Case 1: Arm immobility in an infant

A nine-day-old boy was referred for new-onset right arm immobility of two days’ duration. He exhibited no other symptoms. He was born at term after a normal pregnancy and uncomplicated spontaneous vaginal delivery. There was a negative swab prenatally for group B Streptococcus. On examination, growth parameters and vitals were within normal limits. He had one recorded rectal temperature of 38.5°C on transport to the hospital. He was alert and active, aside from his right arm, which was adducted and internally rotated. He cried with handling. There was spontaneous flexion and extension of the fingers and wrist. Passive flexion and extension at his right elbow caused mild discomfort. Passive extension and abduction of his right shoulder caused severe discomfort, and range of motion was significantly limited. There was no erythema, deformity or swelling of his right arm. Deep tendon reflexes were graded 2/4 in the biceps, brachioradialis and triceps muscles bilaterally. The remainder of the neurological and general physical examination was unremarkable. Investigations revealed a white blood cell count of 16.8×109/L (reference 10.0×109/L to 26.0×109/L) and an elevated C-reactive protein level of 59.7 mg/L (reference 0.0 mg/L to 7.0 mg/L). Further investigations revealed the diagnosis. An x-ray of the right arm and shoulder was normal. Ultrasound of the right shoulder revealed a trace amount of fluid in the glenohumeral joint. Magnetic resonance imaging of the right upper limb (Figure 1) revealed perifascial edema along the right shoulder extending into the upper arm, with increased T2 signal noted in the deltoid, infraspinatus and a portion of the triceps muscle, consistent with a diagnosis of infectious myositis. A blood culture grew methicillin-sensitive Staphylococcus aureus. He was started on a three-week course of intravenous cefazolin and discharged with weekly physiotherapy. At seven weeks of age, he had mildly limited abduction of his right arm compared with the left; however, both arms had equal strength, tone and reflexes. He was playful and did not appear to be in any pain. His C-reactive protein level was 0.2 mg/L. AAxial short tau inversion recovery (STIR) (TR 4070/TE 48.0) andBCoronal STIR (TR 3100/TE 48.0) images of affected patient’s right shoulder and upper extremity revealing perifascial edema along the shoulder extending into the upper arm with increased intramuscular T2 signal noted in the deltoid, subscapularis, infraspinatus and a portion of the triceps. Small joint effusions are also noted Infectious myositis or pyomyositis is an infection of the skeletal muscle involving an insidious progression to abscess formation from hematogenous seeding. In tropical countries, the peak incidence of pyomyositis is between two and five years of age, with a later mean age of 8.4 years in North America. Data from the United States report an incidence of one to two per 4000 pediatric hospital admissions. Pyomyositis has been observed in higher frequencies in temperate climates in individuals with HIV and underlying medical conditions that result in immune compromise (1). Pyomyositis is an uncommon disease because intact skeletal muscle has an innate resistance to bacterial infection. The pathogenesis is not clearly understood. In the presence of muscle injury, there is disruption of the usual sequestration of elemental iron in myoglobin, which may facilitate the provision of critical bacterial nutrition in the form of iron. S aureus is the most common responsible agent in 50% to 90% of cases. Group A Streptococcus is observed in approximately 1% to 5% of cases, with less common occurrences of group B, C and G Streptococcus, pneumococcus, Haemophilus and Gram-negative bacilli. Pyomyositis progresses through three distinct stages. The inciting event often involves a transient bacteremia in the setting of pre-existing or concurrent muscular injury. The initial invasive stage involves increasing edema in the muscle and bacterial seeding. Presenting symptoms at this stage include localized muscular pain, edema and fever, which are sometimes misdiagnosed as muscle contusions. Overlying erythema or warmth, as well as significant elevation in C-reactive protein level, erythrocyte sedimentation rate or white blood cell count, are rarely encountered at this stage. The second stage is the development of abscess, which usually occurs at day 10 to 21 of illness. In the third stage of illness, septicemia and metastatic spread of the abscess involving multi-organ dysfunction results in high mortality. Long-term sequalae and complications of pyomyositis include osteomyelitis of adjacent bones, muscle scarring, residual weakness and functional impairment of the affected muscle group. In neonates with infectious etiologies, there may not always be fever or systemic signs of infection. Perinatal brachial plexus palsy is a common cause of upper limb paralysis in newborns, resulting from traction injury to the brachial plexus nerve roots C5-T1. With perinatal brachial plexus palsy, paralysis and loss of reflexes in the biceps and brachioradialis muscles are evident immediately at birth. Differential diagnosis for perinatal brachial plexus palsy includes consideration of fractures, osteomyelitis, sepsis of the glenohumeral joint, arthrogryposis, brachial neuropathy, congenital varicella syndrome and tumours of the brachial plexus (2). In our patient, the presence of normal reflexes and history of previously normal movement in the affected arm effectively ruled out a diagnosis of perinatal brachial plexus palsy. The presence of a fever raised the suspicion of an infectious etiology. Neurological examination is important for diagnosing perinatal brachial plexus palsy and excluding other etiologies. In the setting of a previously mobile arm and normal neurological examination, a high index of suspicion should be maintained for infectious etiology, such as infectious pyomyositis, even in the absence of fever or other systemic signs of infection, because early diagnosis and treatment is associated with better outcomes and preservation of muscle function. Magnetic resonance imaging is the recommended modality for diagnosis of pyomyositis at early stages by visualization of muscle inflammation revealing signal hyperintensity in T2-weighted images and rim of enhancement on postcontrast T1-weighted images. The authors acknowledge the assistance of Dr M Shepel, Department of Medical Imaging, University of Saskatchewan in selecting the most appropriate magnetic resonance images for this report.

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