Friday, 21 January 2011

General guidance for hearing aid fitting #1: Sloping losses.

Here's my current thoughts on fitting modern hearing aids to forward sloping hearing losses. A forward sloping loss is the most common seen, mainly due to presbyacusis (degradation over time/exposure to life). The degree of the slope varies enormously, from flat-ish 6dB/Octave to a precipitous vertical loss - usually caused by a secondary trauma.

It's not a definitive guide as there are at least two other factors to consider, expected loudness (experience of the patient) and physical characteristics of the ear canal; a normal CIC developing 100dB max output into a 2cc ear canal will produce approx 134dB in a canal with a 0.25cc residual volume - if my maths is correct ( don't count on that though).

LF = Low Frequency = Bass
HF = High Frequency = Treble

Mild/Sloping (0 dB to about 40-50dB in the HF) losses: suitability - RIC devices open dome, open fit IROS vented aids, open fit modular (Fuse/Be).

Moderate/Sloping. (from 30dB LF to about 65dB HF) losses - RIC devices with slightly more closed dome. Most In The Ear aids will be possible, though their acceptability (especially with new users) will depend on the degree of venting which may be constrained by the size of the canal.  

Severe/Sloping. May start at moderate levels, but quickly decline in what's called a 'Ski-slope': traditionally difficult to fit, but there are generally three options:
1. Seal the ear up completely and aid the whole of the loss - attempting to pump lots of sound into the HF end to 'restore' the loss - works for some, but others get problems with the high sounds distorting the good part of their hearing. Moderate-High power BTE/RIC.
2. Abandon (or at least don't attempt to amplify) the highest sounds - concentrating on the idea of fixing what can be fixed easiest to improve the lower frequency hearing. Moderate/High Power ITE/BTE/RIC.
3. Use a device that includes frequency shifting or compression (Phonak Soundrecover) this re-introduces the 'lost' HF sounds at slightly lower frequencies. Moderate/High power BTE/RIC - Possibly ITE power.

Profound/Sloping. Difficult to deal with, bottom corner of the Audiogram. Fitting is usually all about power, though there have been some improvements with Frequency Compression techniques. Usually a high power BTE with a good mould.

Next instalment: Flat losses.

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