The average recovery of hearing both at high and speech frequencies was significantly better and tinnitus persisted less commonly after the HBOT than after the NBOT. Normal hearing at the end of the follow-up period was regained in 42 ears in the HBOT group and in 24 ears in the NBOT group (p<0.01).

We investigated the prevalence, characteristics and subjective perceived handicap caused by long-term tinnitus induced by acute acoustic trauma (AAT) in 418 former military conscripts. They had been treated between 1984 and 1989 because of AAT from exposure to impulse noise caused by firearm shooting. All 418 patients reported tinnitus after the AAT. At discharge from the military service, 122 (29%) still reported tinnitus. In 1999, of these 122 patients, 101 were reached and 66 still had tinnitus. The Tinnitus Handicap Questionnaire demonstrated various difficulties in life because of tinnitus, not attributable only to tinnitus loudness. Psychological factors also seem to play an important role. The effects of AAT and possible compensation have been evaluated so far mostly by audiometric findings, but in some cases tinnitus may be an even more serious threat to life satisfaction than mild hearing impairment.

qfhyperbaric oxygen treutment 011 permanent tlrreshold sli$ in acoustic truu~nu umong rats. Acta Otolaryngol (Stockh) 1997; Suppl 529: 80-82. Impulse noise from firearms is a common cause of acute acoustic trauma (AAT). Recently hyperbaric oxygen treatment has become available in many hospitals treating AAT. We exposed 39 Wistar rats to intense impulse noise of 60 shots from the assault rifle (162 dB SPL). After the exposure IS animals were given hyperbaric oxygen treatment (HBO) by 10 treatment cycles of 90 minutes 100'X) oxygen in 0.25 MPa, one treatment cycle per day. Four weeks after the exposure the hearing thresholds were measured with auditory brainstem response audiometry at frequencies of 1.0, 2.0, 4.0, 6.0, 8.0 and 10.0 kHz. Characteristics for the resulting noise-induced hearing loss were large variations in its severity not only between animals, but also between the ears of a single animal. The largest permanent threshold shifts were found at 6.0, 8.0 and 10.0 kHz. Most of the HBO-treated animals showed less threshold shift than the non-treated animals. The difference between the HBO group and the control group was only slightly statistically significant 0, = 0.067). Key word^: permanent threshold shift, auditory brainstem response audiometry, experimental animals.

The regulations concerning hearing protection during military training in the Finnish Defence Forces were renewed in 1989. The material of this prospective study concerns 912 consecutive conscripts (463 before and 449 after the new regulations) who were referred to the Central Military Hospital for acute acoustic trauma (AAT). We focused on three issues: (1) general habits regarding the use of hearing protection during shooting drills and combat training; (2) hearing protection at the moment of AAT; and (3) the cause of AAT. In combat training, the use of any hearing protectors was only 50% before the new regulations, but after they came into force the proportion was greater than 90%, and more effective protectors were used. However, at the time of AAT the hearing protection was absent in more than 80% of cases in both groups. The most common cause of AAT was small arms in both groups (83%).