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.

Monday, 17 January 2011

Been a bit lazy around Christmas, but here goes 2011

Christmas cheer is well and truly gone: so gladly has the snow.

Business seems to be looking up with a variety of new tests and follow-up from December. 

Still encountering Fuse failures due to the aid splitting. Starkey have a product out called the AMP, which might fix a few. Though the sleeved design is not the smallest. On-line there has been a bit of confusion about audiograms, so I put together this basic guide.

Steve's quick no-nonsense guide to the Audiogram.

Firstly: Air conduction 
Left - Denoted with X and/or Blue
Right - with O and/or Red.

Triangles/Brackets/Right Angle corners - indicative of bone conduction levels.
A gap between these and the air conduction is indicative of conductive loss. 

Frequencies (measured in Hertz/Hz or cycles per second)

Lowest on the left, highest on the right. Each of the principal Octave values are recorded.
To make sense of this: Middle C on the piano is about 250Hz (256Hz), each level either side moves you up or down the Keyboard to the next C.

Intensity or loss. Measured in dB(hl) 0 dB is weighted average of young adult hearing, lots of kids will be better than this. 120dB is getting on for the level of a load rock concert.

Each 3dB increase is a DOUBLING of intensity - this scale is a negative log or geometric progression.

To give a few example: a Normal conversation occurs around 65dB, a whisper at about 45dB, a Shotgun 155dB. Ever wonder why Clint Eastwood is deaf? A .44 revolver 170dB.

Hopefully Clint's lawyers won't be getting in touch. 

That's all for now folks....