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A case of reversible amnesia

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A 43 year old, normally healthy woman was admitted to a general medical ward after collapsing at home. Over the preceding two weeks, her husband had noticed that she had seemed distracted, unusually depressed, and “not herself.” He had found his wife unresponsive and writhing on the floor. She was initially drowsy after the attack but became alert within 30 minutes. On admission, she was amnesic for the episode, disoriented in time, place, and person, apyrexial, and clinically euthyroid, with no meningism or focal neurological signs. Computed tomography of the head was normal. CSF protein was 0.74 g/l (normal range 0.35–0.45 g/l), with normal glucose and no cells. Serum biochemistry and full blood count were within normal limits, apart from her thyroid stimulating hormone (TSH) which was raised at 7.14 mU/l (normal 0.5–3.9 mU/l). Her serum free T4 was normal at 15 pmol/l (normal 10–19 pmol/l). She received 10 days of intravenous acyclovir for a presumed generalised seizure possibly complicating viral encephalitis. Subsequently, CSF and serum viral serology and an autoantibody screen were negative.

During and after treatment she had a fluctuating level of confusion and appeared distant with a flat affect. Three weeks after the initial presentation she had another episode of collapse with writhing movements as before. Her mini-mental state examination was 19/30, with deficits in orientation and attention and total anterograde amnesia. Her neurological examination remained otherwise normal. She was still clinically euthyroid. Her TSH had increased to 25 mU/l with a normal T4 of 15 pmol/l. Her thyroid microsomal antibodies were raised at more than 1:6400. Her thyroglobulin antibodies were negative. Magnetic resonance imaging of the head was normal. Repeat CSF protein was 0.76 g/l with a normal glucose, 4 white cells per high power field, and no oligoclonal bands. An EEG was normal even in the presence of choreiform movements of the arms.

Three days after the second collapse she was started on prednisolone 60 mg daily. Within 24 hours the patient's mental state returned to normal. She had complete retrograde amnesia for the previous four weeks but was able to recall feeling mildly depressed and apathetic for a week before that. Nine months later she remained well on prednisolone 7.5 mg/day.

Questions

(1)
What is the most likely diagnosis?
(2)
Name four other ways in which this disease can present.
(3)
What is the likely prognosis?

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