The British AAIB (Air Accident Investigation Branch) released their bulletin concluding  the probable cause of an incident that British Airways A321 had near London on Dec 20th 2011 :
«The symptoms experienced by the commander may have been the result of the after-effects of a cold, combined with coincidental head and aircraft movement. The temporary symptoms experienced by the co-pilot may have been a reflection of the potentially evolving situation of an incapacitation at an early stage in the flight leading to possible mild hyperventilation.»

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** Incident: British Airways A321 near London on Dec 20th 2011, both flight crew nearly passed out 
Simon Hradecky / 11th-Oct-2012 / The Aviation Herald