The ‘pulsatile’ sebaceous cyst: beware of a superficial temporal artery aneurysm

Abstract

True aneurysms of the superficial temporal artery (STA) are quite uncommon. Only 14 such cases are described in the literature. Ultrasound scan (USS) is an appropriate and easily accessible non-invasive diagnostic modality, as it can show both the anatomical and flow characteristics of the aneurysmal vessel. Other conditions to be included in the differential diagnosis are haematoma, angiofibroma, eroding middle meningeal artery aneurysm, abscess or a parotid mass. Operative intervention is indicated to relieve symptoms as in this case, and to prevent rupture.

BACKGROUND

True aneurysms of the superficial temporal artery (STA) are quite uncommon, though Bartholin described an STA pseudoaneurysm (psA) as early as 1740.¹ Only 14 such cases are described in the literature, Martin et al being the first to document a histologically confirmed true STA aneurysm (STAA) in 1955.² We report a symptomatic true STAA that was mistaken for a sebaceous cyst and discuss the presentation and management.

CASE PRESENTATION

An 80-year-old female presented with a painful swelling in the right temporal region of 3 weeks duration. There was no past history of trauma. This was initially treated as an infected sebaceous cyst with a course of oral antibiotics. Specialist referral was prompted by acute exacerbation of right temporal pain and a short fainting spell.

Clinical examination revealed a non-tender 1×1 cm lump in the right temporal region with expansile pulsation, mobile only along the vertical axis (fig 1A). Cardiovascular and neurological examination was unremarkable. Examination of the limb vasculature and the abdomen did not reveal any obvious aneurysms.

Figure 1.

(A) The appearance of the STAA at initial presentation. (B) USS of the STAA showing the patent lumen, with bidirectional flow (red and blue colour flow indicators).

INVESTIGATIONS

The patient underwent an ultrasound scan (USS), which demonstrated a 10 mm (length) by 8 mm (diameter) STAA arising at its bifurcation (fig 1B).

DIFFERENTIAL DIAGNOSIS

• Sebaceous cyst

• Haematoma

• Angiofibroma

• Eroding middle meningeal artery aneurysm

• Parotid mass

TREATMENT

The aneurysm was excised under general anaesthetic (fig 2A).

Figure 2.

(A) The STAA at operation, showing the inflow trunk (1), frontal (2) and parietal (3) branch. (B) Photomicrograph of the STAA demonstrating the afferent artery (1) showing the internal elastic lamina (arrowheads), loss of this layer (yellow curve) and the aneurysm wall (2) with loss of the internal lamina (elastin van Gieson, ×40).

OUTCOME AND FOLLOW-UP

Histology revealed features consistent with a true aneurysm; a combination of H&E and elastin van Gieson stains demonstrated a paucity of muscle and elastic fibres (fig 2B). A magnetic resonance angiogram (MRA) of her cranial vasculature revealed no further aneurysms. The postoperative period was uneventful and she was discharged the same day. She was well on outpatient follow-up.

DISCUSSION

About 95% of the 300 and more cases of STAAs reported in the world literature are post-traumatic psAs.³ Only the remaining 5% are true aneurysms, with one report of multiple STAAs in the same patient.⁴ The literature is confusing as there are several case reports referring to post-traumatic psAs simply as “aneurysms”⁵ (which have led some authors to mistake Bartholin’s report as pertaining to true aneurysm formation),³ and also some authors reporting a higher number of aneurysms that may have been post-traumatic psAs.⁶ The majority of false aneurysms are due to blunt—including sporting—injuries, accounting for a higher incidence in young men, though older patients may suffer this after falls. Craniotomy, temporomandibular joint surgery and hair transplantation have been described as iatrogenic causes.¹ This is possibly because the frontal branch of the artery runs a relatively exposed course over the temporal bone and is vulnerable to trauma at that level. Most of these patients present within 2–6 weeks of injury. An arteriovenous fistula may also develop because of involvement of the adjacent vein, which then presents in a similar fashion.

Most studies have not reported the final histology, making the incidence of true STAAs difficult to determine. They have been described as isolated or associated with multiple intracranial cerebral aneurysms.⁷

The aetiopathogenesis and natural history of true STAA is unclear, so there are no data as to at what size they may rupture. Though an atherosclerotic STAA has been described,⁶ most other STAAs have been congenital or degenerative in origin. STAAs associated with syphilis, polyarteritis nodosa (PAN) or connective tissue disorders have not been reported, although a possible link with HIV and an associated infective vasculitis has been suggested.⁸

Loss of elastic fibres has been taken to be a conclusive representation of psA,¹ but given this was a true aneurysm it may be that this represents focal elastolysis of unknown aetiology,⁷ which may also contribute to psA, or even true aneurysm formation following trauma to an already weakened artery.⁹ Replacement of the elastic lamellae by fibrous tissue has also been demonstrated.⁸ This is corroborated by the histopathological examination of the specimen in this case.

The usual presenting symptom is of a pulsating swelling in the temporal region or a throbbing headache. Pulsation may be absent in those where the sac has thrombosed completely. A thrill or bruit suggests an arteriovenous fistula. Compression of the proximal STA should result in diminution or cessation of pulsation of the aneurysm distally. Gradual expansion of the lesion raises the index of suspicion.

Angiography, including via direct puncture,^6,9 aids the diagnosis, provided the aneurysmal lumen is patent, and also clarifies the feeding vessel, but gives no estimate as to the size of the lesion.¹ However, we feel USS is the more appropriate and easily accessible non-invasive alternative diagnostic modality, as it can show both the anatomical and flow characteristics of the aneurysmal vessel; in this case, it clearly showed directional forward flow within the main trunk and frontal branch, but contradirectional flow in what was the parietal branch, allowing us to predict that the aneurysm was possibly at branching points, and that more than two vessels would require ligation. It was therefore felt that formal angiography was unnecessary in this case, as MRA in our institution provides excellent views of the cranial vasculature including the circle of Willis. We therefore suggest a combination of USS and MRA as screening modalities in such cases. This is indicated given the described link between STAAs and intracerebral aneurysms.⁷

Operative intervention is indicated to relieve symptoms, as in this case, and to prevent rupture. This latter complication can rarely occur, spontaneously,¹⁰ or from subsequent head trauma or skin erosion. Successful endovascular obliteration of the STA psA has been reported.¹¹ Percutaneuous endo-obliteration of STAAs using agents such as coils, glue or Onyx (ethylene vinyl alcohol copolymer; MicroTherapeutics, Irvine, California, USA) therefore seems a theoretical endovascular alternative, though surgery is a simple, effective and cheap option. Ligation of the afferent and efferent vessels, with excision of the aneurysm, as a day case under local or general anaesthesia, remains the treatment of choice, given that no specific follow-up is necessary as recurrence is unheard of.

As this lesion may be mistaken clinically for a sebaceous cyst, STAA should be included in the differential diagnosis prior to therapy. Application of basic principles of clinical examination of subcutaneous swellings will provide clues to the underlying diagnosis and aid subsequent management.

LEARNING POINTS

• As this lesion may be mistaken clinically for a sebaceous cyst, STAA should be included in the differential diagnosis prior to therapy.

• Operative intervention is indicated to relieve symptoms.

• USS is the appropriate and easily accessible non-invasive alternative diagnostic modality for such cases.