Patients occasionally present with cranial nerve lesions (fifth or seventh) attributed to neurapraxia. These lesions can be caused by the implosive tissue-damaging barotraumatic effects during descent, the distension in enclosed gas spaces during ascent or both. It is possible that air could be forced into the nerve canals as the gas expands with ascent. The nerve damage varies greatly, often transitory but occasionally long-lasting. These presentations are usually associated with barotrauma symptoms and signs, as described earlier (see Chapters 7 and 8).
With the cranial nerve palsies produced by ascent, there may be a delay of many minutes after the dive, and the diver may be aware of the feeling of distension of the gas space. The relief as gas escapes may coincide with improvement in the neurapraxia. This infers that the cause may be ischaemic, with a middle ear or sinus pressure in excess of the mean capillary perfusion pressure. Oxygen inhalation may assist, or even recompression may be indicated.