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Stiff man syndrome (SMS) is a rare, disabling neurological disorder characterised by progressive muscle rigidity and painful episodic spasms of the axial and proximal limb muscles. Diagnosis is based on the recognition of typical clinical features and characteristic EMG findings.1 However, although it is well described, SMS is probably underdiagnosed because of a lack of awareness of its clinical manifestations. It may present to a range of specialties and should be considered in all patients with unexplained back pain, stiffness, and muscle spasms. We present a typical case of stiff man syndrome in a patient who was referred to our unit, having previously been seen by a number of doctors and a neurosurgeon, without a diagnosis.
A 42 year old man with a 28 year history of insulin dependent diabetes mellitus (IDDM) and a two year history of progressive pain and stiffness affecting his lower back and abdomen was seen in the rheumatology clinic. He also had painful, intermittent muscle spasms. His functional ability had significantly decreased and he complained of extreme difficulties in performing tasks requiring back flexion. Bladder and bowel function was normal. Blood sugars were controlled with insulin and he had no diabetic complications.
On examination, he had a marked lumbar lordosis, a prominent thoracic kyphosis, and a protuberant abdomen (fig 1). There was pronounced paraspinal and abdominal wall muscle rigidity. Neurological examination was otherwise normal. Blood tests showed a mildly raised haemoglobin AIc (7.4) and plain radiography of the spine confirmed the clinical findings. The EMG was classical for SMS and showed sustained motor unit activity in agonist and antagonist axial and limb muscles, despite the patient's attempts at relaxation. The diagnosis was supported by the detection of antibodies against glutamic acid decarboxylase (GAD) in his serum. His condition only mildly improved with baclofen 80 mg/day, and diazepam has now been introduced.
SMS has an insidious onset, usually in the fourth or fifth decades, with progressive muscle rigidity and episodic spasms affecting the axial and limb muscles. Muscle rigidity can lead to contractures, and simultaneous contraction of the thoracolumbar paraspinal and abdominal wall muscles causes lumbar hyperlordosis. Episodic muscle spasms are classical in SMS and their absence should raise suspicions about the accuracy of the diagnosis.1 The spasms, which are often provoked by emotional upset or sudden movement, can be extremely painful, generating forces capable of fracturing long bones.2 Rigidity and spasms gradually impair voluntary movements and postural reflexes, resulting in slow, restricted movements and an increased risk of falls. Intellect is not affected, and motor and sensory nerve examination is also normal. However, almost all patients have an abnormal EMG pattern, which shows continuous motor unit activity in affected muscles.1
The cause of SMS is unknown, but an autoimmune pathogenesis is suggested by the presence of autoantibodies and its strong association with autoimmune conditions, such as IDDM and thyroiditis.3 Antibodies against GAD are present in about 60% of patients with SMS.4 GAD is the rate limiting enzyme in the synthesis of γ-aminobutyric acid (GABA), one of the main inhibitory central neurotransmitters. Reductions in GABA production may therefore impair transmission at central nervous system inhibitory synapses, resulting in the continuous motor unit activity seen in this disease. Antibodies against amphiphysin, a nerve terminal protein, have been detected in up to 5% of anti-GAD negative patients. They are strongly associated with paraneoplastic SMS and their presence should prompt careful investigation to exclude malignancy, particularly breast cancer.5 However, approximately 40% of patients have no evidence of autoantibodies, suggesting that the pathogenesis of this syndrome may be heterogeneous.6
Drugs that enhance GABA-mediated central inhibition, such as diazepam (up to 300 mg/day), baclofen, sodium valproate, and vigabatrin, are the mainstay of treatment for this previously refractory disease. Immunomodulatory treatments have also been used and favourable responses to corticosteroids, plasmapheresis, and intravenous immunoglobulin have been reported.7–9 Physiotherapy is also important, offering a valuable adjunct to drug treatment.
SMS should be considered in all patients with unexplained back pain, stiffness and muscle spasms as early recognition and therapeutic intervention can significantly decrease morbidity and improve quality of life.