Auditory evoked potentials in rett syndrome: Peripheral and central auditory function

Rett Syndrome (RTT) is a disorder caused by mutations in the methyl-CpG-binding protein 2 gene (MECP2) located at Xq28 that predominantly affects females. MECP2 mutations account for as many as 96% of cases with the classical features of RTT. RTT is characterized by a progressive loss of cognitive and motor skills, communication disorder, and deceleration of head growth. RTT syndrome is characterized by a period of apparently normal prenatal, perinatal and psychomotor development for the first 6 to 18 months, followed by a period of loss of previously developed language skills and purposeful hand use. Seizures, intermittent hyperventilation, ataxia and stereotypical hand movements develop over time. The marked impairment in expressive and receptive language in patients with RTT has led to a number of studies investigating the peripheral and central auditory status in patients with RTT. These studies have included measures of the peripheral and central auditory status of patients with RTT. Procedures utilized have included tympanometry, otoacoustic emissions, the auditory brainstem response, middle latency response, long latency response, frequency following response, and long latency auditory evoked potentials. In the literature on RTT, there are conflicting reports as to the presence of abnormalities in the interpeak latency intervals of the ABR. The majority of studies have reported no abnormalities in the ABR interpeak latency intervals in RTT. It has also been reported that the interpeak latency intervals do not change over time in RTT, suggesting that RTT is not characterized by degenerative changes over time. However, several studies have reported prolongation in the I-V or the III-V interpeak latency intervals in RTT and an increased rate of ABR abnormalities has been found in patients with RTT syndrome with seizures requiring use of anticonvulsants. The use of sedation may also impact on the ABR in RTT. Evoked potential and other studies have shown that while the majority of patients with RTT have normal peripheral auditory function, hearing loss is present in some patients with RTT. Abnormal or absent middle latency responses and in the late vertex response have been reported and suggest the possibility of central auditory processing disorder in RTT. Atypical developmental patterns in auditory evoked potential responses mediated at brainstem levels (FFR) as well as at cortical levels using a passive oddball task have been reported. These studies will be systematically reviewed in the present chapter. The objective will be to consider the implications of auditory dysfunction as reflected in audiological and evoked potential studies, for the overall speech and language disorder characteristic of individuals with RTT.