Hypothyroidism is the most common prevailing endocrine disorder among all endocrine maladies. It is an altered metabolic state, when the body produces insufficient amount of thyroid hormone. Hypothyroidism is not only a disorder of endocrine system, it also affects almost all the organ system of our body. It shows a wide range of disease severity from asymptomatic state to coma and made it an elusive clinical entity9 .
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Thyroid hormone in addition to governing the rate of metabolism of fats, carbohydrates and proteins, it also regulates the timing and pace of the CNS development. It is extremely important for the growth of cerebral , cerebellar cortex, axonal proliferation, branching of dendrites, synaptogenesis, and myelination . Thyroid hormone enhances the wakefulness, response to various stimuli like auditory sensation, learning and memory capacity1. By enhancing the gene expression it influences the synthesis of myelin. Myelin synthesis is an important factor determining the speed of impulse transmission along the complex neural pathway which mediates the evoked potential3 .
Sometimes hypothyroidism is referred to as “Silent Disease” because the early stage of disease it is asymptomatic. About 1.6 billion people are at risk of getting thyroid disorders worldwide4. According to NHANES III approximately 4 – 5% of population in the developed world is suffering from hypothyroidism and about 4 – 15% of people by subclinical hypothyroidism5 .
In a developing country like India, iodine deficiency is the most common cause of hypothyroidism. In India, hypothyroidism is classified under the group of Iodine Deficient Disorder (IDD) 6. Since 1983 India has been following the Universal Salt Iodization Programme. As a result of it, there has been a decline in the prevalence of IDD 7. In 2004, WHO assessment of global iodine status documented that India has optimal iodine nutrition and now India is undergoing transition from iodine deficiency to sufficiency phase8 . But a nationwide comprehensive epidemiological study done in the eight cities of India found that the prevalence of hypothyroidism was 10.95%. One third of them (3.47%) are not even aware of their disease. Subclinical Hypothyroidism was observed in 8.02% of people. Females are more prone to have hypothyroidism than males (15.86% to 5.02%). Unnikrishnan AG et al has also observed high prevalence of hypothyroidism among the Indian adult population6 .
The well-known symptoms and signs of hypothyroidism are dry skin, weight gain, cold intolerance, muscle cramps, hoarseness of voice and constipation10 . As thyroid hormone is essential for the nervous system maturation , central and peripheral nervous system dysfunctions are also important consequences of hypothyroidism. About 60 – 90% of the patients may have features of peripheral nervous system dysfunction like weakness, paraesthesia and postural imbalance . The CNS manifestations of hypothyroidism seen in 60 to 80% of patients which include delayed mentation, sensory deficits, depression11 . It also produces delay in the neuronal conduction velocity, alterations in hearing threshold and sensation of smell . All these neurological complications of hypothyroidism will resolve completely with thyroid hormone replacement especially when diagnosed at the early stage12.
The peripheral nervous system dysfunction has been studied in hypothyroid patients by a variety of techniques. But the studies in favor of quantification of CNS dysfunction were very sparse. The CNS involvement in hypothyroid patients can be identified with the help of existing advanced electrophysiological studies. Among the electrophysiological studies evoked potentials provide a more reliable and objective measure of the functional integrity of the related sensory pathway13 . Among the various evoked potentials tests, Brainstem Evoked Response Audiometry (BERA) has been emerged as an effective method of revealing the involvement of auditory pathway even in asymptomatic stage itself11 .
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Brainstem Evoked Auditory Response are produced in response to brief auditory stimulation14,15 . In this technique following a brief acoustic stimulus, a series of potentials are generated which corresponds to the sequential activation of peripheral, pontomedullary, pontine and midbrain portion of auditory pathway. So BERA helps to evaluate the integrity of the auditory pathway13 . Persons who are having abnormal brainstem response to auditory stimuli more prone to develop Sensorineural Hearing loss16 .
In 1948 Means states that Hearing loss is one of the troublesome symptom of hypothyroidism and it may be conductive, sensorineural or mixed hearing loss. About 25 to 30% of the hypothyroid patients having loss, but the exact incidence not yet known. So many researchers did study to find the type of hearing loss in hypothyroid patients. Howarth and Lloyd proposed that perceptive deafness is the type of hearing loss in hypothyroidism. (9) Rau et al, Bhatia et al, Von’t Hoff and Stuart , Parving et al and Isam et al were also accepted that sensorineural hearing loss was the predominant type of hearing loss in hypothyroidism people. Malik et al said that the site of lesion in auditory pathway remains speculative , may be at several levels viz in the middle ear, at cochlea and retrocochlear sites 17. The integrity of the auditory pathway is essential for the capture of the acoustic signal by the external ear to the coding of signals in the auditory cortex13 .
Hearing loss is an invisible abnormality which will lead to so many devastating consequences in interpersonal communication, psychosocial wellbeing, quality of life and economic independence. Hearing loss in infants and children due to congenital hypothyroidism results in serious impairment in language, communication skills, cognitive and emotional development. In adults hearing loss will lead to loneliness, social isolation, psychiatric disturbances, depression, occupational stress and relatively low earnings18. There are literatures pointing out that early treatment of hypothyroidism will reverse the hearing loss 19-22.
Electrophysiological testing for finding the functional integrity of auditory pathway is rarely performed in hypothyroid patients in neurology and otolaryngology practice. Now a days BAER is mainly used for screening the preterm infants and also prior to cochlear implantation. But it was proved that the delay in the auditory processing time may provide the information about the subclinical involvement of central as well as peripheral neuropathy in hypothyroid individuals 8. So they can be used to assess both normal and abnormal auditory function in the field of research. So that BERA can be used as a screening test to find the CNS involvement and the hearing loss in hypothyroid patients even in the earlier stage itself.
In this study an endeavor has been made to find the changes in the brainstem auditory evoked responses in newly diagnosed hypothyroid individuals.
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