Worlds of David Darling > Children's
Encyclopedia of Science > Health Revolution > 5. Restoring Sight and
THE HEALTH REVOLUTION:
Surgery and Medicine in the Twenty-first Century
a book in the Beyond 2000 series by David Darling
5. Restoring Sight and Hearing
Until recently, very little
could be done to help people who were blind or profoundly deaf. But now
medical science has developed a number of ways to restore partial hearing
or sight to those who have lost or have never been able to use these senses.
One of the simplest of the new techniques is to replace a defective lens
in the eye with a plastic substitute. For a variety of reasons, the natural
lens in a person's eye may become cloudy. This cloudiness, known as a cataract,
stops light from entering the eye and in severe cases results in almost
total blindness. To correct the problem, with the aid of a microscope a
surgeon opens up the clear part of the front of the eye – the cornea.
The surgeon then removes the patient's diseased lens, which lies behind
the cornea. Next, an artificial lens is put in the place of the natural
A number of other operations are now routinely carried out to repair damage
to the eyes and restore normal vision. At the back of the eye is a light-sensitive
layer called the retina. The cells of the retina transmit vision signals
to cells behind the retina. If the retina becomes torn or detached through
an accident or disease, a person may become partly or completely blind.
However, by using a narrow laser beam, a surgeon can effectively weld the
damaged areas back into place.
|An eye operation being carried out with microsurgical
techniques. A close-up of the patient's eye, as seen through the surgeon's
microscope, appears on the monitor.
Specialized lasers play an important part in the treatment of a serious
eye condition known as glaucoma. In glaucoma, the pressure of the liquid
inside the eyeball increases until it threatens to display a person's sight.
By shining a laser through the cornea and burning a tiny hole in it, a surgeon
can relieve the buildup of pressure without having to make a cut into the
Damage to the cornea – the tough, clear outer covering of the center
of the eye – can sometimes be repaired by a corneal transplant. This
involves replacing the damaged section with a piece of healthy cornea taken
from the eye of a donor. The operation calls for the finest sutures that
are used in surgery, with suture thread no thicker than a human hair and
needles less than a quarter of an inch long.
In 1995, surgeons in the United States began testing a bionic eye, which
contains a miniature black and white TV camera. Signals are carried from
the camera by a wire under the skin to a particular area of the skull. This
area lies over the part of the brain that processes visual signals. The
wire is attached to 256 tiny electrodes, each fitted to a small cluster
of brain cells. In this way, it is hoped, electronic impulses from the camera
will create visual images in the brain.
People who have limited hearing can have sounds made louder by a hearing
aid. But just because the sounds are louder doesn't necessarily mean they
are clearer. The problem is that ordinary hearing aids amplify all the sounds
they pick up by the same amount. They don't allow for the fact that deafness
affects different people in different ways.
Very often, people who are deaf can hear some frequencies of sound better
than others. The frequency of sound determines pitch. For example, a high-frequency
note has a high pitch. A low-frequency note has a low pitch. Deafness usually
affects sounds of higher pitch more than sounds of lower pitch, which makes
understanding speech, in particular, very difficult. To help get around
this problem, researchers have developed hearing aids that can amplify some
sounds more than others.
Important work in this field is being carried out at, among other places,
the University of New Mexico in Albuquerque. The work is based o 30 years
of investigation by scientists in the former Yugoslavia. These researchers
studied the relationship between the understanding of speech and the frequencies
of speech sounds. They also studied how different languages produce different
sound frequencies. Based on their findings, the Yugoslav researchers developed
a hearing aid that amplified sounds only at certain frequencies. Scientists
at the University of New Mexico took this idea further by introducing computer
technology so that the hearing aid could be reprogrammed to meet the needs
of different users or the characteristics of different languages. The scientists
called their invention the digital master hearing aid, or DMHA.
The first version of the DMHA, built in 1992, was about the size of a personal
stereo, weighed just over 2 pounds, and was connected to a headset by a
wire. The hearing aid worked with a special microchip that acted as a signal
processor to boost the level of incoming sounds at selected frequencies.
In the future the DMHA and devices like it will be developed further so
they are smaller and lighter and are linked to the wearer's headset by radio
signals instead of wires. A person requiring a hearing aid will visit a
specialist and have his or her hearing tested across a wide range of frequencies.
The information gathered from the test will be stored in a memory chip.
Then the memory chip will be inserted into the hearing aid to customize
it to the particular needs of the wearer. The memory chip will tell the
signal-processing chip how much to boost the volume at each frequency to
give the user a sense of hearing that is as near to normal as possible.
|Journey of a Sound Wave
The fleshy outside part of the ear, called the pinna, collects sounds
and helps us identify the sounds from which they come. The sound waves
then travel down the auditory canal to the eardrum, a membrane that
forms a barrier between the outer and the middle ear. When sound waves
strike it, the eardrum vibrates, like the skin of a drum. Its vibrations
are transmitted across the air-filled middle ear by the three smallest
bones in the ear – the hammer, anvil, and stirrup. The stirrup
is connected to a thin membrane, known as the oval window, which is
set into the snail-shaped cochlea of the inner ear. Vibrations of
the oval window are detected by over 20,000 tiny hairs inside the
cochlea. As these hairs vibrate, they stimulate nerve cells at their
base. Signals from these nerve cells go up the auditory nerve to the
brain, allowing us to hear. Joined to the cochlea are three semicircular
canals, filled with fluid and set at right angles to one another,
which help control our balance.
|Inside the human ear
The outer part of the ear is the simplest. When something goes wrong with
the complicated parts that are hidden from view – the middle ear and
the inner ear – then corrective action becomes difficult. In fact,
until quite recently, to replace parts of the middle ear or inner ear would
have been beyond the surgeon's power. Now, however, thousands of people
every year are having their hearing at least partially restored, thanks
to artificial implants.
As a result of a development in Japan, patients suffering from inflammation
of the eardrum or middle ear can be given an artificial middle ear. Normally,
sound passes down the channel of the outer ear to the eardrum. The eardrum
vibrates, and the sound vibration is transmitted by three tiny bones in
the middle ear to the cochlea.
The artificial middle ear developed in Japan bypasses the eardrum and the
first two bones of the middle ear. Part of the device hooks behind the outside
of the ear and converts sounds into magnetic signals that are picked up
by a second component implanted within the skull. This second component
transmits the signals along a wire to a vibrator placed in the middle-ear
cavity. The vibrator then moves the third bone, known as the stirrup, which
passes on this movement to the cochlea.
Another new type of implant, called a cochlear implant, bypasses the eardrum
and middle ear altogether and stimulates the cochlea directly. It can help
give people who are totally deaf some form of hearing, provided they have
intact nerve fibers connecting the cochlea to the brain.
The cochlear implant consists of a tiny disk-shaped radio receiver with
a long tail containing very fine wires. Dotted along the tail are a number
of electrodes – little pieces of metal out of which an electric current
can pass. A surgeon places the receiver beneath the skin, just behind the
ear, and pushes the tail of the implant around the snail-like curl of the
Outside the ear is a microphone that picks up sound. The signal is passed
to a signal-processing chip that converts the sound into electrical impulses.
These impulses are then sent to a tiny transmitter that sits behind the
ear directly over the receiver. When the receiver picks up the processed
sound signals, it passes them down to the electrodes inside the cochlea.
The earliest type of cochlear implant had only one electrode on the tail
and was known as a single-channel implant. It stimulated the nerve fibers
and sent just one electrical signal to the brain. The signal matched the
sound waves picked up by the microphone, but it was left to the brain to
try to sort out the frequency pattern, a job normally done by the cochlea.
More recent implants have a string of 22 or more electrodes along the tail,
resting at different places in the cochlea. These mutlichannel devices depend
on more sophisticated signal-processing chips that analyze the frequency
pattern of sounds picked up by the microphone. Signals representing different
sound frequencies are then sent to different parts of the cochlea, so the
implant works in a manner closer to that of the natural ear.
Developments to Come
As miniature computers become ever more powerful, it will be possible to
make replacement parts for ears and eyes that give a better and better quality
of hearing and vision. Cochlear implants, for example, may eventually be
able to analyze sounds in as much detail as a healthy human choc lea and
so restore a person's hearing to normal. Similar advances in technology
that allow visual scenes to be broken down into fine parts and turned into
signals that can be fed along the optic nerve directly to the brain will
allow blind people to see clearly.
Over the next few decades, it will become increasingly common for people
to have defective or worn-out natural body parts replaced by artificial
ones. This will allow accident victims, disabled people, and elderly people
to live fuller, healthier, and longer lives.
But advances in surgery and medical care will also create a number of problems.
The proportion of people of advanced age will steadily rise, which will
place a strain of facilities and care for the elderly. The ability of people
to pay for expensive implants may determine who is treated and who is not.
And, finally, there is the ethical question of how much of a human being
we are willing to replace by machinery. When it becomes possible, as it
almost certainly will, to replace eyes, ears, even parts of the brain by
electronic devices, will we want to place limits on what the surgeon should
and should not do? If such limits are not imposed, will the time eventually
come when, after repeated surgery, some people are more artificial than
natural? And should that matter? These are questions that will need to be
given serious consideration as we move through this century.