a1 Lt.-Col. S. W. Ahmad, Military Hospital, Agra cantt–282001 (UP).
a2 Colonel G. V. Ramani, Sr. Adviser (Otology) Command Hospital (East Command) Calcutta.
70 Patients with dry central perforations have been studied to assess the hearing loss. The hearing loss is greater at the lower frequencies and increases with the size of the perforation. Malleolar perforations cause more hearing loss than the non-malleolar unless the perforation involves less than 10 per cent of the tympanic membrance surface area. Perforations of the postero-inferior quadrant cause more hearing loss than those in the antero-inferior quadrant.
The large effective surgace area of an intact and normally vibrating tympanic membrane plays a major role in the middle ear transformer mechanism. The loss of this surface area by perforation is bound to cause deafness (Mawson, 1962). The degree of hearing loss has been extensively studied in human cadavers (Békésy, 1939) and in cats (Bordley and Hardy, 1937; Payne and Githler, 1951) but not much published work is available on any detailed clinical study in human beings. Most authors have generally stated that ‘small perforations’ have little effect on hearing, and only low tones are affected (Wever and Lawrence, 1954). In two individuals, Bordley and Hardy (1937) found average losses of 12 dB over frequencies ranging from 256 to 4,096 Hz. 10–30 dB loss is quoted by Simpson et al. (1967), and 20–45 dB loss in the speech frequencies by Thorburn (1971). It has been a general view that the hearing loss increases with the size of the perforation, more so it if is located in the postero-inferior quadrant (Mawson, 1962; Simpson et al., 1967; Thorburn, 1971). In a retrospective study of audiograms in 103 cases, Anthony and Harrison (1972) found that the maximum average loss occurred at 250 Hz., the loss being less in ‘small’ perforations (less than 2 mm. diameter) than in large ones, less in perforations touching the manubrium than in those away from it, and also less in perforations of the antero-inferior quadrant than in those in the postero-inferior quadrant.