July/August 2016
5
feature
The sense of hearing is truly remarkable.
From birth,
the human cochlea can distinguish sounds ranging from a frequency
of 20 hertz (Hz) to 20,000 Hz—almost 10 octaves. Dogs and cats
can hear even higher frequencies, up to 40,000 Hz, and bats and
dolphins can detect up to 150,000 Hz. The elephant and the mole
can hear very low frequencies, below our range.
1
Human speech
typically falls between 500 and 2,000 Hz.
2
The audiogram measures the lowest threshold at which a sound
of a particular frequency is detected. Air and bone thresholds in
normal hearing are equal. Mild hearing loss is defined as thresh-
olds between 20 and 40 dB, moderate is 41 to 70 dB, severe 71 to
90 dB, and profound is thresholds greater than 90 dB.
3
With senso-
rineural hearing loss, both air and bone conduction thresholds are
abnormally elevated. (The “sensory” part of “sensorineural” refers to
the sensory cells in the cochlea, the hair cells.) In conductive hearing
loss, air conduction is affected more than bone conduction, leading
to the description of the audiogram as having an “air-bone gap.”
Hearing loss in newborns is not common, but when infants are born
with mild to severe hearing loss it can have a profound effect on
their development. It can be part of a syndrome with other serious
health problems or it can be seen in isolation. Loss of cochlear
function with sensorineural hearing loss is the main problem in
congenital hearing loss.
Normal hearing
The sensory cells of the cochlea are topped by cilia, which makes
them appear to have a head of hair. A mere 3,500 inner hair cells
(IHC) assisted and amplified by 12,000 outer hair cells (OHC)
are arranged along the coiled strip called the organ of Corti. The
cilia of the hair cells float in endolymph, a specialized fluid that is
high in potassium, low in sodium, and contains almost no calcium.
The organ of Corti is surrounded, protected, and supported by two
other fluid-filled tubes, the scala tympani and the scala vestibuli.
The fluid in the two scalae is very much like extravascular fluid.
4
A sound wave entering the pinna strikes the tympanic membrane
and triggers the hammer, anvil, and stirrup. The bony series within
the middle ear is designed to prevent the sound reflection that
1.
www.cochlea.eu/en
, accessed May 30, 2016.
2. Rauch SD, “Idiopathic Sudden Sensorineural Hearing Loss.”
NEJM
, 2008;
359: 833–840.
3. Kral A and O’Donoghue G, “Profound Deafness in Childhood.”
NEJM
, 2010;
363: 1438–1450.
4. w
ww.cochlea.eu/en
.
5. Papsin B and Gordon K, “Cochlear Implants for Children with Severe-to-Profound
Hearing Loss.”
NEJM
, 2007; 357: 2380–2387.
6. “Profound Deafness in Childhood.” For comparison, Down Syndrome occurs
in 1.5/1,000 live births, and congenital heart defects are seen in 1 percent of
births.
http://www.cdc.gov/ncbddd/birthdefects/types.html
.
7. Morton C and Nance W, “Newborn Hearing Screening—A Silent Revolution.”
NEJM
, 2006; 354:2151–2164.
8. “Profound Deafness in Childhood.”
9. “Newborn Hearing Screening.”
occurs when a wave transfers from air to liquid. Without the bony
ossicles, humans would lose 30 dB of hearing.
Sound travels almost five times as fast in water as it does in air.
Once it reaches the cochlea, it does not have far to go. The stirrup
vibrates against the oval window of the scala vestibuli, and the sound
wave passes along the fluid-filled scala, where it vibrates the basilar
membrane of the organ of Corti. (The videos at www.cochlea.eu/
en/cochlea/function are helpful in visualizing this.) High frequency
sounds are detected by IHCs at the base or outermost portion of
the cochlea, while the innermost aspect of the basilar membrane,
at the apex of the cochlea, responds to lower frequency sounds.
5
When the basilar membrane moves at a particular location, the cilia
of hair cells wiggle, opening stretch-sensitive potassium channels.
Potassium from the endolymph pours into the hair cell. The IHC
depolarizes and sends a specific tone nerve signal to the auditory
cortex. The OHC’s neural signals serve as an amplifier or can have
a dampening effect. A system of supporting cells recycles potas-
sium back into the endolymph, ready for reuse.
Etiology of congenital hearing loss
Permanent hearing loss is found in 1.2–1.7 cases out of every
1,000 live births. Between 20 and 30 percent of these cases have
profound hearing loss.
6
While mild to severe hearing loss can be
treated with hearing aids, hearing aids are not very effective for
profound hearing loss.
Between 50 and 60 percent of cases have a genetic cause,
7
and
300–400 different mutations have been identified. Most are auto-
somal recessive, but 15 percent are autosomal dominant. Less than
one percent are X-linked or mitochondrial in nature.
8
The GJB2 gene has been found with at least 100 mutations in deaf
children, but one mutation, 35delG, is by far the most common.
When the GJB2 gene is faulty, the channels recycling potassium
are abnormal. Instead of potassium moving from the hair cells
through the supporting cell network back into the endolymph for
reuse, it accumulates in the cells, which then die.
9
The 3,500 IHCs
are already differentiated at 10 weeks’ gestation, and they do not
divide further. Death of these cells means permanent hearing loss.
Environmental causes of cochlear damage are another common cate-
gory of hearing loss in infants and children. This category includes
infections, pharmacologic toxicity, prematurity, and neonatal asphyxia.
Cytomegalovirus (CMV) infection has replaced rubella as the most
prominent environmental cause of infant and child hearing loss in
developed countries. CMV underlies 21 percent of hearing loss
detected at birth. This infection leads to hearing loss that may fluc-
tuate. CMV can affect one or both ears, with deafness sometimes
appearing only after several years. About half of CMV infections
are undetected until hearing loss is noted.
