A 25-year-old man is referred by his family doctor for neurological assessment. He comes from a family who rarely seek medical attention. His brother has had hearing difficulties for the past 5 years and uses a hearing aid. The patient has been experiencing increasing unsteadiness and slurred speech over the past 6 months, but is still performing well at work.
During the examination, both optic discs appear pale and there is fine nystagmus. The patient’s speech is dysarthric and there is bilateral intention tremor. There is also muscle wasting in the distal limbs, absent tendon reflexes, and extensor plantar responses. Romberg’s test is positive. The patient’s blood pressure is 110/70 mmHg with no orthostatic hypotension.
What is the most probable diagnosis?