Perceptive Deafness Associated with Severe Neonatal Jaundice

Auditory impairment in infants at risk for bilirubin-induced neurologic dysfunction

Classical and subtypes of kernicterus associated with bilirubin toxicity can be differentiated in part with physiological auditory measures that include auditory-evoked potentials and measures of cochlear integrity. The combination of these auditory measures suggests that bilirubin exposure results in auditory system damage initially at the level of the brainstem, progressing to the level of the VIII cranial nerve and then to greater neural centers. There is no evidence of neural damage at the level of the cochlea. Auditory neural damage from bilirubin toxicity ranges from neural timing deficits, including neural firing delays and dyssynchrony, to neural response reduction and even elimination of auditory neural responses. This condition is comprehensively described as auditory neuropathy spectrum disorder. Independent measures of cochlear function and auditory neural function up to the level of the brainstem can effectively diagnose auditory neural damage resulting from bilirubin neurotoxicity. Intervention, including cochlear implants can be effective.

Delayed development of sensorineural hearing loss after neonatal hyperbilirubinemia: a case report with brain magnetic resonance imaging

Sensorineural hearing loss has long been known to be a clinical consequence of kernicterus. Brainstem auditory evoked potentials (BAEPs) that occur in hyperbilirubinemic infants, can be reversed in the neonatal period by exchange transfusion. The case was reported in an infant with neonatal hyperbilirubinemia from hemolysis due to glucose-6-phosphate dehydrogenase (G6PD) deficiency and napthalene exposure. BAEPs showed that the baby had normal hearing at 30 decibels at 13 days of age, after exchange transfusions, but had developed profound bilateral sensorineural hearing loss by 7 months of age. The brain magnetic resonance imaging (MRI) findings at 7 months are also presented.

Risk factors for sensorineural hearing loss in extremely premature infants

OBJECTIVE

To identify potentially preventable risk factors for sensorineural hearing loss (SNHL) in extremely premature infants.

METHODOLOGY

A case control study of survivors with gestational age (GA) < 28 weeks or birthweight (BW) < 1000 g using data collected prospectively in our Neonatal Intensive Care Unit database. Each subject with bilateral SNHL > 40 dB was matched according to GA, BW and sex with two controls who had neither sensorineural nor conductive hearing loss.

RESULTS

Infants with SNHL had increased mean (+/- s.d.) days ventilated (53 +/- 21 vs 37 +/- 23 days, P = 0.006) and in oxygen (107 +/- 44 vs 69 +/- 28 days, P = 0.02) compared with controls. The risk for SNHL was increased for infants who spent > 90 days in oxygen (OR 4.0 [95% CI 1.1-15.6]), had maximum FiO2 > 0.90 (5.6 [1.2-26.9]), minimum plasma Na < 125 mmol/L (5.6 [1.1-27.8] or maximum pH > 7.60 (5.6 [1.1-89.0]). Neither maximum serum bilirubin nor exposure to ototoxic drugs was associated with SNHL.

CONCLUSIONS

Avoidance of severe hyponatraemia and extreme alkalosis, as well as use of surfactant to minimize the severity of hyaline membrane disease, may result in a decreased incidence of SNHL in extremely premature infants.

Cochlear microphonics in the jaundiced Gunn rat

INTRODUCTION

Bilirubin toxicity causes brain damage and deafness. Brainstem auditory areas are damaged, but the effects of bilirubin toxicity on the peripheral auditory system are less well defined. As a first step in the study of cochlear physiology, we performed studies of scalp-derived cochlear microphonic (CM) responses in the jaundiced (jj) Gunn rat model of acute bilirubin toxicity.

MATERIALS AND METHODS

CMs were obtained from scalp needle electrodes in response to acoustically delayed 500-, 1,000-, 2,000-, 4,000-, 6,000-, and 8,000-Hz tonebursts delivered by overhead speakers, and brainstem auditory-evoked potentials (BAEPs) were obtained to clicks. At 18 days of age, one nonjaundiced (Nj) and two jj Gunn rats in each of four litters were anesthetized. CMs and BAEPs were performed before and 4 hours after either (1) sulfonamide was injected into jj rats to produce acute bilirubin toxicity, (2) jj controls were injected with saline, or (3) Nj controls were given sulfonamide. In a second experiment, 16-day-old jj-sulfa and Nj-saline littermates were studied with insert speakers at 60 and 75 dB hearing level (HL) at baseline and 6 hours later, and CM amplitude was analyzed with a fast fourier transformation.

RESULTS

No statistically significant differences were found by repeated measures analysis of variance in the CMs in either experiment between groups or after sulfonamide, despite BAEP changes of decreased amplitude of waves II and III and increased latency of I-II and I-III interwave intervals in jjs given sulfa.

CONCLUSION

Alterations of CM after acute bilirubin toxicity did not occur at a time when there was electrophysiologic evidence of brainstem dysfunction. Although more subtle effects might be detected with a larger sample studied at longer intervals after the insult, CM seems insensitive or less sensitive to acute bilirubin toxicity than brainstem auditory function. These results suggest that retrocochlear tests of central auditory function may be more sensitive to the effects of hyperbilirubinemia than tests of peripheral auditory function in humans.

Auditory nerve of the normal and jaundiced rat. I. Spontaneous discharge rate and cochlear nerve histology

Hyperbilirubinemia is a major problem in neonatal intensive care. Hearing impairment is one of its sequelae. Although lesions of the central auditory pathways are known to be associated with this disorder in both humans and homozygous Gunn rats, the presence of cochler pathology is still controversial. The purpose of this study was to examine the functional integrity of the peripheral auditory system in the Gunn rat. The Gunn rat is a mutant of the Wistar strain with congenital deficiency of the liver enzyme uridine diphosphoglucuronyl transferase which is essential for bilirubin conjugation. This deficiency is inherited as an autosomal recessive trait, with the homozygous animals (jj) showing evidence of bilirubin encephalopathy. The heterozygotes (Jj) have 50% enzyme deficiency and are not jaundiced. The Long-Evans rat served as a control. The approach was to study the discharge characteristics fo single auditory nerve fibers using standard procedures in a closed and calibrated sound system. Various response measurements which would reveal pathological processes in the cochlea were analyzed. In this study, spontaneous discharge rate distribution and interspike interval statistics derived from Gunn rat auditory nerve recordings were found to be within the normal range, and cochlear nerve histology showed no evidence of neuropathy.

Auditory brainstem responses (ABR) in neonates with hyperbilirubinemia

The technique of ABR testing was applied to 25 infants with neonatal hyperbilirubinemia at levels exceeding that for exchange transfusion, in an attempt to study potential influence of bilirubin toxicity on the auditory brainstem pathway. The test was performed at a mean conceptional age of 40.4 +/- 0.6 weeks just after discharge. Twenty normal term neonates of comparable birth weights and conceptional ages, who had no hyperbilirubinemia, were also examined for comparison. Fifty six percent (n = 14) of the hyperbilirubinemic neonates had some abnormality in the ABR pattern, the major one being a transient increase in the threshold of wave V (7, fail-30; 5, fail-45). Wave V, however, was consistently present at 30 dBnHL click stimulus in all the normal neonates (pass-30; normal threshold). Further, mean ABR latencies (wave III, V) and 1-V interpeak latency (brainstem conduction time) were significantly prolonged in jaundiced neonates as compared with controls (P less than 0.01). ABR changes were strongly correlated with the serum bilirubin levels (P less than 0.001). On follow up retesting at 3 months, however, all infants were found to have normal ABR latencies and threshold. Neonatal jaundice was associated with significant transient aberrations of ABR, suggestive of a transient toxic brainstem encephalopathy.

Brain stem auditory evoked potentials in jaundiced Gunn rats

Bilirubin encephalopathy causes potentially preventable brain damage and hearing loss. The site of auditory dysfunction is controversial, despite pathologic studies showing damage to brain stem auditory nuclei in humans and experimental animals. We studied the effects of bilirubin toxicity on the auditory system of homozygous jaundiced Gunn rats by use of brain stem auditory evoked potentials. Small but statistically significant abnormalities were found for wave latencies, interwave intervals, and amplitudes.

Pathology as it relates to ear surgery. VI. Cochlear implantation

The surgical anatomy and pathology of the cochlea have been reviewed in relation to cochlear implant surgery. Animal experimentation, as well as human temporal bone studies, have shown that the implant electrodes were well tolerated by the cochlea. The possible chemical and mechanical trauma induced by the electrodes can be avoided by better choice of shape, size, length and material of the implants. Long-term electrical stimulation did not seem to cause any deleterious effects on the neuronal population of the cochlea. In the present state of the art, cochlear implantation seems justified in well chosen cases.

ABR in patients with congenital/early acquired sensorineural hearing loss, abnormal stapedius reflex thresholds and speech retardation

The auditory brain stem response (ABR) was recorded in 14 young subjects with mild to moderately severe congenital/early acquired sensorineural hearing loss and abnormal stapedius reflex thresholds. The speech problems of these patients as evaluated by experienced examiners were considered to be more pronounced than could be explained from the hearing loss as measured by the pure tone audiometry. Psychological testing yielded intelligence scores within normal limits or above average for age in 11. They all gave abnormal ABR, indicating dysfunction of the auditory brain stem pathways or, in a few cases, of the cochlear part of the auditory nerve. Presumably, the electrophysiologically demonstrable pathological changes were caused by perinatal complications or early life infectious disease. ABR may prove valuable in the evaluation of children with speech retardation.

The Gunn rat: an experimental model for central deafness

The Gunn rat which develops neonatal hyperbilirubinemia has been used as an experimental model to evaluate the effect of bilirubin on the auditory system. Electrocochleographic and morphological studies (including light microscopy, surface preparations and transmission electron microscopy) did not reveal any cochlear abnormality in homozygous Gunn rats. Brainstem auditory evoked potentials showed morphological and amplitude changes suggesting a functional damage in the brainstem auditory pathways. These results suggest that hearing loss, when observed in kernicterus, is primarily due to neuronal damage at the level of brainstem auditory nuclei.

Pathology of congenital deafness. Present status and future priorities

The pathology of congenital hearing loss is as varied as the etiologies. Severe external ear anomalies, such as microtia, and middle ear defects have a significant incidence of accompanying inner ear aplasias, more often of the vestibular than of the cochlear end-organs. Classic inner ear aplasias are discussed, including Michel, Mondini, Alexander and Scheibe. A case illustrating close resemblance between a genetically induced Scheibe malformation and rubella inner ear deformities is present. A variety of ear disorders acquired during intrauterine or neonatal life, including cytomegalic inclusion disease, hypoxia, erythroblastosis fetalis and later-onset genetic disorders are also discussed. Illustrative case histories are presented as well as priorities for future investigations into the pathology of congenital and genetic hearing loss.

Effect of middle-ear dysfunction and disease on hearing and language in high-risks infants

The effect of middle-ear dysfunction and disease on hearing and language development at one year of age was evaluated for 143 high-risk infants. These infants were categorized as normal or abnormal based on otologic history, otoscopic examinations, and on tympanometric examinations. Language was significantly related to gestational age, being delayed by approximately the amount of prematurity. Language scores were therefore adjusted for gestational age. Speech-detection threshold was not related to gestational age, and was used as the measure of hearing. Hearing levels were negatively correlated with adjusted language quotients. Infants with abnormal otologic histories reported were not different from infants with normal histories in either hearing or language development. Infants with bilateral otoscopic abnormalities had significantly higher speech-detection thresholds, but did not differ in language development from those with bilaterally normal otoscopy. Infants who were abnormal bilaterally by tympanometric examination had significantly higher speech-detection thresholds as well as significantly delayed language development. A significant effect on both hearing and language was found among those infants bilaterally abnormal by tympanometry for whom evidence of middle-ear disease was not visualized by otoscopic examination. Implications of these findings are discussed.

Cochlear and brain stem responses in hearing loss following neonatal hyperbilirubinemia

The site of lesion in hearing loss following neonatal hyperbilirubinemia is unclear. Histopathological studies have implicated the brain stem auditory nuclei while other investigations have hinted at a lesion in the cochlea. In order to clarify this issue, attempts were made to record responses from the auditory pathway in 13 patients with hearing loss following neonatal hyperbilirubinemia. The neural response from the auditory nerve was absent in 11 of the 13 patients and present only in response to high intensity stimuli in 2 patients. However, the response of the cochlear hair cells (cochlear microphonic potential) was present in 9 of the 13 patients. In most other cases of sensorineural hearing loss, with no history of hyperbilirubinemia, the hair cell response was absent. This is functional evidence for auditory nerve damage in cases of hearing loss following neonatal hyperbilirubinemia while the hair cells are spared.

The combined correlated audiohistogram. Incorporation of the superior ventral cochlear nucleus

The state of the spheroid cells of the superior ventral cochlear nucleus (SVCN) must be accounted for along with that of stria vascularis, hair cells, and spiral ganglion neurons in the development of the combined correlated audiohistogram in cases of sensorineural hearing loss. For the stated evaluation, transverse sections through SVCN are marked off into ventrodorsally progressive zones corresponding to the frequency levels of the audiometric chart, and the loss of spheroid cells in each level is estimated on a percentage basis. The resultant "best fit" of pattern of cell injury with audiometric curve suggests that the anatomic frequency scale of SVCN parallels the logarithmic frequency scale of the standard audiometric chart.

Effect of hyperbilirubinemia on the inner ear in Gunn rats

The effect of hyperbilirubinemia on the inner ear was studied in fifteen jaundiced homozygous Gunn rats. Seven non-jaundiced heterozygous littermates served as controls. Histopathological examination of the temporal bone in these animals showed normal sensory and neuronal structures of the inner ear. This cannot be taken as conclusive evidence that a central, rather than a peripheral, lesion is the cause of hearing loss in hyperbilirubinemia.