Symposium on basic ear research. II. Strial atrophy in clinical and experimental deafness

The hair cells and the first-order neurons of the cochlear nerve are usually considered to be the primary targets of traumatic, ototoxic, and degenerative processes affecting the inner ear. The purpose of this paper is to show that several forms of sensori-neural deafness are associated with atrophy of the stria vascularis, and that strial atrophy often precedes and may indeed be an important cause of hair cell loss. Strial atrophy was found: 1. in presbycusis; 2. in hereditary deafness in Dalmatian dogs; 3. after noise exposure; and 4. after ototoxic drugs. The technique used was that of dissecting the cochlea, stained with Osd4, into surface preparations. Special attention was given to the spiral ligament and the-stria vascularis. In man, sensori-neural degeneration with aging was invariably associated with strial atrophy. There appeared to be a direct relation between severity of strial atrophy and hair cell loss. The findings in two old animals were similar to those in man. We found no cases with obvious strial atrophy and a normal hair cell population. Four litters of deaf Dalmatian dogs were studied. Two litters were from deaf parents and two were from a deaf bitch and a hearing male. Animals sacrificed at various ages displayed a series of changes involved in cochleosaccular degeneration. The following phases were observed: 1. strial atrophy; 2. a. sagging of Eeissner's membrane and onset of hair cell degeneration, b. collapse of Reissner's membrane, ductus reuniens, and the membranous wall of the saccule; 3. complete hair cell degeneration, involving first the outer hair cells, then the inner hair cells; and 4. nerve degeneration. The offspring of deaf parents showed bilateral degeneration, occurring mainly during the first two to three weeks postnatally. In the litter from a deaf female and a hearing male, several pups had complete cochleo-saccular degeneration on one side, with partial involvement on the opposite side. Four of these pups had strial atrophy in the middle and apical turns, and recent outer hair cell degeneration in the corresponding area. This finding suggests a close relation between strial atrophy and hair cell degeneration. The capillaries in the atrophic stria were narrow and had markedly thickened walls. Cats were given high doses of neomycin and ethacrynic acid. Guinea pigs were given gentamicin, also in high doses. All three drugs caused strial atrophy, which was most severe in the basal turn but was also pronounced in the apical turn in the neomycin and ethacrynic acid-treated animals. The neomycin- and gentamicin-treated animals showed severe to total hair cell degeneration, but ethacrynic acid-treated cats showed virtually no hair cell loss despite severe strial changes. With one exception the neomycin-treated animals had the most severe changes, with only an epithelium and network of empty, atrophic capillaries and strands remaining in the most affected areas. The stria of the ethacrynic acid-treated animals had a distinctive appearance, with swollen osmiophilic cells clumped around the capillaries and severe atrophy between the blood vessels. Twelve guinea pigs were exposed to a wide-band noise at 120 db SPL for eight, 30 and 110 hours. In one 110-hour animal and two 30-hour animals, ultrastructural changes were observed in the stria vascularis. The marginal cells displayed marked loss of cell processes and their nuclei were pyknotic. The basement membrane of the strial capillaries was thickened. Strial atrophy appears to be a common denominator in sensory degeneration. It is suggested that the atrophy alters the composition of the endolymph, and may thus cause hair cell degeneration; however, the temporal relationship between stria atrophy and hair cell loss is still to be ascertained.