MRCP2-3102

A 65-year-old retired librarian presented to her general practitioner with a gradual onset of unsteadiness of gait, resulting in her having to hold onto furniture to move around her house. She also experienced difficulty in reading her books and frequently lost her place while scanning the text. Her husband noticed a slowing of performing simple tasks such as making a meal, washing, or eating, and now needed to accompany her to the shops and help in most tasks because of her unsteadiness.

Her medical history included hypertension, hypothyroidism, and osteoarthritis. She was taking bendroflumethiazide 2.5 mg daily and thyroxine 75 mcg daily. On examination, her blood pressure was 102/65 mmHg (lying) and 97/55 mmHg (standing). Her pulse was 67/minute and regular, and she had marked dysarthria. On examination of eye movements, there was normal smooth pursuit, but slow vertical saccades and evidence of square wave jerks. She also had slow spastic tongue movements. On examination of the upper limb, there was marked neck rigidity with retrocollis and evidence of symmetrically increased tone and bradykinesia. On examination of the lower limb, there was symmetrically increased tone, bradykinesia, and gait instability.

Investigations revealed:
– Serum sodium 135 mmol/L (137 – 144)
– Serum potassium 3.8 mmol/L (3.5 – 4.9)
– Serum urea 5.4 mmol/L (2.5 – 7.5)
– Serum creatinine 100 mol/L (60 – 110)
– Serum thyroxine 60 nmol/L (58 – 178)
– Serum thyroid stimulating hormone 6.5 mU/L (0.4 – 5)

An MRI scan of her brain showed midbrain atrophy. What is the most likely cause of this patient’s symptoms and signs?