March 2018 Hearing Health Care of Louisiana
You’re probably familiar with the many telltale, well-known signs of hearing loss — asking people to repeat themselves frequently, turning up the TV to uncomfortable levels for others in the room, or leaning into a conversation on one side to use your “good ear.” But what if speech is clear to you and you never turn up the TV — but you can’t hear whether the car you’re standing next to is running? This is an actual type of hearing loss, called reverse-slope hearing loss (RSHL), and people with this type often don’t realise they have a hearing impairment.
The most common type of hearing loss — the kind most people think of when they think of hearing loss — is characterised by loss of sounds at higher frequencies and is sometimes called high-frequency hearing loss. These frequencies correspond to what we think of as high notes or high-pitched voices. As such, when someone first notices this type of hearing loss, it’s usually because they’re having trouble hearing women’s voices or those of the children in their life, and having difficulty hearing conversation in a restaurant.
Because this particular kind of hearing loss doesn’t affect lower frequencies but does affect mid-level and high frequencies, it has a distinct appearance on an audiogram like a ski slope.
RSHL is so named because its shape on an audiogram is the reverse of ski-slope hearing loss. The low frequencies are affected far more than the higher ones. RSHL is rare: It affects only 3,000 people in the U.S. and Canada. Put differently, for every 12,000 cases of hearing loss, only one person has RSHL. Like ski-slope hearing loss, there are different degrees of RSHL.
Many people don’t suspect they have RSHL unless someone in their family already has it, which underscores one of the main sources of RSHL: genetics. Wolfram syndrome, Mondini dysplasia, and inheritance through a dominant gene have all been identified as sources of RSHL. Certain diseases have been implicated as well, mainly those affecting the hair cells, which are responsible for sending sound information from the inner ear to the brain. Examples include sudden hearing loss, Ménière’s disease, and viral infection. The third most common source of RSHL is anything that causes a change in the pressure of the endolymph, a fluid in the inner ear. This includes things such as spinal or general anesthesia, intracranial hypertension, and a perilymphatic fistula.
Hearing care is focused chiefly on ski-slope hearing loss, so RSHL can be difficult to recognise, diagnose, and treat. Because it is often hereditary or genetic — that is, because they were born with it — many who have RSHL don’t realise that the way they hear is different, so they may never seek out a hearing appointment.
- Difficulty understanding speech on the phone. The aspects of speech that give it clarity (the consonants) are in the higher frequencies, the treble side of sound, but the aspects of speech that give it volume (the vowels) are in the lower frequencies, the bass side. Because RSHL involves the lower frequencies, speech loses its volume but retains its clarity. Face-to-face conversation, therefore, is not usually a problem. But the phone mainly delivers the low and middle frequencies, so it can pose a problem for RSHL.
- Ease understanding women and children but not men. Again, because RSHL affects the lower frequencies, those with RSHL more clearly understand higher-frequency speech — that of women and children — than lower-frequency speech, such as that of men.
- Inability to hear low-frequency environmental sounds. Thunder and a refrigerator humming are examples of low-frequency environmental sounds. Because the click of a refrigerator is a high-frequency sound, someone with RSHL might hear their fridge click, but they wouldn’t know if the hum was the fridge turning on or off, even if they were standing right next to it.
Because of the prevalence of ski-slope or other high-frequency hearing loss, diagnostic tools focus on that type. Therefore, many with RSHL may “pass” a hearing screening or are treated as though they have other issues. Naturally, this leads to frustration for all involved. Key to diagnosis is a well-educated patient. Because this condition is rare, many in the hearing care field simply haven’t encountered it. RSHL has a distinct set of characteristics that an audiologist will look for but is not limited to:
- Unusually good speech
- Sensitivity to high-frequency environmental sounds
- Poor speech perception in the absence of visual cues
- High speech-detection thresholds
- Pure-tone hearing losses
- Inability to adjust to standard ski-slope hearing technology settings
A simple test any hearing care provider can use as an initial screening for RSHL is the Ling sound test performed while standing behind the patient. RSHL is most likely present if the “s” and “sh” sounds are heard at a much softer sound level than the other sounds.
Those with RSHL tend to have high expectations of hearing aids, which can lead to frustration. An audiologist who hopes to successfully fit an aid for RSHL has to build the settings from the ground up, for several reasons.
- Manufacturer-recommended hearing aid settings are meant for high-frequency hearing loss. As previously mentioned, only 3,000 people in the U.S. and Canada have RSHL; many millions have high-frequency hearing loss. It makes sense that the industry would weigh toward the type of hearing loss with the highest incidence but still allow audiologists to customise individual aids for rare types of hearing loss.
- Hearing aids may be programmed based on computer settings that are based on the audiogram. These computer settings assume the most typical situation: ski-slope hearing loss. These settings rarely work for RSHL.
- Hearing aids are built with the expectation of a high-frequency hearing loss. Often high-frequency losses need amplification in the high frequencies. The shape of the aid complements the shape of the typical ear canal, and this combination dependably treats high-frequency loss very well. But RSHLs require different amounts of amplification across a different range of frequencies.
- People with RSHL have already successfully adapted to their speech needs. Having been born with this condition, many with RSHL develop the ability to navigate speech easily.
Treating RSHL means parking industry standards and theoretical fitting curves. It requires taking time to really listen to the patient, and then build the settings channel by channel, frequency by frequency, to what they find comfortable, audible, and helpful.
But there are certain starting points that may help in the treating of an RSHL. A study by Kuk et al. determined that
- A digital, multichannel, nonlinear hearing aid is optimal
- Wide dynamic range compression, a low compression threshold, and high-level compression might more effectively preserve hearing and comfort
- Amplification in the lower frequencies is preferred, but gain may vary depending on input levels
- A broad bandwidth with individualised amplification customisation is desirable
- The paired comparison technique may help customise individual settings
You may well be thinking, “Those symptoms don’t sound so bad — why bother putting myself through all the frustration of getting diagnosed and fitted?” The key reason is safety. Much of what you lose with RSHL is environmental sound. If you can’t hear a car coming, you can’t avoid it. If someone some distance from you is trying to warn you away from something, you might not hear it, because volume is a product of the lower frequencies. Another reason is enjoyment. There are many aspects and nuances in music that you might be missing out on if you have RSHL, because you’re missing the low-frequency sounds — for example, much of what is below middle C.