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It has been a medical mystery as to what caused his slowly progressive hearing loss to the point of deafness, but a recent publication suggests it was due to chronic low-level lead exposure consumed from wine tainted with lead.
FACT: High levels of lead were found deep in the bone suggesting chronic lead exposure over a long period of time.
FACT: Beethoven's physicians noted that he had alcohol dependence (profound liver damage was found on autopsy).
FACT: It is not unusual that wine in that time period was tainted with lead.
More significantly, the paper disputed the more common thought that his lead poisoning was due to taking excessive doses of lead-based medications administered by his doctor: lead was found deep in the bone indicating lead exposure over a long period of time and absence of physical exam signs/symptoms (ie, wrist drop) indicative of acute lead poisoning (lead consumed over a short period of time).
The paper also did rule out other causes of hearing loss including otosclerosis, autoimmune hearing loss, autoimmune inflammatory conditions, mercury poisoning, and syphilis.
Lead and the deafness of Ludwig van Beethoven. Laryngoscope. 2013 May 17. doi: 10.1002/lary.24120. [Epub ahead of print]