3-30 U.S. Navy Diving Manual — Volume 1 oxygen on the built ­ in breathing system (BIBS) mask. Recompress the diver if there is doubt about the cause of post ­ dive hearing loss or vertigo. CAUTION When in doubt, always recompress. Frequent oscillations in middle ear pressure associated with difficult clearing may lead to a transient vertigo. This condition is called alternobaric vertigo of descent . Vertigo usually follows a Valsalva maneuver, often with the final clearing episode just as the diver reaches the bottom. Symptoms typically last less than a minute but can cause significant disorientation during that period. Descent should be halted until the vertigo resolves. Once the vertigo resolves, the dive may be continued. Alternobaric vertigo is a mild form of inner ear barotrauma in which no lasting damage to the inner ear occurs. 3-7 MECHANICAL EFFECTS OF PRESSURE ON THE HUMAN BODY--BAROTRAUMA DURING ASCENT During ascent gases expand according to Boyle’s Law. If the excess gas is not vented from enclosed spaces, damage to those spaces may result. 3-7.1 Middle Ear Overpressure (Reverse Middle Ear Squeeze). Expanding gas in the middle ear space during ascent ordinarily vents out through the eustachian tube. If the tube becomes blocked, pressure in the middle ear rela tive to the external water pressure increases. To relieve this pressure, the eardrum bows outward causing pain. If the overpressure is significant, the eardrum may rupture. If rupture occurs, the middle ear will equalize pressure with the surrounding water and the pain will disappear. However, there may be a transient episode of intense vertigo as cold water enters the middle ear space. The increased pressure in the middle ear may also affect the inner ear balance mechanism, leading to a condition called alternobaric vertigo of ascent . Alter­ nobaric vertigo occurs when the middle ear space on one side is overpressurized while the other side is equalizing normally. The onset of vertigo is usually sudden and may be preceded by pain in the ear that is not venting excess pressure. Alter­ nobaric vertigo usually lasts for only a few minutes, but may be incapacitating during that time. Relief is usually abrupt and may be accompanied by a hissing sound in the affected ear as it equalizes. Alternobaric vertigo during ascent will disappear immediately if the diver halts his ascent and descends a few feet. Increased pressure in the middle ear can also produce paralysis of the facial muscles, a condition known as facial baroparesis . In some individuals, the facial nerve is exposed to middle ear pressure as it traverses the temporal bone. If the middle ear fails to vent during ascent, the overpressure can shut off the blood supply to the nerve causing it to stop transmitting neural impulses to the facial muscles on the affected side. Generally, a 10 to 30 min period of overpressure is necessary for symptoms to occur. Full function of the facial muscles returns 5 ­ 10 min after the overpressure is relieved.

The PDF viewer opens on the cited page and searches this passage for highlight.