A 65-year-old retired builder presented to his GP with a progressive history of stiffness and weakness affecting both legs. He had recently started dragging his right leg and had noticed some urinary incontinence. His symptoms had come on gradually over a period of four months.

There was no history of trauma; however he had had a bout of gastroenteritis in the last few weeks, which he had attributed to eating a take-away curry. His past medical history included rheumatoid arthritis and tension headaches, which had been more frequent of late, and borderline hypertension. He was a smoker of 20 cigarettes per day and drank 20 units of alcohol per week.

On examination he was alert and orientated. His blood pressure was 142/89 mmHg, pulse 89/min and temperature was 36.7°C. On examination of cranial nerves, no abnormalities were found. On examination of the peripheral nervous system, upper limb was entirely normal, however on examination of the lower limb there was marked spasticity, hyperreflexia with extensor plantar responses. Power was grade 4/5 on the left and 3/5 on the right with a pyramidal pattern of weakness. There did appear to be some sensory neglect of the right lower limb and diminished vibration and light touch on the left lower limb. No sensory level could be detected. There was no cerebellar dysfunction. Chest and abdominal examination was normal.

He was investigated with an MRI thoracic spine, which was entirely normal, and lumbar puncture.

Lumbar puncture showed:
Opening pressure 13 cm H2O (5-18)
CSF protein 0.6 g/L (0.15-0.45)
CSF white cell count 20 per ml (> 5)
CSF red cell count 4 per ml (>5)
CSF glucose 3.4 mmol/L (3.3-4.4)
CSF oligoclonal bands Present –
Serum oligoclonal bands Present –

What is the likely diagnosis in this 65-year-old patient based on the history and findings?