Accounting for presbycusis is not mandatory but it is often done in the legal setting.  What is actually sought is to account for non-occupational hearing loss.  Seven states allow deductions for presbycusis.  

NIOSH does NOT recommend accounting for presbycusis when looking for Medical Causation of a progressive hearing loss, NIOSH states in DHHS (NIOSH) Publication No. 98-126 Section 5.1:  "NIOSH does not recommend that age correction be applied to an individual's audiogram for significant threshold shift calculations. Although many people experience some decrease in hearing sensitivity with age, some do not. It is not possible to know who will and who will not have an age-related hearing loss. Thus, applying age corrections to a person's hearing thresholds for calculation of significant threshold shift will overestimate the expected hearing loss for some and underestimate it for others, because the median hearing loss attributable to presbycusis for a given age group will not be generalized to that experienced by an individual in that age group."

1)  Measuring Presbycusis:

Two researchers have pooled a variety of research study data to determine the level of presbycusis and have developed two equations to calculate presbycusis.  Both of these studies found differences in presbycusis between male and female subjects.

In addition, OSHA (Occupational Safety and Health Administration) has published correction values for aging.  These values will correct threshold frequencies of 1000 Hz to 6000 Hz.  Since no values are given for 500 Hz, this method can only be used with the NIOSH and Wisconsin equations.  These values are greater than those given by Robinson Sutton's Equations.  However, when used to evaluate hearing loss progression, they are subtracted from baseline values.  For age 20 and below these values are not zero but vary from 3 dB to 8 dB.   In addition, the correction values also do not change above the age of 60, and there is no noise-presbycusis compression factor.   The lack of a compression factor will tend to cause this method to predict higher values for aging in audiometric data.

Several researchers have studied primitive cultures to determine the influence of aging on hearing acuity in the absence of occupational noise (1,7).  These studies found aging effects on hearing, but observed no sex difference in the hearing loss (1,7).  Animal studies have also found no sex difference (2).  It has been proposed that the difference between male and female thresholds as a function of age, is due to environmental factors.  The most important of which is noise exposure (4). 

Goycoolea (1) studied the natives on Easter Island and found equal hearing thresholds in males and females of similar ages.  Males, from the island, who lived and worked in modern society had a greater hearing loss.  The natives on the island, both male and females, had hearing acuity similar to females who lived in the United States. 

However, Spoor's Equations (10) and Robinson Sutton's (6) Data in ISO-1999 Annex A were derived from populations screened for noise induced hearing loss and there is still a marked difference between the sexes.  Rosenhall (9) also found a gender difference when noise exposure was compensated for.  

Thus,  the choosing of a method of compensating for presbycusis is a complex one and we offer two equations to calculate presbycusis, both Spoor's and Robinson Sutton's.  In addition, if you believe that the gender difference in male and female subjects is due to noise exposure, the Master Calculator also offers an option to project male patients with female presbycusis data. Use the "PROJECT MALE WITH FEMALE PRESBYCUSIS DATA" check box to accomplish this.  I would only consider doing this if all other risk factors such as hunting, use of lawn equipment, alcohol or tobacco are not present.  

2)  Handicap Determination:

Determination of a hearing impairment from an audiogram is performed by one of the predefined equations.  The percentage of handicap due to presbycusis is usually determined by ISO-1999 Annex A (Robinson Sutton's Data).  It is assumed that presbycusis and noise trauma are additive but one can argue in severe hearing losses that this is not true.  However, legally this is a mute point.  Since, either the two are additive and the percentage of presbycusis is predicted by Robinson Sutton's Equations or if they are not additive then some of the noise trauma prevented some presbycusis from taking place, or visa versa.  Thus, this percentage of noise trauma caused no harm and therefore would still not be compensatible.  Therefore the "Calculators" determine the percentage of the hearing handicap "expected from" presbycusis and NOT "caused from" presbycusis, the ISO-1999 compression factor or other modifications of the data are not used.

After the hearing handicap is adjusted for presbycusis, the remaining handicap may still be accounted for by a variety of factors including, but not limited to, other sources of acoustic trauma, smoking, diabetes, heart disease, etc.

3) Projection of Audiogram:

The projection calculators were programmed to help give guidance to the following question.  An employee enters employment with a preexisting hearing loss.  Now he leaves the job with a more severe hearing loss.  Is the increase in his hearing loss due to the employment or to the effects of aging and his environment? 

One must remember that these equations are looking at LEGAL CAUSATION, what is more likely than not to be true, in order to reach a legal settlement.  The discussions that follows are of legal arguments and  should not be used to determine MEDICAL CAUSATION.

It is assumed with all of the calculators that hearing loss due to noise and presbycusis are additive.  

Hearing Loss = ARL + NIL - (ARL * NIL)/120

The NIH is estimated by following equation:  NIH = 120*(HL-PHL)/(120-PHL)
  If the hearing loss from NIH is less than expected from presbycusis then the NIH is set to zero. 

ARL:  Age Related Loss,  NIL:  Noise Induced Loss,  HL: Hearing Loss,  PHL:  Expected Hearing Loss From Presbycusis

The calculators will apply the ISO 1999 compression factor to all projected thresholds and truncate all losses at 100 dB.  The threshold projection may underestimate the hearing loss in patients with other risk factors.  For example, in patients who hunt without ear protection, since ISO-1999 Annex A eliminated subjects exposure to gun fire in determining presbycusis hearing thresholds.

In some cases, the patient's hearing loss will be less than predicted by the presbycusis calculations (better than average for his/her age).  In this case the projected hearing loss will be that of the expected hearing loss from presbycusis for the patient's age for the projected audiogram.   One could argue that the expected patient's hearing should be better than this, however, we know of no studies to guide estimations of future thresholds in individuals with above average hearing.
 

References