**1. Introduction**

156 Hearing Loss

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Hearing impairment is one of the most frequent sensory deficits in humans, affecting more than 250 million people in the world. Consequences of hearing loss include inability to interpret speech, often resulting in a reduced ability to communicate and delay in language acquisition. Untreated hearing loss may also cause economic and educational disadvantage, social isolation and cause stigmatization.

There are three basic types of hearing loss based on the part of auditory system with the damage: conductive hearing loss, sensorineural hearing loss, and mixed hearing loss.

Conductive hearing loss is the most common cause of hearing impairment both in children and in the adults and the incidence is significantly higher in children. In conductive hearing loss, the inner ear functions normally, but sound vibrations are blocked from passage through the ear canal, ear drum or across the tiny bones located in the middle ear. Patients with conductive hearing loss hear perceive bone-conducted sounds presented with a small vibrator to the skull with better thresholds than sounds presented through earphones. Conductive hearing loss is usually mild to moderate in degree and can be unilateral or bilateral and in most cases unilateral. Most type of conductive hearing loss is correctable by relatively minor medical or surgical treatments. More significant conductive hearing loss may be associated with skull and/or facial malformations which may require surgery for its correction.

Trauma generally is a major cause of morbidity and mortality in any society [Paul & Peter 2001]. Generally, trauma to the ear may result in fracture of the external auditory canal, tympanic membrane perforation, fracture to the ossicular chains, fracture of the temporal bone itself, damage to the cochlea or the facial nerve. Lesser bone trauma causes damage to the ossicular chains [Fradis & Podoshin 1975.] Hearing loss from trauma occurs in 22.5% of cases of temporal bone trauma and of these cases 16-30% have conductive hearing impairment [Fradis & Podoshin 1975, Ghoyareb B.Y et al 1987]. Hearing loss is defined as the averaged hearing loss at 1000, 2000 and 4000 Hz, measured by pure tone audiometry. This definition, although different from WHO/ISO definitions, is currently used in Nigeria because of its higher relevance to speech discrimination. It also corresponds to the standard proposed by the British Association of Otolaryngology and the British Society of Audiology (1983). Hearing is said to be impaired when the hearing level is above 25dB in the best ear. Hearing loss can have a profound impact on an individual's emotional, physical, and social well-being. People with hearing loss are more likely to report symptoms of depression,

Conductive Hearing Loss Due to Trauma 159

causes deflection of the stereocilia, and stimulation of the cochlear nerve. Injury to middle ear causes hearing loss. The is due to the problem of impedance: the resistance of transfer of energy between two media. This can be understood by analogy with transmission of light through water. When sun shines on water some of the light is reflected, while the remaining is transmitted through the water. This is manifested as a glare on top of the water and also visibility below the surface. Without the middle ear, most of the sound would bounce off of the surface of the oval window and less sound would be transmitted into the cochlea. This is because sound must pass from one medium (air) into another (liquid) in order to stimulate

The middle ear reduces the problem of impedance mismatch through several mechanisms. The effective ratio of these areas of the tympanic membrane to the oval window is about 14:1 (Roger and Maurice 1992). A second mechanism by which the middle ear overcomes impedance mismatch is through the ossicles. The malleus is 1.3 times longer than the incus. The ossicles also constitute a lever mechanism with a mechanical advantage of 1.3:1 (Roger and Maurice 1992, Lee KJ 1995). The product of these areas and lever ratios represent the transfer ratio of the whole mechanism. 14 x 1.3 = 18:1 (Roger and Maurice 1992). The vibration of cochlea fluids are processed and analyzed in such a way that data representing frequency, intensity and phase are transmitted as impulses along the auditory nerve via

Thus injury to either the tympanic membrane or the middle ear ossicles from trauma will result in loss of this physiological function causing impedance mismatch ultimately

The ear can be stimulated either with sound pressure waves or via vibrations applied directly to the skull. The latter is noted on audiograms as bone conduction and is used to distinguish the sensorineural component of hearing. There are three mechanisms in which

1. The distortional mechanisms is due to vibration directly distorting the skull. As the cochlea is part of the skull, it would be distorted as well. Because the round window yields more than the oval window, the scala vestubuli and scala tympani have different compliances. This results in the deflection of the basilar membrane and deflection of steriocillia with stimulation of the auditory nerve. However in fractured temporal bone (longtitudinal or transverse fracture) affecting either the oval window, cochlea or

2. The inertial-ossicular mechanism of conductive hearing relates to the ability of vibration energy directed to the skull to cause motion of the ossicles. Vibration to the skull will cause the ossicles to move. Ossicular movement imparts its energy to the oval window which stimulates the cochlea. However, this is dependent on the direction of vibrations in the skull. If the direction of vibration is parallel to the axis of the movement of the ossicles, inertial-ossicular mechanism of conductive hearing takes place. If there is is

vibratory energy placed directed onto the skull will cause stimulation of the cochlea:

semicircular canal, this smooth mechanism is interrupted (Stanley 2009)

auditory pathway to brain for interpretation (Gibson 1978).

resulting in impaired hearing (Roger 1992, Lee KJ1995).

the cochlea.

This can be through i. Distortional ii. Inertia-Ossicular

iii. Osseo-tympanic mechanism

dissatisfaction with life, reduced functional health and withdrawal from social activities. This chapter aims to profile hearing loss due to trauma and its aetiology.
