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November 29, 2003

Hearing loss

From: The Australian, Australia - Nov 29, 2003

By Justine Ferrari

As if short people didn't have it hard enough, having to ask the tall stranger next to them in the supermarket to please pass the corn chips on the top shelf.

Now, scientists say short people are more likely to become deaf than their tall friends. "What?" I hear you say (and not just because I'm tall). Are their ears too small? Do the sound waves flow over the tops of their heads?

The study published in the British Medical Journal by a group of Swedish scientists (all those tall Viking genes) examined the hearing of almost 500 men aged 20 to 64 years old who were exposed to noise in their jobs.

The study also tested the hearing of 500, 18-year-old youths selected at random.

Among the men who suffered hearing loss, twice as many were short as men with normal hearing. Short men also had worse hearing for their age than expected, three times more often than taller men.

The study also found short people were more likely to inherit hearing loss, but their exposure to noise did not affect their risk of deafness.

The reason for deficient hearing in people with lesser stature is the thrifty phenotype hypothesis, or thrifty gene.

The hypothesis says that what happens while we are in our mothers' wombs can influence the diseases we develop as adults, and even how long we live. The theory, which is still being debated in scientific circles, holds that when a foetus endures adverse circumstances, mainly malnutrition, they reprogram their development.

Early growth is restricted and the programming can become permanent, changing the structure and physiology of the body.

Some research has linked the thrifty phenotype to the development of diabetes in adulthood as well as heart disease, obesity and now, the Swedish researchers suggest, hearing loss.

Everyone loses their hearing as they age as the hair cells in the inner cochlea become damaged.

Sound waves travel through to the inner ear to the actual organ of hearing, called the organ of Corti.

When the sound waves reach the inner ear, they cause the cochlear fluid to move, pushing against the basilar membrane in the organ of Corti, causing it to move.

The hair cells on the basilar membrane slide against a membrane above, called the tectorial membrane, creating nerve impulses in the fibres attached to the hairs.

These impulses are transmitted by the cochlear nerve to the temporal lobe in the brain, which interprets the signals as sounds.

The structures of the organ of Corti are very delicate and easily damaged by a range of factors, including sustained loud noise and plain overuse that comes with ageing. This type of hearing loss is known as sensorineural hearing loss.

The Swedish researchers suggest that short people are programmed for sensorineural hearing loss while they are still in the womb.

The link appears to be insulin-like growth factor I, which is crucial in the development of several organs including the size of the cochlea and auditory nerves and the development of hearing cells. It could be that short people end up with fewer hearing cells and nerves at birth and so suffer hearing loss quicker than their taller counterparts.

© The Australian