The bone plate separating the ICA from the middle ear is absent.

The MRI and magnetic resonance angiography (MRA) yielded a better view on the exact vessels’ anatomy. In addition to the reduced diameter of the native ICA, a duplication of the ICA was identified. The medial, thin vessel presented the native hypoplastic ICA, whereas a wider ascending artery with a more lateral course corresponded to the enlarged inferior tympanic artery. The horizontal petrous portion of the ICA was mainly the continuation of the enlarged inferior tympanic artery, with an anastomosis with the hypoplastic native vertical ICA (figures 3 and 4).

Figure 3.

Axial MRI in time of flight (TOF) sequence from caudal to cranial. Duplicated right internal carotid artery (ICA) (native ICA: black arrow, aberrant ICA: white arrow), both with reduced diameter compared with the left normal ICA (black arrowhead).

Figure 4.

Coronal view of magnetic resonance angiography. Duplication of the right internal carotid artery (ICA). The medial thin vessel represents the native ICA (black arrow) and the lateral vessel represents the aberrant ICA (white arrow). White arrowhead: left normal ICA.

The haemodynamic properties of the carotid vessels were evaluated using color-coded duplex ultrasonography. The only remarkable findings were a small effect of compression of the aberrant ICA and a reduced diameter of the right common carotid artery (CCA) compared with the left one (right 5.49 mm, left 7.19 mm). Other haemodynamic measurements were regular, with normal blood flow and absence of atherosclerotic plaques.

Differential diagnosis

Apart from the aberrant ICA, the differential diagnosis of a retrotympanic mass causing pulsatile tinnitus should include glomus tumours, other vascular malformations such as aneurysms, pseudoaneurysms and haemangiomas, high jugular bulbs, as well as cholesterol granulomas.^{1 2 4} Definitive diagnosis is made after radiological assessment. Caution is needed, not to proceed to any manipulation or intervention in the affected ear before the diagnosis is confirmed.^{5 6}

The otomicroscopic picture of the presented case was very suggestive for a vascular anomaly. After completing the radiological examination final diagnosis of an aberrant ICA was confirmed. However, careful examination of the CT and MRI pictures was necessary in order to identify the ICA duplication.

Treatment

The patient was informed about the diagnosis and the possible complications of middle ear interventions. Recommendations included regular ENT checks.

Outcome and follow-up

At the 6-months and 1-year follow-up the patient’s complaints of pulsatile tinnitus and aural fullness were persisted without significant changes. The ear microscopic finding was stable as well, so that a new follow-up in 1 year was planned.

Discussion

Usually, the ascending ICA enters the petrous bone medial to the styloid process via the carotid canal. Anterior to the cochlea the ICA is separated from the middle ear cavity by a thin bony plate of mean thickness of 0.24 mm. This point corresponds to the ICA genu, where the vessel turns anteriorly and transits to its horizontal segment. Thereafter it traverses the foramen lacerum and enters the medial cranial fossa.^{1 7}

The anastomosis between the internal and external carotid arteries in the middle ear is of particular importance for the pathogenesis of the aberrant ICA. Originating from the external carotid artery (ECA) network, the inferior tympanic artery is normally a small branch of the ascending pharyngeal artery. It runs vertically, enters the tympanic cavity with the Jacobson’s nerve through the inferior tympanic canaliculus or Jacobson’s canal. The caroticotympanic artery originates from the horizontal portion of the ICA and enters the middle ear cavity via a small bony dehiscence. The two tiny vessels, the inferior tympanic artery from the ECA and the caroticotympanic artery from the ICA anastomose on the promontory, are responsible for the blood supply of the middle ear.⁸

The above described local vessel anatomy is essential for understanding the most popular hypothesis regarding the aetiology of aberrant ICA. This hypothesis was first described by Lasjaunias et al.^{9 10} This implies that the initial phenomenon is an aplasia of the cervical part of the native ICA during embryogenesis. As a result a compensatory collateral anastomotic pathway is developed via the ascending pharyngeal artery, the enlarged inferior tympanic artery and retrograde via the caroticotympanic artery. The latest joins the horizontal portion of the ICA. Two additional congenital mechanisms are still being discussed about aberrant ICA aetiology. First, the bony plate along the vertical segment of the ICA is missing, which, in turn, leads to lateral displacement of the vessel. Second, the persistent stapedial artery—which normally involutes during embryonic stage—or its fibrous remnant pulls the ICA laterally towards the middle ear cavity. In addition to the congenital mechanisms, acquired causes are also discussed but are supposed to be less common. Acquired causes include trauma, chronic ear infection, cholesteatoma, thrombophlebitis and granulomatous disease.1–3 8–13

The special feature of the presented case was the existence of a hypoplastic native ICA additionally to the aberrant ICA. As already mentioned, the majority of the aberrant ICA cases present with an aplasia of the native ICA, whereas a hypoplastic native ICA is much more rare.^{8 14}

Aberrant ICA remains asymptomatic or is associated with non-specific symptoms such as pulsatile tinnitus, aural fullness and hearing loss. Otomicroscopy typically reveals a retrotympanic red mass, but a normal finding is possible as well.¹ These non-specific or sometimes absent clinical symptoms and signs make clinical diagnosis difficult. Pathologies such as otosclerosis, glomus tumours, other vascular diseases, like stenosis of the ICA, aneurysms, pseudoaneurysms and haemangiomas, high jugular bulbs, as well as cholesterol granulomas may present in a similar way and they have to be considered during the differential diagnosis procedure. The features of the retrotympanic mass may indicate an aberrant ICA: anterior position, pulsatile nature and white or rosy colour.^{1 2 4} Rarely, an asymptomatic aberrant ICA is diagnosed during middle ear surgery; therefore, high clinical suspicion and sufficient diagnostic assessment is needed.^{1 15}

The diagnosis is primarily made by imaging assessment. Already in 1971, Lapayowker et al established diagnostic criteria with typical angiographic findings. These criteria included a lateral extension of the ICA beyond a ‘vestibular line’, which was defined as a vertical line at the level of the ear’s vestibule.¹⁶ Today, CT is the most common modality for diagnosis; however, in some cases more extensive radiological assessment such as MRI/MRA and computed angiography is used.^{1 2} CT scan features are characteristic for an aberrant ICA (mentioned as seen from caudal to cranial): (a) ICA in a posterior position compared with the native one, running adjacent to the jugular bulb and having a reduced diameter; (b) enlarged inferior tympanic canaliculus; (c) aplastic or hypoplastic vertical segment of the carotid canal; (d) enhancing mass in the hypotympanum; (e) absence of the bony plate separating the ICA from the middle ear.^{1 2 17} The MRA offers additional information since it enables an excellent visualisation of the vessels, including reduced diameter of the aberrant ICA and the aplastic or hypoplastic native ICA. Further MR findings are a lateral and superior location of the carotid genu in the middle ear on the anteroposterior projection and a posterior and superior location of the genu on the lateral projection.1 2 16 18 Conventional angiography is only needed in complicated cases of aberrant ICA such as in iatrogenic or incidental trauma, in infections, local diseases or tumours.²

Ultrasonographic characteristics, although not often described in case of an aberrant ICA, include a significant difference in the diameter of the CCA and a small effect of compression of the suggested aberrant ICA with no other pathological haemodynamic features.¹⁹ This suggests that color-coded duplex ultrasonography should be a part of the investigations performed when an aberrant ICA is suspected.

In the past, surgical treatment has been proposed to achieve complete resolution of troublesome symptoms (tinnitus and hearing loss) and prevent possible destruction of the middle ear structures and formation of an aneurysm.^20–22 Some of the recent literature is also in favour of surgical treatment, provided that a correct diagnosis before surgery has been established.^{3 23} However, later reports suggested that surgery was neither necessary nor indicated because of the risk of bleeding, infection, aneurysm formation, endovascular embolisation and stroke.^{24 25} In general there is broad consensus that in case of an uncomplicated aberrant ICA (ie, without bleeding complications), a conservative approach with observation is recommended, since this situation is not followed by any future complications. ENT surgeons and especially otologists have to be aware of this variant, because surgical injury of the aberrant ICA during myringotomy or middle ear surgery can lead to dramatic complications such as bleeding and extensive neurological deficits.1 2 26–28

Learning points.

• A duplication of the internal carotid artery (ICA), with an enlarged inferior tympanic artery and a narrower collateral hypoplastic native ICA, is a special form of the rare vascular anomaly of the aberrant ICA.

• Diagnosis is confirmed with CT scan, whereas MRI and magnetic resonance angiography offer additional information about vessels’ diameter and anastomoses.

• In case of an uncomplicated aberrant ICA a conservative approach with observation is recommended, since this situation is not followed by any future complications.