A 59 year old, retired metal worker presented with a 3 month history of difficulty walking. He had initially developed left leg weakness, followed 1 month later by numbness of both legs, a burning sensation in his anal region and penis, constipation, urinary frequency, retention, and incontinence. At that stage he presented to another hospital with abdominal pain, but all investigations for this were normal. On examination there was some weakness of his left leg, but no objective sensory loss in the legs or perineum. The weakness progressed to involve his right leg over the next 2 months such that he needed a stick to walk.
For 6 years before presentation he had had unremitting paraesthesiae of both shins, for which no cause had been found. He had been judged unfit to work 4 years previously due to depression. Ankylosing spondylitis had been diagnosed at the age of 30, followed by proctitis 10 years later for which he had been successfully treated with sulphasalazine.
Cognitive, cranial nerve, cerebellar, and general physical examination were unremarkable. Upper arm and pectoralis muscles were atrophied, but not weak. His gait was broad based, stiff, and he dragged both feet. There was bilateral MRC grade 4 weakness of knee flexion and MRC grade 4+ weakness of the left extensor hallucis longus muscle. Jaw jerk was absent, but deep tendon reflexes in the arms and legs were brisk with clonus at the right ankle, an absent left ankle jerk, and up going plantar responses. Pain and touch sensations were diminished over both forearms, as they were below both knees with additional impairment of vibration sense.
Initial abnormal investigations were C reactive protein 43.7 mg/l (reference range (RR) 0-8 mg/l), lactate dehydrogenase 638 [micro]mol/l (RR 300-620 [micro]mol/l), serum angiotensin converting enzyme (SACE) 28 U/l (RR 8-21 U/l) rising to 36 U/l two months after presentation, and protein electrophoresis showed two paraproteins, one band of IgG-K and three bands of IgM-[lambda]. There were occasional leucocytes (never more than 20 per high power field) and some erythrocytes in the urine. Autoantibody screen, erythrocyte sedimentation rate, HIV and Lyme serology, tuberculin test, and the rest of the serum biochemistry and urine examination were normal. Examination of the CSF on three occasions demonstrated a maximal protein of 1.25 g/l, maximal white cell count of 13x[10.sup.6]/l, IgG index of 0.50 (normal[less than]0.60), no growth from bacteriological cultures, and unremarkable cytology and serology.
Magnetic resonance imaging of the lumbar spine was initially normal, but on two subsequent examinations, within a month of presentation, a small contrast enhancing lesion was seen adjacent to a spinal root in the vicinity of the medullary conus (fig 1). Brain MRI soon after presentation showed small, scattered lesions in the subcortical and periventricular white matter (fig 2), with a larger lesion in the right frontal lobe (fig 3); these appearances were unchanged 3 months later. Initial EMG showed denervation in the left gastrocnemius and biceps femoris muscles. Nerve conduction studies revealed normal velocities, action potential amplitudes,...