Table 1

Focal causes of asymmetric muscle enlargement

Myopathic conditionsSome forms inflammatory muscle diseases—for example, granulomatous myositis—see text.
Neurogenic disordersRadiculopathies or poliomyelitis as well as ‘focal myositis’—see text.
TumoursSarcomas present as a gradually enlarging painless mass; they are a rare heterogeneous group of malignant tumours of mesenchymal origin.
Vascular causesIntramuscular haematoma, caused by inherited or acquired coagulopathy; haematomas can cause pain, including ischaemic pain, from increased intramuscular pressure.
Diabetic muscle infarction manifests as acute painful swelling muscle (often but not invariably quadriceps) in someone with poorly controlled diabetes mellitus (more often type 1 than 2; frequently with other diabetic complications), moderately increased serum creatine kinase (but can be normal) with little weakness and sometimes bilateral.
It often presents after a 3–4 week history of agonising pain at rest, worse with exercise. The muscle is hard and warm with swelling; ultrasound scan for deep venous thrombosis is negative. MR scanning is very helpful. However, a muscle biopsy is often avoided unless there are atypical features or diagnostic uncertainty because of the risk of complications, including delayed wound healing, haematoma, infection, nerve palsy, heterotopic calcification and need for blood transfusion. A biopsy usually shows muscle necrosis and oedema.
Rest, immobilisation, good glycaemic control, low-dose aspirin and analgesia usually allow resolution but up to one-third of patients develop a recurrence. There is a rare risk of developing compartment syndrome with the need for surgical intervention. The differential diagnosis includes thigh pain characteristic of diabetic amyotrophy.30 31
Deep vein thrombosis often presents with calf warmth, tenderness and swelling, sometimes erythema and oedema. The diagnosis is helped by imaging, particularly compression ultrasound scanning.
InfectionsNecrotising fasciitis. This is a deep-seated fulminant infection of subcutaneous tissue, which destroys fascia and fat. It typically presents with acute severe localised pain that is out of proportion to the initial physical signs, but profound shock often soon follows. It is usually caused by Lancefield group A β-haemolytic streptococci (high serum creatine kinase with localised streptococcal myositis). The bacterium elaborates a range of potent exotoxins that behave as superantigens and there is a brisk release of cytokines that probably accounts for the rapid and often irreversible shock syndrome. The management includes debridement of necrotic skin to deep fascia as well as intensive care unit support and antibiotics.
Pyomyositis. This is a purulent infection of skeletal muscle (often the thigh) that most often arises from haematogenous spread, usually with abscess formation. In temperate regions, most patients are immunocompromised adults (eg, HIV, diabetes mellitus, cirrhosis, organ transplants, immunosuppressive drugs, intravenous drug abuse). About 25%–50% have a history of trauma (some cases follow vigorous exercise) possibly because of increase perfusion and/or release of iron (normally sequestered by myoglobin), a vital nutrient for bacterial growth. In temperate regions, the infecting organism is usually staphylococcus (75%) or group A streptococcus. Early diagnosis is difficult since most patients have only minor pain and localised swelling with very few systemic features and no erythema (because the infection is deep). MR scanning, ultrasound or enhanced CT scans help in the diagnosis. Most cases require incision and drainage as well as antibiotics.32
Other‘Baker’s cyst’. This arises through fluid from a knee effusion escaping to form a popliteal cyst in a subgastrocnemius bursa. The diagnosis requires imaging with ultrasound or MR scanning.