Suzanne de Bruijn

24 Chapter 1.1 pumps, ion channels and gap junctions located in the hair cells, the supporting cells, the lateral wall of the scala media and the stria vascularis are responsible for recycling of the potassium ions back to the endolymph. 31 The inner ear: Vestibular system The vestibular part of the inner ear, the vestibular labyrinth, includes three semicircular canals and two otolith organs (utricle and saccule) ( Figure 3A ). The semicircular canals allow detection of rotational movements whereas the otolith organs can sense linear accelerations. Similar to the cochlea, vestibular structures are filled with endolymph or perilymph fluids and contain supporting cells and vestibular hair cells with stereocilia at their apical surface. The hair cells are present in the otolith organs and in the ampullae, that are located at the base of the semicircular canals. The two otolith organs are responsible for detection of head displacement and linear accelerations. A gelatinous layer (the otolithic membrane) that contains calcium carbonate crystals (otoconia), the so-called ear stones, is overlying the hair cells. Gravity can cause the otolithic membrane to shift towards the sensory epithelium, this shearing motion leads to displacement of the hair bundles. The semicircular canals are filled with endolymph fluid. The stereocilia of the hair cells that reside in the ampullae of these canals are deflected due to fluid movements caused by head rotations. 32,33 As in the cochlea, stereocilia deflection leads to hair cell depolarization that induces neurotransmitter release to the vestibular nerve. 32 HEARING LOSS According to the World Health Organization (WHO), 460 million people suffer from disabling hearing loss (HL), among which 34 million children. 34 Based on these numbers, it is the most frequent sensory disorder worldwide, which has enormous negative consequences for society (economical challenges) in addition to those on the personal level (educational and social disadvantages). In most developed countries, newborn hearing screening programs have been implemented that allow the earliest possible identification of HL and intervention. 35,36 Disabling HL is defined as a loss that is greater than 40 dB in the better hearing ear in adults, or greater than 30 dB in children. Several different causes can be underlying that include both acquired and congenital causes. Early detection and treatment or rehabilitation of HL is a key element in effective disease management. It is of benefit for the patients as it is essential to prevent and adverse the impact of HL. 3 Nevertheless, numbers suggest that HL is still significantly undertreated. 37 Although no cure is currently available for sensorineural HL, hearing aids and cochlear implants can mitigate HL. Several studies have shown that the use of cochlear implants consistently leads to improvement of quality of life (reviewed in (37)).