Biology Reference
In-Depth Information
1.1 Defining and Quantifying the Hearing
Loss Phenotype
The findings that result from a clinical examination are an individual's phe-
notype. They may include biochemical, physiological, and morphological
characteristics. This phenotype can result from genetic factors, environ-
mental factors, or a combination of both. The genetic factors are the indi-
vidual's genotype. Hearing loss may be just one of the features of the
phenotype, in which case the disorder is known as a syndrome; if no other
clinical anomalies are apparent, the hearing impairment is called nonsyn-
dromic. Gorlin et al. (1995) describe 30 syndromes in which hearing loss
is associated with abnormalities of the external ear, 40 with the eye, 87
with the musculoskeletal system, 23 with the kidney, 56 with the skin, 63
with the central nervous system, 51 with endocrine and metabolic condi-
tions, 12 with chromosomal anomalies, 8 with oral and dental problems, and
35 with miscellaneous disorders, as well as 22 forms of nonsyndromic
hearing impairment. The phenotypes and genotypes of many of these syn-
dromes, as well as those of nonsyndromic hearing loss, are described in
Chapters 6 and 7. The reader is able to keep updated on new hearing
loss genes by accessing the Hereditary Hearing Loss Homepage (URL:
http://dnalab-www.uia.ac.be/dnalab/hhh/).
The hearing loss phenotype has been most commonly quantified by
obtaining a pure tone air conduction audiogram. The work that led to the
development of the equipment necessary to make these measurements was
done over 70 years ago. Georg von Békésy had just reported his theory on
the traveling wave in the cochlea and its role in frequency analysis in 1929
(see Békésy 1960), when Crowe et al. (1934) published their paper showing
that high-frequency hearing loss was associated with loss of hair cells at the
base of the cochlea. The convergence of these independent lines of inves-
tigation led to considerable excitement in the field. Up to that time, hearing
loss had been estimated with mechanical devices such as a monochord (a
bow-activated, carefully calibrated string), whistle (Bunch 1943), and, of
course, a tuning fork. What was unmistakable, however, was that Crowe
et al. (1934) had used what was then a revolutionary instrument to quan-
tify hearing loss. Their tool was the Western Electric 1-A audiometer, a
battery driven device with an oscillator that produced pure tones up to
16,384 Hertz (Hz), and an interrupter, attenuator, and black bakelite ear-
phones, whose output verification was difficult to quantify at the time.
However, the voltage to the terminals of the phones was quite constant, and
hearing losses were expressed both in decibels (dB) of attenuation and dB
relative to what was thought to be normal sensitivity at octave intervals
from 64 to 16,384 Hz. Thus, Crowe et al. (1934) were able to quantify
hearing loss in dB, although at the time they were less interested in clinical
issues than in harvesting bones with quantifiable hearing loss. It was some
time before norms were developed, but this early 1930s electronic audiome-
