Hearing Dysfunction Associated with Congenital Sporadic Hypothyroidism

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Abstract
A study of the effect of congenital sporadic hypothyroidism (CSH) on hearing dysfunction was undertaken. Fifteen female and six male CSH patients were evaluated audiometrically. All of these patients had a protein bound iodine quotient (PBI) of less than 4 mg% at the time of diagnosis and were currently under treatment. The patients ranged in age from four and one-half years to 40 years old; eight had sensorineural hearing loss; three had mixed hearing loss; only two had conductive loss and eight had essentially normal hearing. Special audiometric studies were performed including SISI, Rosenberg threshold tone decay, Békésy tracings, and evoked response audiometry (ERA). The SISI and tone decay test results were equivocal. However, statistically abnormal Békésy tracings, with unusually wide excursions were noted. This may be explained by a prolonged nerve conduction time. Evoked response audiometry, which is objective and is independent of cortical neuromuscular function, demonstrated that the unusually wide Békésy excursions were not the result of sluggish motor responses alone. A statistically significant prolonged latency period measured by ERA (without sedation) was found in four patients who were under compensatory hormonal treatment. A parallel preliminary animal experiment was performed in an attempt to confirm these clinical ERA findings. Guinea pigs were induced to hypothyroidism by propylthiouracil. After two months of continuous treatment the ERA was measured. All six guinea pigs had prolonged nerve conduction times at two frequencies. The latency periods were generally 2.5 to 3 times greater than control guinea pigs. A case report is presented of a patient who developed increased hearing loss when replacement therapy was stopped. The hearing levels improved when a euthyroid state was medically reestablished. The results of ERA and Békésy tracings suggest that abnormally prolonged nerve conduction time can play an important role in the hearing dysfunction of CSH patients.