Clinical Practice Guideline

for

HEARING LOSS

Developed for the

Aerospace Medical Association

by their constituent organization

American Society of Aerospace Medicine Specialists

 

Overview: Aviation medicine specialists typically encounter questions about hearing loss related to either changes in annual audiograms and/or changes in the hearing profile category of aircrew.  Aircrew must meet hearing standards from their governing agencies and are considered to be exposed to hazardous noise during most aviation activities.  This Clinical Practice Guideline addresses questions related to changes in hearing profile category and asymmetry.

 

A.  Epidemiology and classification.

 

Hearing loss (HL) is common in the general population with estimates of 4% of people under age 45 and as many as 29% of people over age 64 suffering from “handicapping loss of hearing,” defined as “severe enough to interfere with effective conversation in an adult – approximately 25 to 30 decibels (dB).”  Most audiologists use the following guideline when describing the severity of hearing loss.8

 

Through World War II, hearing loss among aviators was so common that acquired hearing loss was referred to as “aviator’s ears” or “aviator’s deafness”; implying that hearing loss among aviators was expected or routine.  A 1985 study of 777 aviators in the Israeli Air Force found that “13.5% of the examined population suffered from hearing loss,” that was at least mild to moderate as described above.

 

HL is commonly classified by type:

 

Sensorineural hearing loss (SNHL) is the most common type of HL in the general population and is usually related to long-term exposure to noise.  However, a short blast of loud noise (generally greater than 120 – 155 dB) can also cause severe to profound sensorineural hearing loss, pain, or hyperacusis (pain associated with loud noise).  In either case, the HL is related to direct mechanical damage of the hair cells lining the cochlea resulting in permanent loss of a number of these cells specialized to sense sound at a given frequency.  All auditory information is transduced by only 15,000 hair cells, of which the so-called inner hair cells, numbering 3500, are critically important, since they form synapses with approximately 90 percent of the 30,000 primary auditory neurons.  Thus, damage to a relatively few cells in the auditory periphery can lead to substantial hearing loss.

 

Therefore, HL related to noise exposure (short or long term) can be permanent and irreversible.  Think about the effect of losing a number of adjacent rods and cones in the retina and the resulting visual field defect.  Losing adjacent hair cells in the cochlea is similar in that it results in loss of hearing in specific frequencies that can grow larger as more nearby hair cells are lost with continued noise exposure.  Just as glasses cannot reverse a visual field defect, modern hearing aids are not capable of restoring the function of lost hair cells.

 

Clinically, an individual’s hearing limitation is described in terms of decibels (dB) of HL.  The threshold of hearing at a given frequency by a “normal” person is 0 dB HL, and numbers higher than zero on an audiogram indicate how much louder a sound at a given frequency must be in order for that individual to perceive it fifty percent of the time.  Normal conversation levels are 45 to 60 dB, while a jet engine at 100 ft is 140 to 150 dB.

 

HL as the only symptom of a systemic illness is unlikely except in the case of latent syphilis or immune-mediated SNHL.  However, numerous other systemic illnesses (e.g. diabetes, blood cell dyscrasias, hyper or hypothyroidism) can result in hearing loss.

 

B.  Office evaluation of hearing loss.

 

Whether the aviator presents with acute hearing loss or an audiogram that demonstrates worsening HL, in order to direct further evaluation/treatment, classifying the hearing loss (conductive, SNHL, mixed) is started with the history and physical exam.  Some pertinent questions are:

 

Physical examination portion of an office hearing evaluation includes:

 

These tests are not intended to replace a thorough audiology evaluation (see below) but do provide some objective findings to document the status of the individual’s hearing at the time of the visit.  (A recent meta-analysis listed the sensitivity of the whispered voice test as 90-100 % while the specificity was 70-87 % as a screening tool.)  They also provide more effective communication with specialists at a (potentially) distant site (e.g., when trying to make decisions concerning air evacuation for further evaluation).

 

C.  Formal audiologic assessment.

 

The following tests are part of a complete audiologic evaluation.

 

Pure tone air and bone conduction thresholds – This is the audiogram.  Hearing is tested with both air and bone conduction.  Air conduction measures thresholds of sensitivity to sounds that travel to the inner ear through the external auditory canal and middle ear.  These values are compared to the thresholds of sensitivity to sounds that travel to the inner ear directly through the bone of the skull.  Any difference between the two thresholds is consistent with conductive hearing loss.

 

Speech reception thresholds (SRT) – This is the softest level at which a person can correctly repeat 50 percent of presented spondee words (words should be presented by professional recorded test).  Spondee words are two-syllable words where each syllable is stressed, such as airplane, armchair, or pancake.  SRT results are recorded in dB HL and should correlate with the audiogram:  equal to the pure tone air conduction average (average dB score at 500, 1000, and 2000 Hz) to within 10 dB as long as the scores from the three frequencies are similar.

 

Speech discrimination testing, to include high intensity discrimination – (aka word recognition score) percentage of phonetically balanced words correctly repeated at a given dB level, the most comfortable level (MCL) for speech.  Generally classified as normal (>90%); slight difficulty; comparable to listening over a telephone (75-90%); moderate difficulty (60-75%); poor discrimination, difficulty in following conversation (50-60%); and very poor discrimination, difficulty in following running speech (<50%).  It is important to understand that hearing aids do not improve word recognition scores.  This is a significant test for an aviator with bilateral hearing loss as it directly relates to comprehension of what is being said (on a radio, for example).  All speech testing should use professionally recorded materials, not live voice (e.g., NU-6 by difficulty, W-22, Harvard-50).

 

Immittance audiometrygenerally consists of three separate tests:

 

Otoacoustic emissions (transient evoked or distortion product) – this is a test of cochlear function and requires no input from the patient.  Simply stated, a normal cochlea produces sound that can be measured by ultra-sensitive microphones placed in the outer ear (requires normal middle and outer ear function).  This test can help specify whether sensorineural hearing loss is related to the sensation of sound (cochlear function) or neural transmission of the sound (acoustic nerve).

 

If the audiology testing listed above excludes conductive and retrocochlear disease, the audiologist may defer ENT evaluation.  If the results are equivocal, additional testing and ENT evaluation is recommended.  Some additional tests to consider include:

 

Aeromedical Concerns: Clearly it is essential that aviators have hearing adequate to recognize and understand verbal communications and warning tones.  This includes adequate binaural hearing in aircraft with warning tones presented specifically to the left or right sides.  Significant tinnitus may also interfere with communications as well as sleep.  Hearing loss can be an early symptom of other medical problems, for example, an acoustic neuroma which could directly impact vestibular function and flight safety.  Lastly, aviators with noise induced hearing loss will likely experience some degree of worsening hearing loss secondary to continued noise exposure.

 

Normally hearing aids are not worn in hazardous noise (flight environment).  Hearing aids are not hearing protection, and if exposed to hazardous noise hearing protection must be worn.  However, if necessary the only type of hearing aid that may work is custom-made feedback phase cancellation hearing aids that fit in the ear and hearing protection is provided by David Clark type muff (not helmet).  Hearing aids behind or over the ear cannot be worn due to comfort issues with the hearing protection, breaking hearing protection seal and feedback issues.  If double protection is required than hearing aids are not allowed.  Cochlear implants or implantable amplification devices are not allowed in any hazardous noise environment and thus not allowed in aviators.  Battery life varies with the shortest being about 4 days; changing a battery can be disruptive to aircrew duties, thus batteries should be changed prior to flying if hearing aids are worn while performing aircrew duties.

 

Individuals with otosclerosis or other causes of conductive hearing loss may actually hear better in noise/flight.  This is due to a phenomenon called the Paracusis of Willis; the otosclerosis filters out the background noise and allows the individual to hear communications better.  In this unique situation hearing aids may be used on the ground but not recommended/needed in flight.

 

The Attenuating Custom Communication Earpiece System (ACCES) earphone is the shape of the pilots' external auditory canal and blocks out much of the ambient noise; 35 to 50 dB attenuation occurs with the combination of ACCES and David Clark headset.  ACCES may improve communication capability in individuals that otherwise may have failed the hearing proficiency validation tests.

 

Medical Work-up: The aviator needs to submit a complete history related to hearing loss (including noise exposure history).  If hearing aids used include if worn while flying and address the ability to wear hearing protection.  Also needed are the baseline and latest audiograms along with the current completed audiology evaluation.  Each agency will need some sort of validation of hearing proficiency which is commonly accomplished through some sort of in-flight hearing test.  Refer to specific military or FAA standards.  A complete HEENT exam is also necessary as well as an ENT evaluation, if audiologist does not state conductive or retrocochlear disease is ruled out.

 

Aeromedical Disposition:

 

Air Force: In the US Air Force, the following table outlines the definition for H-1, H-2, H-3 and H-4 hearing profiles.  The hearing profile is based on an unaided audiogram (no hearing aids) and removal from hazardous noise for at least 14 hours.

 

 

500 Hz

1000 Hz

2000 Hz

3000 Hz

4000 Hz

6000 Hz

H-1 Profile

If no single value exceeds (dB):

25

25

25

35

45

45

H-2 Profile

If no single value exceeds (dB):

35

35

35

45

55

--

H-3 Profile

Any hearing loss exceeding at least one value for H2 profile

H-4 Profile

Hearing loss precluding safe and effective performance of duty, despite the use of hearing aids, as determined by hearing proficiency validation.*

*Hearing Proficiency Validation

Inflight hearing test

– OR –

Written validation of ability to safely perform all assigned aircrew duties in flying environment signed by flying SQ/CC or Operations Officer, supplemented by the flight surgeon’s written MFR stating that Speech Discrimination Levels (from the audiology report) are adequate to perform flying duties (>60%).

Asymmetry

≥25 dB difference comparing left and right ear, at any two consecutive frequencies

 

For all flying class physicals, a hearing profile that exceeds H-1 is disqualifying.  Trained aviators with H-2 profiles should have a full audiology evaluation sufficient to exclude conductive or retrocochlear pathology, but do not require waivers.  Trained aviators with H-3 profiles or asymmetric HL are disqualified.  Waivers are valid for no greater than three years (indefinites will not be granted) or until a shift of 10 dB or greater on the average of 2,000, 3,000 and 4,000 Hz in either ear from the previous waiver’s audiogram, whichever occurs first.

 

Army: Army aeromedical concerns parallel those articulated in this clinical practice guideline.  Adequate hearing is essential for communication in flight and for rapid and accurate assessment of warning tones and sounds in the cockpit. 

 

Acceptable screening audiometric hearing levels for Army aircrew members and ATC

Class

500 Hz

1000 Hz

2000 Hz

3000 Hz

4000 Hz

6000 Hz*

Flt Applicant

25

25

25

35

45

45

Rated Aircrew

25

25

25

35

55

65

*Isolated hearing loss at 6000 Hz will not require full audiology work-up unless recommended by the local FS or audiologist

 

Given the plethora of etiologies, the evaluation of hearing loss, especially newly diagnosed loss must be sufficiently thorough to insure obscure causes are not overlooked.  In addition to the pure tone screening tests, the evaluation of hearing loss must include tympanometry, acoustic reflex threshold testing, speech reception threshold testing, and speech recognition (discrimination) testing.  Aircrew members with a speech recognition score of less than 84% may receive a waiver, but are generally handled on a case-by-case basis. Patients who have an Army H4 profile will invariably be disqualified.

 

Navy: Waivers will be considered depending on the degree of hearing loss, and the member’s functional capability. Waivers following surgical treatment of conductive hearing loss may or may not be necessary, depending on the final hearing result and the nature of the surgery. For instance, repair of a traumatic eardrum perforation resulting in full correction and normal hearing would not require a waiver. However, a stapedectomy done to treat otosclerosis is CD and requires a waiver. Designated aviators are grounded for three months following stapedectomy, before waiver being recommended to SG1. For NFO and other Class II personnel, a waiver is also considered for duty involving flying after three months.  Waiver criteria include:

1. Asymptomatic

2. Passes a current flight physical

3. Prosthesis used was not a wire loop/gelfoam (a piston prosthesis and tissue graft is preferred versus a blood seal)

 

No waiver will be recommended if there are signs of vestibular dysfunction, spontaneous nystagmus, or sudden/progressive neurosensory hearing loss is present. Bilateral stapedectomy is not waived. Applicants with a history of stapedectomy are CD, no waiver.

 

Class I Hearing Standards

 

Frequency (Hz)

Better Ear (dB)

Worse Ear (dB)

500

35

35

1000

30

50

2000

30

50

 

Civilian: In the FAA’s medical certification the airman must only pass one of 3 acceptable hearing tests. The most common one is called the Conversational Voice test and requires the aviation medical examiner to speak to the airman in a conversational voice with the airman 6 ft away with his/her back turned to the examiner. The other acceptable tests are either an audiogram with a better ear/worse ear:

 

Frequency (HZ)

500 HZ

1000 HZ

2000 HZ

3000 HZ

Better Ear (Db)

35

30

30

40

Worse Ear (Db)

35

50

50

60

 

If the airman is unable to pass either the conversational voice or the pure tone audiogram, then an audiometric speech discrimination test should be administered with passing score at least 70% obtained at intensity no greater than 65Db.  This usually requires the airman to be seen by an Audiologist.

 

If an airman wears hearing aids he/she may take the test with them and if they pass a restriction is placed on their medical certificate: MUST WEAR HEARING AMPLIFICATION.

 

If an airman who wears hearing aids informs the AME that they can hear perfectly well while wearing their headset [versus the hearing aids], they are given permission for a medical flight test and if they pass are issued a Statement of Demonstrated Ability. 

 

Airmen who are completely deaf can attain medical certification but they are issued a medical certificate with the restriction: NOT VALID FOR FLYING WITHIN RADIO CONTROLLED AIRSPACE. 

 

Waiver Experience:

 

Air Force: Query of the AIMWTS database showed 20 cases of hearing aid usage and all but one were granted a waiver; this person was a pilot training applicant.  During the same time period there were 1,977 waivers for some degree of hearing loss; 172 were disqualified (8.7%).  Of the 172 disqualified, 105 were reviewed.  Of the 105, 79 (75%) were disqualified for the hearing loss and 26 (25%) were disqualified for other medical conditions (TIA, stroke, Meniere’s disease, coronary artery disease, diabetes mellitus, back pain, psych, etc.).  Of the 79 disqualified for hearing loss, the majority (~ 95%) were due to non H-1 hearing (H-3>>H-2).

 

Army: The Aeromedical Epidemiological Data Repository (AEDR) catalogs all Army flight physicals since 1960.  There have been approximately 160,000 individual aircrew entered in this database.  During this period of time, there were 530 requests for waiver among pilot applicants.  Of those 354 were granted an exception to policy and retained.  During the same period there were 607 aeromedical summaries for rated aviators were submitted, for which 553 waivers were granted.  Additionally, there were 477 requests for waiver in non-rated aircrew; 413 were granted waivers and retained in aviation.

 

Navy: not available at this time

 

Civilian:  As of July 31, 2010 the number of airmen currently issued with a restriction on their medical certificate MUST WEAR HEARING AMPLIFICATON: First-class: 65, Second-class: 220 and third-class: 1,145.

 

ICD 9 Codes for Hearing Loss

389.0

Conductive hearing loss

389.1

Sensorineural hearing loss

389.16

Sensorineural hearing loss, asymmetrical

389.2

Mixed conductive and sensorineural hearing loss

V53.2

Hearing aid

 

References:

 

Air Force Occupational Safety and Health (AFOSH) Standard 48-20, Occupational Noise and Hearing Conservation Program, 30 June 2006.

 

Della Santina CC, Lustig LR.  Chapter 157 – Surgically implantable hearing aids.  In Cummings CW, Flint PW, Haughey BH, et al, (eds) Cummings:  Otolaryngology:  Head and Neck Surgery, 4th ed.  Philadelphia; Mosby, Inc, 2005.

 

Gasaway D.  Noise levels in cockpits of aircraft during normal cruise and considerations of auditory risk.  Aviat Space Environ Med.  1986; 57(2):  103.

 

Goetzinger CP.  Chapter 13 – Word discrimination testing.  In Katz J (ed), Handbook of Clinical Audiology, 2nd ed.  Baltimore; Williams& Wilkins Co, 1978.

 

Nadol JB.  Hearing loss.  N Eng J Med.  1993: 329(15):  1092-1102.

 

Pirozzo S.  Whispered voice test for screening for hearing impairment in adults and children: systematic review.  BMJ.  2003; 327:  967.

 

7.  Ribak J, et al.  The association of age, flying time, and aircraft type with hearing loss of aircrew in the Israeli Air Force.  Aviat Space Environ Med.  1985; 56(4):  323.

 

Wayner DS.  Hear what you’ve been missing.  www.healthyhearing.com, last updated April 4, 2002.

 

Weber PC.  Etiology of hearing loss in adults.  UpToDate.  Online version 15.3.  www.uptodate.com.  January 11, 2007.

 

Weber PC.  Evaluation of hearing loss in adults.  UpToDate.  Online version 15.3, July 24, 2007.

 

 

 

2/19/11

Prepared by Drs. Howard Givens and Karen Fox