Complete or partial absence of hearing. Genuine deafness is a serious handicap which is not well understood, probably because it does not show superficially. Hearing has a number of very important functions for the human being. It can give warning of danger, and a deaf person is thus very exposed; he is also cut off from one of the most important means of contact with his fellow men, conversation. Deaf people are often very lonely – it is hard for normal people to imagine what it is like to live one’s life in complete silence. Hardness of hearing is of two kinds: conductive deafness and perceptual deafness. Conductive deafness is caused by impaired conduction of sound from the outside world to the cochlea. Thus the cause may be in the auditory canal (blockage with ear-wax, ruptured eardrum) or in the middle ear (otitis media, otosclerosis, low pressure caused by closure of the Eustachian tube or a cholesteatoma). These conditions can often be treated by medication or surgery to restore conduction of sound. Perceptual deafness is caused by damage to the cochlea, the auditory nerve or parts of the brain. The most common form is deafness in old age, and another important cause is noise damage. Meniere’s disease also causes a kind of perceptual deafness, and hearing can be further impaired by inflammation of the inner ear, the use of certain medicines, tumour of the pons, haemorrhage in the auditory organs, or meningitis. If the condition exists at birth it can cause deaf-mutism. Sudden deafness can be caused by labyrinthitis, the stricture of a blood vessel or haemorrhage in the inner ear, or fracture of the base of the skull. The condition is rare. Hearing tests can establish severity of hearing loss and the source of the problem. A very general method is the whisper test, in which the tester whispers from a set distance. More precise is audiometry, in which sounds of various frequencies are used to establish the threshold of hearing, the degree of loudness at which notes can still be heard. It is possible to establish whether the patient is suffering from conductive or perceptual deafness by the difference between air conduction (through headphones) and bone conduction (with a microphone behind the ear on the bone).