A post-traumatic pulsatile nodule in the right pre-auricular region

Answer

A pseudoaneurysm of the superficial temporal artery, true aneurysm, arteriovenous malformation, vascular tumour, and mass overlying the artery with transmitted pulsations.

Discussion

From clinical assessment and duplex ultrasound examination, the patient was diagnosed as having a pseudoaneurysm of the right superficial temporal artery. Duplex ultrasound examination of the right pre-auricular region (fig 1⇓) showed a nodule of pulsatility adjacent to the superficial temporal artery and right parotid gland.

Fig 1 Duplex ultrasound image of the right pre-auricular region showing a hypoechoic mass (arrowheads) containing central turbulent arterialised blood flow (C). Tortuous afferent (A) and efferent (B) arterial vessels are seen lying adjacent to the right parotid gland (P). The anatomical location of the vessels makes them consistent with the superficial temporal artery branch of the right external carotid artery

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The superficial temporal artery is a terminal branch of the external carotid artery, which divides with the parotid gland en route to the scalp to provide arterial perfusion to the pinna, auricular area, parotid region, and masseter muscle. These branches of the artery are divided into frontal and parietal depending on their anatomical position. The frontal branches of the vessel, located in a gap in the muscle on the periosteum at the superior temporal line, are highly susceptible to injury because of a lack of cushioning from cranial vault muscles.1

A pseudoaneurysm is a collection of blood contained by haematoma that is formed when trauma to an artery creates a partial or complete break in the intimal layer of the arterial wall. Trauma can be relatively minor. Leakage of blood from the intimal defect creates a tissue sac at the site of the defect, composed of organised haematoma with a surrounding pseudocapsule. The capsule expands, and the haematoma is absorbed, leaving a cavity, which is connected to the main artery through a neck—the pseudoaneurysm.2

Pseudoaneurysms of the superficial temporal artery are rare, with fewer than 200 cases reported in the literature.3 They can present to a variety of specialties because they are caused by a wide range of injury processes, including blunt trauma, sporting injuries, falls, and even hair transplantation.4

Patients typically present with a compressible pulsatile mass two to four weeks after trauma. They may also have associated ear pain, headache, and, rarely, facial nerve palsy. There is no age or sex preference.5 Most pseudoaneurysms are solitary, but multiple pseudoaneurysms have been reported.6

Pseudoaneurysms differ from true aneurysms in that blood is contained by the clot alone, whereas a true aneurysm is contained by the focal weakening of the arterial wall.2 True aneurysms of the superficial temporal artery are extremely rare, accounting for only 5% of aneurysms at this anatomical site—only 21 or so cases have been published in the literature since 1954.7 It is thought that the traumatic mechanism behind a true aneurysm of the superficial temporal artery is the same as for a pseudoaneurysm, but that pre-existing vessel disease (such as atherosclerosis or congenital arterial deformities) determine the extent to which the arterial walls are affected.8

Congenital vascular anomalies (such as high flow haemangiomas) or arteriovenous malformations should also be considered as differential diagnoses. These are often suspected on visual inspection because of pigment changes to the skin, but they can be differentiated from a pseudoaneurysm by their continuous thrill and bruit. By contrast, pseudoaneurysms often have arterial pulsations and thrills that correspond to systole.9

Pulsation within the mass may be absent because of complete thrombosis of the aneurysmal sac, and non-pulsatile differential diagnoses include epidermal inclusion (sebaceous) cyst, lipoma, and abscess.9 In addition, some non-pulsatile masses (such as haematomas) may overlie an artery and demonstrate transmitted pulsations. The final diagnosis should be confirmed through a combination of clinical history, physical examination, and radiological imaging.

Answer

Duplex ultrasound, computed tomography angiography, magnetic resonance angiography, and digital subtraction angiography.

Discussion

Duplex ultrasound is widely accepted as a non-invasive, non-ionising, and cost effective real time dynamic imaging modality that can enable rapid diagnosis. Although this test can raise the suspicion of a pseudoaneurysm of the right superficial temporal artery, other imaging modalities may be needed to confirm the diagnosis and assess the association between the pseudoaneurysm and adjacent vessels.10

Computed tomography angiography is a minimally invasive diagnostic technique that can be used to diagnose vascular aneurysms and pseudoaneurysms on the basis of the location, attenuation, and enhancement of a visible lesion (fig 2⇓).

Fig 2 Coronal post-contrast computed tomography angiogram at the level of the external auditory canal showing the normal, more distal, superficial temporal artery (arrowheads), contrast in the pseudoaneurysm lumen (arrow), and the right parotid gland (P)

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It can assess the external carotid artery and the main superficial temporal artery, as well as its distal branches. Advances in post-processing software enable the reconstruction of post-contrast computed tomography images into multiple two dimensional and three dimensional assessment planes known as MIP images (maximum intensity projections) (fig 3⇓). The size of the pseudoaneurysm—for example, the size of lumen and the neck—can be measured accurately with this test, as can the degree of thrombosis.10

Fig 3 Reconstructed MIP (maximum intensity projection) from the computed tomography angiographic sequence showing the pseudoaneurysm and its vascular associations. A=right external carotid artery, B=right superficial temporal artery, C=afferent vessel, D=efferent artery, E=pseudoaneurysm sac (note the tortuosity of the afferent arterial supply)

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Computed tomography angiography is particularly useful in trauma to evaluate the extent of the trauma sustained. The adjacent calvarium can be assessed for fractures, as well as any communication with the intracranial compartment. The position of the pseudoaneurysm in relation to osseous landmarks such as the external auditory canal and the zygomatic arch can also be easily assessed.10

The test has risks associated with the use of ionising radiation and contrast. It may also be hard to assess the size of the neck of the pseudoaneurysm, and in such cases magnetic resonance angiography can be a useful addition to, or an alternative to, computed tomography angiography. Digital subtraction angiography performed using fluoroscopic imaging techniques is rarely used as a first line diagnostic imaging modality but may provide additional information.11 It can also be undertaken immediately before endovascular intervention to avoid delay in treatment.

Answer

Endovascular micro-coil embolisation, ultrasound guided percutaneous thrombin injection, surgical ligation, and excision.

Discussion

Several options should be considered when treating a pseudoaneurysm of the superficial temporal artery. An intervention of some sort is generally indicated to prevent rupture and subsequent haemorrhage. However, the choice of treatment depends on the patient’s performance status and preference, and the clinical presentation and morphology of the pseudoaneurysm.

Endovascular micro-coil embolisation should be considered as the first line treatment of pseudoaneurysm of the superficial temporal artery. Patients who present to institutions without access to interventional radiological facilities should be transferred to units with this expertise. Under fluoroscopic guidance, a metal micro-coil (usually made of soft platinum) is positioned within the artery, proximal and distal to the pseudoaneurysm, sealing off the neck.12 Treatment success can be determined immediately using an iodinated contrast agent to delineate the artery. This outpatient procedure is increasingly favoured for its minimally invasive approach, quick recovery time owing to avoidance of parasthesiae, and reduced risk of subsequent scarring when compared with surgical techniques.13 However, associated complications include recanalisation of the pseudoaneurysm, transient ischaemic attack, stroke, infection, contrast reaction, and pseudoaneurysm of the groin secondary to femoral puncture.12 The micro-coils are left in place permanently, but a palpable lump in the temporal region will always be present. One study reported the percutaneous injection of NCBA (N-butyl cyanocrylate) into the embolised pseudoaneurysm to increase the therapeutic effects of the micro-coil. The injection can be administered during the coil embolisation procedure.14

Thrombin injections are also commonly used to treat pseudoaneurysms, because thrombin acts as a catalyst in the process of blood clotting. When directly injected into a pseudoaneurysm, it causes a thrombotic effect, which effectively seals off the aneurysmal cavity. The residual mass of the thrombosed pseudoaneurysm usually resolves in two to three months.15

The success of this outpatient procedure can be immediately assessed after injection by investigating blood flow within the pseudoaneurysm using colour Doppler ultrasound. One major complication associated with percutaneous thrombin injection is the potential to occlude the patent superficial temporal artery. The injection is therefore performed under ultrasound guidance to ensure that the needle inserted into the pseudoaneurysm is directed away from its neck, thus avoiding spillage into the artery.16 Although highly unusual, there is also a risk of non-target embolisation with this technique. A case of induced seizure and ischaemia of the scalp has been attributed to the migration of injected thrombin after percutaneous thrombin injection for a pseudoaneurysm of the superficial temporal artery.17 Other complications include allergic reaction and risk of recanalisation of the pseudoaneurysm.18

Surgical ligation and excision has now largely been replaced by newer techniques, although it may still be used as a reserve treatment when initial attempts at micro-coil embolisation have failed, or as an alternative approach in patients with contraindications to endovascular intervention (such as allergies to iodinated contrast).18 This procedure can be performed under local anaesthetic, although general anaesthetic is recommended in children or if the pseudoaneurysm is close to the facial nerve or parotid gland.19 During the procedure, the afferent and efferent vessels of the pseudoaneurysm are ligated with a permanent suture, until pulsation in the aneurysmal sac ceases. The sac is then dissected and excised before the incision is closed.9 This approach cures the pseudoaneurysm in most cases, relieving associated headaches, resolving the cosmetic defect, and preventing recurrence or haemorrhage from further trauma.1 However, the surgical approach has several disadvantages, including the risk of damage to the frontal and zygomatic branches of the facial nerve and facial scarring.20

One new surgical approach to treatment of this condition was recently reported.21 Using ultrasound, the afferent and efferent branches were identified and, after administration of local anaesthetic, the branches were ligated through skin incisions. The pseudoaneurysm was then decompressed by needle aspiration. Although the outcome in this case was successful, currently this is the only report of such an approach, and needle aspiration for diagnostic or therapeutic purposes in pseudoaneurysm is generally not recommended owing to increased risk of rupture and subsequent haemorrhage.1