Weak at the knees

Abstract

A 71-year-old man was referred to hospital in March, 1998, by his general practitioner with a 6-month history of leg weakness. He reported progressive difficulty over 2 months in climbing stairs or rising from a chair without using his arms, and his legs felt numb. He had no difficulty walking on the flat and was otherwise well. His diabetes had been diagnosed when he was aged 58 years; he was changed from oral hypoglycaemics to insulin after 11 years because of poor glycaemic control. He had uncomplicated myocardial infarctions aged 60 and 68, and had a successful transurethral resection of the prostate for benign hypertrophy at age 69. He complained of low lumbar back pain, worsened after exercise or standing for long periods, for the past 10 years. He was taking mixtard insulin, lowdose aspirin, and coproxamol as required. He stopped smoking 20 years ago, and drank 14 units of alcohol weekly. On examination, he was obese. His blood pressure was 140/90 mm Hg. Tone, power, and sensation in his arms were normal but reflexes could only be elicited with reinforcement. No wasting, fasciculation, tenderness, or abnormal tone was found in his legs. Hip flexion and extension, and knee extension were mildly reduced on both sides. He was unable to rise from a squat unaided, but muscle power was normal distally. No leg reflexes were present. Proprioception and vibration sense were absent below his knees. Lumbar lordosis was maintained, but lumbar-spine movements were reduced. He had a smooth, moderately enlarged prostate with normal anal tone and sensation. The rest of the examination, including cranialnerve testing and fundoscopy was normal. Blood tests showed a normal full blood count, urea and electrolytes, liver transaminases, serum vitamin B12 and folate, alkaline phosphatase, calcium, thyroid function, and complement concentration. Random glucose was 9·9 mmol/L, glycosylated haemoglobin 7·3%, erythrocyte sedimentation rate (ESR) 32 mm in the first hour, C-reactive protein 1·3 mg/dL (normal <5 mg/dL), and creatine phosphokinase (CKP) 496 IU/L (normal 24-195 IU/L). He was antinuclear antibody positive 1:80, rheumatoid factor positive 1:80, double stranded DNA and VDRL negative. Urinalysis revealed glucose +, and no protein. His chest radiograph and abdominal ultrasound scan were normal. Computed tomography scan of his lumbar spine showed a degenerative disc at L4/5, with no evidence of root or spinal cord compression. Electromyography (EMG) showed moderate sensory-motor neuropathy, with an active myopathy. A muscle biopsy specimen showed necrosis of muscle fibres with a perivascular mononuclearcell infiltrate. At initial assessment a diagnosis of diabetic amyotrophy (proximal motor neuropathy) with peripheral sensory neuropathy was made. However, the elevated CPK and EMG and muscle-biopsy findings suggested polymyositis as the cause of weakness, with a coincident