Behavioral and Audiological Manifestations of Noise-Induced Hearing Loss
Sufficiently intense sounds have the potential of disrupting all parts of the peripheral and central auditory system. Noise can have direct mechanical effects on the middle ear, such as ossicular and discontinuity, tympanic membrane perforation, or fistula of the oval window, and on cochlear structures. The outer hair cells are particularly vulnerable to the effects of excessive noise exposure, followed in vulnerability by the inner hair cells. The cochlea, once damaged, cannot be repaired; the subsequent loss of sensory cells and neural changes produces an auditory pathology that represents the morphologic substrate for the loss of hearing threshold, referred to as a noise-induced sensorineural hearing loss, or simply a noise-induced hearing loss (NIHL). A similar set of cochlear changes can be induced by lower levels of noise that continuously stress the metabolic processes of the cochlea. While these changes may initially produce a temporary loss of threshold, with repeated exposures they may lead to permanent changes.
Hearing loss resulting from noise exposure can be separated into three distinct categories: acoustic trauma, temporary threshold shift (TTS), and permanent threshold shift (PTS). A single, relatively intense noise exposure is referred to as an acoustic trauma and is usually followed by tinnitus and a change in hearing threshold. While hearing may improve slightly over time, if the exposure is sufficiently intense a PTS will result. One or both ears may be involved. Those who experience an acoustic trauma may also suffer from tympanic membrane perforation(s) and disarticulated or fractured ossicles. Such middle-ear disorders are more likely to appear, if at all, once the peak noise exposure level exceeds approximately 160 dB SPL. In general, however, any acute sound exposure that causes any of the following symptoms represents a hazard to the auditory system and could result in an acute acoustic trauma: immediate pain, a tickling sensation in the ears often occurring if the SPL exceeds approximately 120 dB, vertigo, tinnitus, hearing loss, or reduced communication skills.
Lower levels of noise (<85 dB[A]) are potentially hazardous and may result in an NIHL if, following exposure, there is a transient shift in the threshold of hearing that recovers gradually (a TTS). While the onset of hearing loss in acute acoustic trauma is instantaneous, the onset and progression of NIHL is far more insidious since it accumulates, usually unnoticed, over a period of many years of exposure to noise on a daily basis. During the initial stages of NIHL, the temporary hearing loss recovers within a few hours or days following removal from the noise. However, if the exposure to this noise is repeated often enough, the hearing loss may not recover completely (that is, permanent sensorineural hearing impairment will begin).
Reprinted with permission from Occupational Health: Recognizing and Preventing Work-Related Disease, Levy and Wegman (eds.), 328–31, Copyright 1995, Little, Brown and Company.
During a plant visit to review the company’s hearing conservation program and resolve medical referrals resulting from the review of hearing tests, an unscheduled employee presented with an interesting hearing difficulty. The gentleman was in his early fifties and he reported that last evening, after cutting his grass he went to accompany his wife on the front porch with a glass of iced tea. He reported being tired and wanting to relax a bit before calling it a day. Suddenly, he felt fullness in his left ear. He knew something had happened but did not understand exactly what. His wife spoke to him and he could not understand. He thought at first she was not speaking clearly. He asked her to repeat her comment and she did. He realized that it was the hearing in his left ear that was a problem. He could not hear out of it. The employee went to bed as he was tired and was hoping the problem would go away over night. When he awoke the next morning he still could not hear in his left ear. He reported to the medical department at the first opportunity soon after he arrived at work. I just happened to be there at the time.
Once the employee was interviewed, we conducted a pure-tone hearing test. It confirmed a significant hearing change in the left ear and stable hearing in the right ear. A tuning fork test indicated the hearing loss was sensorineural. An otoscopic evaluation revealed normal eardrums (tympanic membranes) and no impacted wax. The employee did not wear hearing aids. A test of eardrum mobility (tympanometry) showed normal middle ear functioning. An evaluation of the function of the outer hair cells of the cochlea (Otoacoustic Emissions) revealed normal functioning in both ears. The situation now took a very serious turn as testing and history strongly suggested "Sudden Sensorineural Hearing Loss" (SSHL).
SSHL is described as an abrupt sensorineural hearing loss of 30 dB HL or greater over at least three audiometric frequencies occurring within 72 hours or less.
What are the chances of SSHL occurring?
Approximately 4,000 cases of SSHL are diagnosed each year. That’s 1 in 5,000.The highest incidence is of adults between the ages of 50 and 60 years old with 75% being over the age of 40. Single ear losses occur in 90% to 98 % of the cases. A minority present with loss in both ears.
What are the presenting conditions?
50% to 70% of the cases report tinnitus (head noises) or a change in tinnitus, fullness in the ear and dizziness (vertigo), which is reported by 40% of the cases. The most important of all symptoms are patient reports of sudden hearing loss and the onset or a change in the perception of tinnitus. Patients frequently present with head trauma, lifting incident, flying, scuba diving, loud noise, recent viral illness (flu), upper respiratory illness (30%), surgeries, and/or change in medications or dosage. Awakening from sleep is common. Therefore securing a good history is important.
What can cause SSHL?
There are five areas of possible cause in differential diagnosis:
- Autoimmune; as AIDS, Lupus, Meniere’s
- Vascular; as Cardiopulmonary bypass, Sickle Cell
- Neurologic and Neoplastic Lesions; as auditory tumor and/or surgery, Migraine, Multiple Sclerosis
- Trauma; as Concussion, Ototoxicity, Perilymph Fistula, Temporal Bone Fracture
- Infection; as Herpes (Shingles), Mumps, Measles, Syphilis, Lymes Disease
What kind of audiological and medical procedures are used?
Standard audiological evaluations as air conduction, bone conduction, speech audiometry, immittance, otoacoustic emissions (OAE) and auditory evoked response testing show a sensorineural hearing loss. One of the more important audiological tests is the OAE evaluation as it is significant in its prognostic value. Where the cause is unknown (idiopathic), the medical evaluation will be negative for physical findings. Blood work will be done to implicate conditions as Sickle Cell, urinalysis for conditions as presence of blood or triglycerides, serologic tests for conditions as syphilis or HIV and imaging (CT and MRI) for fractures and soft tissue lesions as Multiple Sclerosis.
What are the medical treatments?
If the cause is autoimmune, vestibular or idiopathic steroids as Prednisone (1mg/kg/day for 2 4 weeks) are prescribed. If the loss does not recede, the dosage is tapered at a slower rate. Additional measures include diuretics, low-sodium diet, restricted caffeine, nicotine and alcohol, limited physical activity and limited noise exposure. If a vascular pathology is suspected the patient may be treated with Carbogen, a mixture of 95% oxygen and 5% carbon dioxide. This is an attempt to increase oxygen supply to the cochlea. Head trauma or tumor surgery may require additional surgery.
What is the prognosis?
The individual’s hearing loss may improve, fluctuate, stabilize or progress. Approximately 90% of the cases are diagnosed as "Sudden Idiopathic Sensorineural Hearing Loss" (SISHL) or unknown cause sensorineural hearing loss. In only about 10% of the cases is an actual diagnosis found. The good news is that 65% of the SSHL cases will recover spontaneously. 90% of the recovery will occur in the first 6 weeks. The bad news is that for those who do not have spontaneous recovery and SSHL is not medically treated within two weeks; the prognosis for any recovery is very poor. Success in recovery is seen in 70% of patients treated within 10 days. Actually, if there is no improvement within 12 days of occurrence, improvement is unlikely. The OAE testing is the best prognostic indicator. If testing indicates functioning of the outer hair cells in the cochlea, then an average of 33 dB improvement in the speech range hearing can be expected.
Because of the urgency for treatment, the employee’s primary physician was contacted and an appointment was set for that afternoon. He saw an otolaryngologist (ENT) the next day. The employee was given a 10-day course of prednisone and asked to avoid noise. In instances when he could not avoid noise on/off job, he was asked to wear hearing protection with a good seal. In discussing the case with the ENT physician, we discussed the dilemma of starting steroids when many times hearing will recover spontaneously. He explained that once he has ruled out any known causes, he discusses with the patient the possible steroid side effects and the chances of spontaneous recovery.
Together, they make the decision. In this case hearing began to recover in five days and in two-and-one-half weeks was almost complete with 92% word recognition ability. One year after the incident, recovery was complete returning to pre-incident levels.
NOTE: Whereas this case is based upon actual events, the names, conditions, and incidences may have been fictionalized to complete the story and protect the innocent.
Acknowledgement for the discussion on case treatment, follow-up and select additional readings goes to Cheek, E. Lou, ASHS 800: Auditory and Vestibular Disorders, Sudden Idiopathic Hearing Loss, July, 2003 (unpublished paper).