ERA in the ‘difficult-to-test child’

13


ERA in the ‘difficult-to-test child’



CLINICAL PROBLEM


Until the infant is 4 months of age, the quality of the auditory responses is little dependent on mental development, and only later can auditory behaviour and developmental landmarks differentiate the normal child and the child whose functioning and behaviour are abnormal (Northern & Downs 1984). A battery of audiological tests is applied according to the auditory and mental development of the child. Some children’s behaviour and functioning is such that it is difficult to judge their auditory responses to conventional audiometric behavioural tests. Precise audiometric threshold estimation may be desirable in such a child, and objective hearing assessment, using ERA tests, may greatly contribute. Delaying the detection of hearing loss and therapy may deprive the child of critical time in learning auditory skills, and may further compromise the development of his speech and behaviour.


Identification of children who would benefit from ERA is primarily the responsibility of the clinician. It is also important to select appropriate tests in order to obtain the information on the degree of hearing loss in low and high audiometric frequencies. This is important for the correct selection of a hearing aid.



Population


Catlin (1978) reported that 50% of cases of childhood deafness occur within the first year of life, and most of these cases are congenital. Martin (1982) reported the prevalence of hearing loss of 50 dB or more, in all children aged 8 years in the European Economic Community (EEC), as ranging from 0.74–1.48 per 1000.


Behavioural audiometry requires great skill and experience from the tester. In a small proportion of tests, the credibility of the results is questionable, usually because of inadequate co-operation of the child, and reliable pure-tone audiograms are rarely available before a mental age of 4 years.


A wide variety of clinical problems in children of various ages leads to referral for an objective test of hearing, in order to verify the degree of hearing loss and institute the best rehabilitation programme. It has been shown that the time interval between objective hearing estimation with ECochG or ABR and the first ‘reliable’ pure-tone audiogram may be several years, and that this was obtained at some stage in 31% of 841 tested children (Bellman et al 1984). Some important single features or combinations of salient features in children of various ages, exhibiting both normal intelligence and mental handicap, who may require objective hearing assessment, are listed below:



A more extreme group of children can be identified, whose functioning and behaviour are such that they are sometimes untestable, or about whom it is difficult to make a conclusive judgement using standard behavioural audiometric techniques. This group of children is known as the ‘difficult-to-test’ children, including mentally retarded, brain-damaged or centrally disordered, autistic-like, and deaf-blind children.


Neither mental retardation, nor central auditory disorder, nor autism result, in themselves, in hearing loss, but when behavioural auditory responses suggest hearing loss, or pure-tone audiograms are not credible, these conditions have to be verified. However, it is possible for such children to have peripheral hearing loss. For example, in mental institutions, the prevalence of hearing loss and ear disease ranges from 10 to 45% or higher (Northern & Downs 1984). Brain-damaged, autistic, or hyperactive children who have difficulty in paying attention for any length of time may be difficult to test with conventional audiometric methods; they are prime candidates for ERA.


Some children with neurological handicaps and visual disability have delay of language and of other developmental characteristics. They often fail behavioural audiometric tests and are classified as ‘the deaf-blind’ child; however, on testing them later, it may appear that some of them have normal hearing (Stein et al 1981). Various eye–ear syndromes, congenital neuromuscular disorders, and rubella can be associated with deafness.



CHOICE OF ERA TESTS


An appropriate behavioural audiometric test is chosen to match the mental development of the child. One has to possess considerable experience and expertise to assess difficult-to-test children. If the auditory responses correspond to the behavioural landmarks, then the hearing is considered normal.


Impedance audiometry in a child of 6 months of age or older is a useful additional test in assessing middle-ear function and acoustic reflexes. The absence of acoustic reflexes suggests hearing loss. However, one should be aware that normal responses may be present in a mild hearing loss with recruitment. Auditory behavioural testing and impedance audiometry may be difficult to attain in ‘the difficult-to-test child’. ERA should be used, if necessary, to clarify the hearing level and audiometric contour. However, one should remember that ERA is not a simple procedure, and that precise results depend on the expertise of the tester.


ERA requires minimum gross body movement. Testing with scalp-attached electrodes in natural sleep following feeding is suitable in infants under 4 months of age.


In older children, the investigation is carried out under sedation or general anaesthesia, and testing time becomes an important factor.


Several different types of sedation to induce sleep have been recommended, including, for example, oral and rectal administration of chloral derivatives; a phenobarbital suppository (5 mg/kg body weight) in infants older than 3 months; Vallergan forte syrup (4 mg/kg body weight) administered half an hour before the test; cocktails of chlorpromazine (5 mg) and promethazine (4 mg/5 ml solution administered 1 ml/kg body weight, repeating half of the dose if there is no effect after 20 minutes, and maximally administering another half of the dose if there is still no effect after another 20 minutes). In some difficult-to-test children, the effect of sedation is often inadequate and difficult to predict.


General anaesthesia guarantees very good recording conditions, and the tests are carried out after administering either an intramuscular injection of ketamine or inhalation anaesthesia with halothane (occasionally Ethrane) with nitrous oxide and oxygen. Such anaesthesia enables relatively invasive ECochG using a transtympanic needle electrode.


The clinician has to decide what the most important information is that he wants to obtain, and choose the procedure that avoids a long testing time.


ECochG is a very useful procedure when applied in ‘difficult-to-test’ children (Fig. 13.1). Assessment of hearing at the level of the cochlea is obtained from the stimulated ear only, and there is no contribution from the better ear at high-intensity stimulation level when there is a sound crossover effect. Click-evoked ECochG is the most sensitive test, especially for moderately severe to severe hearing loss, and the correlation with the pure-tone audiogram at 1000, 2000, and 4000 Hz is very good. Correlation between the ECochG threshold and the behavioural threshold in free field in difficult-to-test children was found to be good, although the differences between the two values increased with increase of hearing loss (Bergholtz et al 1977). ECochG is a comparatively quick test, and responses can be obtained in 128 to 256 samples. Its disadvantage is difficulty in assessing hearing at lower frequencies, and the responses correlate best with the hearing level for audiometric frequencies above 1000 Hz.


Related posts:

  1. ERA in non-organic hearing loss (NOHL)
  2. Other long latency responses
  3. ERA in hearing screening in neonates and infants
  4. Basic principles of instrumentation and signal-processing

Stay updated, free articles. Join our Telegram channel
Tags:
Apr 10, 2017 | Posted by in PATHOLOGY & LABORATORY MEDICINE | Comments Off on ERA in the ‘difficult-to-test child’

Full access? Get Clinical Tree
Get Clinical Tree app for offline access