A fine pair of buttocks
S. Bawa,
K. Gaffney and
T. J. Marshall
Department of Rheumatology and Radiology, Norfolk and Norwich University Hospital NHS Trust, Norwich, UK
Correspondence to: S. Bawa, 26 George Morland House, Coopers Lane, Abingdon OX14 5GA, UK. E-mail: sbawa{at}talk21.com
SIR, the following two case reports provided a diagnostic challenge and highlight the contribution of MRI and interventional radiology to musculoskeletal medicine.
Patient 1, a 27-yr-old male, previously fit and well, presented to his general practitioner with a 9-month history of left hip pain. He complained of nocturnal pain, sleep disturbance and curtailed physical activity, and had been off work for 2 months. His symptoms began when he fell off a trampoline. On examination there was diffuse swelling and tenderness overlying his left greater trochanter, suggestive of trochanteric bursitis. Investigations, including full blood count, erythrocyte sedimentation rate (ESR), C-reactive protein (CRP) and X-rays of his lumbar spine and left hip, were normal. The left trochanteric bursa was injected with 40 mg methylprednisolone. At review 2 months later, an ultrasound scan was arranged as there was no improvement. This also suggested a diagnosis of trochanteric bursitis. A second injection of 80 mg methylprednisolone with 3 ml bupivacaine was administered under ultrasound guidance. Due to a lack of clinical improvement, an MRI was arranged (Fig. 1) and revealed an arteriovenous (A-V) malformation. There was also an associated bursitis. He successfully underwent embolization of his gluteal A-V malformation under X-ray guidance.

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FIG. 1. Axial T2-weighted sequence with fat suppression reveals high-signal serpiginous structures within the left gluteal musculature, consistent with an arteriovenous malformation.
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Patient 2, a 19-yr-old-male, previously fit and well, presented with a 2-month history of a twinge in his back, spreading to the left buttock. Treatment with diazepam and diclofenac was ineffective. Investigations showed ESR 92 (110), CRP 41 (010), platelets 759 (150400) and white cell count 24 (411). X-rays of his lumbar spine and sacroiliac joints were normal. He was referred to the rheumatology department and admitted as an emergency because of progressive pain, sleep disturbance, pyrexia, night sweats and weight loss. Further questioning suggested a self-limiting episode of olecranon bursitis 6 months previously. On examination he had lumbar scoliosis and an indurated, tender swelling overlying the left sacroiliac joint (Fig. 2). An ultrasound-guided aspirate yielded thick, creamy caseous fluid. Sequential ultrasound biopsies were negative. An echocardiogram was normal. Serial blood cultures grew Staphylococcus aureus sensitive to flucloxacillin. MRI confirmed the diagnosis of infective sacroiliitis. He was treated with intravenous flucloxacillin with fusidic acid for 2 weeks. This was followed by 6 weeks of oral antibiotics. At follow-up 6 weeks later he was asymptomatic, CRP had returned to normal, and he had returned to work.

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FIG. 2. Axial T2-weighted sequence with fat suppression reveals left-sided infective sacroiliitis with fluid collections in the pelvis and left buttock.
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Persistent buttock pain is commonly due to sacroiliitis, fracture, nerve entrapment or psoas abscess. Rarer causes reported in the literature include persistent sciatic veins [1], gluteus medius tendon tear [2], osteochondroma [3], strangulated sciatic hernia [4], leiomyosarcoma [5], synovial osteochondroma [6] and persistent sciatic artery [7]. Unless these are considered, costly investigations and prolonged suffering may be endured.
A-V malformations can occur anywhere in the body and the symptomatology depends on the site, size and degree of shunting. They may present as a pulsatile mass, symptoms of urinary retention and sciatica, or a buttock abscess. They can cause disfigurement, discomfort and danger to many patients [8]. A-V malformations that are mistakenly incised and drained could lead to life-threatening blood loss, hence the importance of diagnostic imaging techniques to first establish the correct diagnosis [9], the extent of the lesion and its relation to surrounding structures. Embolization is a palliative procedure and may have to be repeated in some individuals.
Staphylococcus aureus is the commonest organism causing septic arthritis. MRI is useful in making a diagnosis of infected sacroiliitis in diagnostically difficult cases, as X-rays only become abnormal when joint destruction is advanced [10].
These two cases not only demonstrate the importance of musculoskeletal imaging in rheumatology practice, but also highlight the need for a broad knowledge of the possible differential diagnosis of buttock pain. This in turn influences the correct investigations of these patients and hasten the initiation of appropriate treatment.
The authors have declared no conflicts of interest.
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Accepted 12 November 2003