VETS at the Animal Health Trust in Newmarket could not believe their ears. They had just removed a tumour from a pony鈥檚 lip, when they heard a strange high pitched humming noise coming from its right ear.
Tinnitus 鈥� the sound of ringing in the ears 鈥� is familiar to anyone who has heard an explosion or listened to loud music. The sensation usually wears off, although some people 鈥渉ear鈥� constant noises which disrupt their concentration and can cause chronic health problems. This is called 鈥渟ubjective tinnitus鈥�. In rare cases, other people can also hear the noises, a condition called 鈥渙bjective tinnitus鈥�.
The pony described in the current issue of the Veterinary Record seemed oblivious 鈥� to the sound. But people standing as much as a metre away could hear it, and though the humming varied in intensity, it stayed at a constant pitch of 7 kilohertz.
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The owners of the five-year-old Welsh pony had not mentioned the noise, although it had probably been there for some time. 鈥淭he stable maid who was cleaning the stable first reported it. We told her that she should muck out her ears,鈥� said Tim Phillips, one of the vets at the trust. But then the entire surgical team heard it 鈥� apart from one who suffers from hearing loss 鈥渞elated to an ongoing, intimate association with electric guitar amplifiers鈥�.
Joe Mayhew, the head of the team, suggests that the noise was caused by a reversal of the normal hearing pathway. Normally, sound is transmitted through the eardrum and the tiny bones in the inner ear, called ossicles, and stimulates nerve cells in the ear to send messages to the brain. In this case, the nerve cells seemed to fire spontaneously, causing the hair cells of the inner ear to vibrate and make the ossicles and the eardrum resonate 鈥� generating the sound outside the ear.
But Ellis Douek, an ear, nose and throat surgeon at Guy鈥檚 Hospital, London, thinks this an unlikely explanation. Spontaneous nerve activity can cause sounds, he says, but these can only be picked up by powerful microphones. He says audible sounds are 鈥渧ery rare indeed鈥� and are usually caused by spasms in the muscles of the inner ear or throat, or by resonance due to abnormalities in the ear鈥檚 blood supply.
Mayhew says it is impossible to tell whether animals experience the same sensations as tinnitus sufferers. But he thinks that the condition could explain why some horses repeatedly shake their heads for no obvious reason.