Pyomyositis

Received 2 September 2002; Accepted after revision 4 April 2003

Pyomyositis of the obturator muscles is a rare condition, characterised by pain in the hip and features of systemic infection. It may follow minor trauma to the hip, sometimes in the presence of an apparently innocuous infective source. All previously reported cases have been diagnosed conclusively on the initial CT or MR scan. We present a case of obturator pyomyositis in a 21-year-old football player in which the first MR scan was misleading. A radiolabelled, white blood cell scan was also negative and the resultant delay in diagnosis proved dangerous. The crucial importance of careful and repeated clinical examination is emphasised.

Patients with a painful hip and pyrexia often require an urgent orthopaedic opinion. Early diagnosis of bacterial septic arthritis is essential as the consequences of delay are potentially devastating for the joint. The diagnosis is not always immediately obvious, however, and early use of MRI is increasingly common. Consequently, during the past decade there has been an apparent increase in the incidence of other causes of a painful hip and pyrexia, which presumably, were previously treated empirically without a firm diagnosis being reached. To date, all cases of primary obturator pyomyositis reported in Western literature have been diagnosed conclusively by the initial CT or MR scan. We report the first case which was not.

Case report

A 21-year-old man presented with a painful right hip and a pyrexia. While playing football three days earlier, he had sustained a "slight groin strain", but had completed the match. He awoke the following day with pain in the hip and an inability to bear weight. The pain was constant and exacerbated by movement. Over the next two days, he had become progressively unwell with rigors and vomiting. He was otherwise healthy, with no history of foreign travel, and was not taking any medication. He described several episodes of dysuria during the previous months, but had no urethral discharge and no unprotected intercourse.

On examination he was unwell with a temperature of 39.8[degrees]C and a pulse of 110 per minute. Examination of his chest and abdomen was normal, although there was tenderness towards the right side on rectal examination. There was some discomfort in the right groin on deep palpation, tenderness in the right buttock lateral to the ischial tuberosity and a few inguinal lymph nodes were palpable. There was some limitation of movement of the right hip, extension 10[degrees], flexion 70[degrees], abduction 30[degrees], adduction 20[degrees], internal rotation 30[degrees] and external rotation 30[degrees], with pain at the limits of movement. Active, straight leg-raising was to 5[degrees] and limited by pain. There were eczematous lesions on the dorsum of both feet.

Initial laboratory studies revealed ESR 31 mm/hr, CRP 288 mg/l, white blood cell count 6.6 x 10^sup 9^/l, with lymphopaenia. An anteroposterior (AP) radiograph of both hips was unremarkable. He was admitted for bed rest and analgesia; antibiotics were withheld. MRI was performed on the following day (Fig. 1), and demonstrated slightly increased signal in the right adductor region consistent with the history of groin strain, and small bilateral hip effusions. Mild bone marrow oedema was noted, suggesting ostietis pubis, although osteomyelitis could not be excluded. The right perivesical plexus appeared slightly prominent, but this was not considered significant. No abscess cavities were seen. The radiological appearances were thought to be those of a soft tissue strain rather than significant infection.

Blood cultures, however, grew Staphylococcus aureus, and he was given high-dose intravenous (IV) flucloxacillin. Despite this, he remained unwell and unable to bear weight. An abdominal CT which was performed on the third day after admission revealed bilateral, small, pleural effusions and hepatosplenomegaly, but no psoas, pelvic or obturator abscess.

He continued to deteriorate despite IV antibiotics and on the fourth day was transferred to intensive care with Acute Respiratory Distress Syndrome. He required 48 hours of continuous positive airway pressure ventilation, and IV rifampicin was also administered. A radio-labelled, white cell scan, performed on the eighth day, failed to identify any cause for the sepsis. Pain in the right hip without loss of rotation persisted, and a repeat MR scan was performed on the tenth day (Fig. 2). This demonstrated confluent abscesses in the obturator internus, obturator externus and adductor brevis muscles, the largest measuring 5 cm in the obturator externus. Marked inflammatory changes were also seen in the surrounding tissues, including the pubic symphysis. Urgent surgical exploration was undertaken through a medial approach to the hip. The adductor longus was released to improve the exposure proximally. There was a small amount of pus and a little surrounding muscle necrosis within obturator externus, and this was drained. Samples of pus and muscle were sent for microbiological investigation. Prolonged culture demonstrated Staph. aureus, consistent with a partially treated abscess, and relatively small numbers of organisms persisting due to previous antibiotics.

His condition improved dramatically after operation and he was discharged ten days later. He continued to take oral flucloxacillin and rifampicin for two weeks postoperatively and made a complete recovery. The inflammatory markers returned to normal. A repeat MR scan performed at four months showed a little residual bone marrow oedema within the pubic symphysis and minor soft tissue changes in the obturator region.

Discussion

Primary infection of muscle is usually regarded as a tropical disease,1-4 and is rare although becoming more common in temperate climates.1,4,5 This is only the third case of primary obturator pyomyositis reported in the United Kingdom.6,7 All reported cases have occurred in the past decade, but it is unclear whether this increase in incidence is real, or reflects the use of modern imaging techniques.

A review of the 22 cases of primary obturator myositis which have been reported (Table I), shows that 82% were in children under 18 years of age. The median age was 9.5 years (3 to 46), with a male:female ratio of approximately 3:2. The median duration of symptoms prior to presentation was three days (1 to 21 ), and the most common symptoms were fever, hip or thigh pain, and inability to fully bear weight.

In 41% there was a history of recent trauma to the hip. such as a fall8 or strenuous exercise.9 Our patient sustained an injury to the groin playing football the day prior to becoming unwell. Local trauma is a recognised initiating factor for pyomyositis1,2,5 and is documented in between 21% and 66% of cases.2,5 A potential source of bacteraemia is also frequently identified, such as the eczema of the feet seen in our patient. It has been suggested that primary myositis begins when a haematoma in muscle from a recent injury is colonised during an episode of bacteraemia.4,6 One previously reported patient had an established obturator internus haematoma, as demonstrated by CT scan.10 Miyake11 found that local muscle trauma is necessary prior to the IV administration of Staph. aureus in order to produce experimental myositis in rabbits.

The principal findings upon examination were similar to those seen in septic arthritis of the hip. These included signs of sepsis, a tendency to hold the hip flexed and externally rotated, and an inability to bear weight. There are, however, a number of distinguishing features. Obturator pyomyositis does not reduce hip movement so dramatically, and is associated with more discrete areas of tenderness, including pain on rectal examination when the obturator interims is involved. There may be pain radiating to the leg if the sciatic nerve is involved,1,10 and in females, there may be oedema of the ipsilateral labia.8,12

Haematological investigations revealed an elevated ESR in all cases and a raised white cell count in over 50%. Blood cultures are invariably positive. Plain radiographs and ultrasound are usually unhelpful, but an isotope bone scan demonstrates some abnormality in approximately two thirds of cases. A radiolabelled white cell scan eight days after admission in our patient misleadingly failed to detect any infection. In the previously reported cases, a firm diagnosis was made in every case on the initial CT (16 patients) or MRI (nine patients), with MRI providing clearer anatomical detail. In our patient, the first MRI showed only changes consistent with a history of groin strain.

The causative organism is usually Staph. aureus (81%), but group A Streptococcus, E. coli and Enterococcus have also been implicated. Others have documented similar bacteriological profiles for pyomyositis in general.2,5 Involvement of the obturator muscles has also been reported with N. gonorrhoea in women,1,6 but it seems likely that these were secondary to local pelvic spread. In 54% of cases the infection resolved with intravenous antibiotic treatment alone. In five patients (23%) percutaneous aspiration was performed, and in another five (23%), incision and drainage were required. There is usually a full recovery without long-term sequelae, although two patients developed ischial osteomyelitis, which was successfully treated with antibiotics.8,13

The case presented here highlights many of the characteristic features of primary obturator pyomyositis. It is, however, the first reported case where the initial MRI was not diagnostic. Yuh et al14 stated that MRI is superior to CT scanning in diagnosing pyomyositis, and Mazur et al15 found MRI to be 97% sensitive for acute musculoskeletal infections in children. MRI is regarded as the investigation of choice for acute pyomyositis.6-8 The scans, however, must be interpreted with caution as there is often a recent history of local trauma in obturator myositis and groin strain is well demonstrated on MRI, and may not be clearly distinguishable from early infection. Our patient presented with a history of trauma, and the MRI findings were entirely consistent with this. The negative isotope labelled white cell scan was also misleading. These false negative results both proved to be dangerous. This case clearly demonstrates that despite sophisticated modern imaging techniques the diagnosis and management of obturator pyomyositis may depend on a careful history and clinical examination, and previously negative investigations should be repeated if clinical suspicion remains.

No benefits in any form have been received or will be received from a commercial party related directly or indirectly to the subject of this article.

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R. J. King, D. Laugharne, R. W. Kerslake, B. J. Holdsworth

From Queen's Medical Centre, Nottingham, England

R. J. King. AFRCS Ed, Specialist Registrar

D. Laugharne, MRCS, Senior House Officer

R. W. Kerslake, FRCR, Consultant Radiologist

B. J. Holdsworth, FRCS, Consultant Orthopaedic Surgeon

Queen's Medical Centre, University Hospital NHS Trust, Nottingham NG7 2UH, UK.

Correspondence should he sent to Mr R. J. King.

(C)2003 British Editorial Society of Bone and Joint Surgery

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