A 70 year old woman is brought in by her caretaker for decreased mental status. Her caretaker noted lethargy and confusion at home, but now she is more obtunded. Her caretaker says that toxins are impossible and that there have been no new drugs. She has no liver, kidney, or psychiatric disease and is not a drinker. There was no seizure activity or trauma. Her past medical history includes a head and neck cancer treated with radiation and a cardiac arrhythmia controlled on amiodarone.
On exam, temperature is 34 degrees Celsius, heart rate is 50, blood pressure is 90/40, respiratory rate is 10, oxygen saturation is 97% on room air. Labs show a low sodium and low sugar. BUN, creatinine, calcium, magnesium, CBC, lactate, and cortisol are all normal. RPR, blood cultures, U/A, and CXR are negative. An LP shows only modest elevation of protein, not consistent with a CNS infection. Scans of the head are negative for structural abnormalities or stroke. B12, thiamine, and glucose are given, but do not help. She is rewarmed and her vital signs are corrected, but she still has persistent altered mental status.
Challenge: You've ruled out most of the causes of coma; what's left?