In part 1 of this blog series, several general issues were touched on regarding the simplistic nature of the first and second PCAST recommendations. In summary, both were predicated on referring to a “basic” hearing aid as one that is of a mild to moderate degree and one that could equally (or almost equally in the eyes of the PCAST membership) be fit with over the counter Personal Sound Amplification Products or PSAPs). These “basic” hearing aids would be available in an over the counter fashion, bypassing the audiologist.
The error in that statement is that the word “basic” was meant to mean “conductive”.
In their recommendations, there were a series of statements, perhaps based on the principle of universality – that a “basic” hearing aid was similar, and should be treated similarly, to eyeglass prescriptions and devices. This eyeglass/hearing aid comparison is erroneous on a number of levels and part 2 of this blog series will focus more on the difference between a conductive and a sensory/neural hearing loss. This information is of course known to the field of audiology but is intended to act as a primer for those readers who do not possess a hearing health care background.
In part 1 of this blog series, I referred to myopia that requires corrective lenses as a “conductive” problem whereas the vast majority of those with hearing loss have a “sensory/neural” type of loss. This is not just a difference in jargon, but actually described two different types of hearing loss that have little in common with each other.
People with myopia that require corrective lenses have a conductive pathology. Light is conducted through the cornea and other structures of the eyeball, finally arriving at the retina at the back of the eye. It is the retina that is the sensory organ that assists in the transduction of light waves into their neural form that is carried to the visual centers of the brain. By far, the vast majority of visual deficits are conductive in the sense that the light waves have not properly been conducted to the retina. Reshaping of the eyeball and/or cornea will allow for the proper focus of the light waves on the retina. This can also be accomplished by a reshaping of the incident angle of the light waves refracting off of an external lens and in the vast majority of cases, is corrective.
Eye glasses can be corrective because as long as the refracted light impinges on the retina in the proper fashion, eyeglasses can replace the less than optimal conduction of light through the eye to the retina. The range of sensitivity, in most cases, is the same- acuity for low level light to tolerance for higher levels of light. Myopia, in the vast majority of situations, results in a dynamic range that can be corrected by external lenses in eye glasses, contact lenses, or surgery that alters the conductive pathway.
“Tuning” of the light waves is also something that is maintained with the vast majority of visual deficits. Does the color red still look like the color red? Color is defined by the frequency range of the light that is impinging on the retina. Except for rare cases of retinal pathology (or neural pathology on the optic nerve), this frequency of light tuning is maintained. Red will still continue to appear to be red, and all other colors will maintain their hue characterization.
In the case of conductive hearing loss, similarly the dynamic range is maintained and as long as the amplifier is appropriate for the hearing loss (as are the eye glasses for the visual deficit). And tuning of the sound waves is not altered. A conductive hearing loss is similar to saying that the volume on a radio has been reduced. Hearing aids simply turn up the volume to the required level.
The vast majority of visual deficits that require corrective lenses or surgery are conductive. Only a very small minority of hearing loss is conductive.
Sensory (or sensory/neural) loss:
Sensory (or sensory/neural) loss is extremely rare with those who wear corrective eye glasses. In contrast, it makes up a large majority of those how have non-medically treatable hearing loss.
With a sensory/neural pathology both the dynamic range and the “tuning” of the signal become altered or distorted. An appropriate analogy is that a sensory (or sensory/neural) hearing loss is one where the radio station is off-station- no amount of amplification can retune the station. This can be found with mild or moderate sensory/neural hearing loss as well as hearing loss of greater degree, although these uncorrectable issues become gradually more significant as hearing loss progresses.
This can also be found with significant retinal damage as well as damage to the optic nerve. In these cases, eye glasses, contact lenses, or surgery are either not appropriate or not possible. Eyeglasses will simply not correct the myopia. In these rare conditions, assistive visual devices may be required.
Unlike a conductive pathology, the dynamic range of the light in the case of vision, or sound in the case of hearing, is significantly reduced. In these sensory (or sensory/neural) conditions light can be not perceived, and then a slightly brighter light is perceived as being too bright. Similarly with hearing loss, even for those with a mild to moderate sensory (or sensory/neural) hearing loss, sound may not be audible, and then if slightly louder, would be considered to be too loud. That is, the dynamic range of a person with a sensory (or sensory/neural) pathology would be severely reduced. This cannot be easily corrected even with appropriate magnification or amplification. In many cases, training (auditory and perhaps visual) is required as well as the use of assistive visual or auditory devices.
A hall mark of a sensory (or sensory/neural) loss is altered “tuning”. By the time that a signal reaches the visual cortex, the visual input may have little resemblance with the actual structure being seen. Similarly, a C musical note may sound like a B or Bflat. Speech recognition degrades and while this does increase, on average, with greater hearing loss, it can be significantly degraded, especially in the presence of background noise.
And what is a mild hearing loss?
There really is no good definition of what constitutes a mild hearing loss. Is it the average at 500 Hz, 1000 Hz, and 2000 Hz being between 25 dB HL and 40 dB HL that Goodman and his colleagues defined in 1961? And is a high frequency mild hearing loss the same as a flat 40 dB HL hearing loss? The definition and descriptions of hearing loss are by their very nature simplistic. Should that define the route by which potential consumers can obtain hearing aids?
In the PCAST report I suspect that the membership confused “basic” with “conductive”. A mild to moderate hearing loss can be conductive; it can be sensory/neural; and it can be a mixed hearing loss with elements of both types of pathology. The use of the term “basic” is an error- I can see an argument for conductive, but this is a rather rare type of loss that is seen clinically in a typical work week. For many types of myopia, these conductive pathologies can be surgically corrected. Similarly, for many types of conductive hearing loss, these can also be surgically (or medically) corrected, but conductive hearing losses constitute a very small proportion.