Because the external auditory canal is usually open to the environment, water enters and replaces the air in the canal during descent, equalizing the pressures.
If the external ear is occluded, water entry is prevented. Contraction of the contained gas is then compensated by tissue collapse, outward bulging of the tympanic membrane, local congestion and haemorrhage. This is observed when a pressure gradient between the environment and the blocked external auditory canal is +150 mm Hg or more, i.e. 2 metres descent in water (Figure 7.2).
The common causes of blockage of the external auditory canal include wax or cerumen, large exostoses, foreign bodies such as mask straps, tight-fitting hoods and mechanical ear plugs.
Clinical symptoms are usually mild. Occasionally, a slight difficulty in equalizing the middle ear is experienced. Following ascent there may be an ache in the affected ear and/or a bloody discharge.
Examination of the external auditory canal may reveal petechial haemorrhages and blood-filled cutaneous blebs that may extend onto the tympanic membrane. Perforation of this membrane is uncommon.
Treatment for this condition includes maintenance of a dry canal, removal of any occlusion, possibly cleansing of the canal with an antiseptic solution warmed to body temperature and prohibition of diving until all epithelial surfaces appear normal. Secondary infection may result in a recurrence of the pain and may require antibiotics and local treatment (see Chapter 29).
This condition is easily prevented by ensuring patency of external auditory canals and avoiding ear plugs or tight-fitting hoods that do not have apertures over the ear to permit water entry.
External ear barotrauma of ascent is theoretically possible.