otolaryngology
PHYSIOLOGY OF THE EAR: HEARING & BALANCE
Dr Haider Alsarhan
The Hearing System consists of:
1. Peripheral Auditory System:
. External Ear
. Middle Ear
. Inner Ear
. The Cochlear division of the 8
th
Cr. Nerve
2. Central Auditory Pathways:
FUNCTION OF THE MIDDLE EAR:
1. Pressure equalization by Eustachian tube
2. Impedance matching by :
A. Lever mechanism
B. Area difference of TM & oval window
THE FUNCTION OF THE INNER EAR
The inner ear transform the physical properties into electrical neural
impulses. Sound waves pass from the environment through the external
and middle ears to cause vibrations of the cochlear perilymph. These
vibrations produce travelling waves in the basilar membrane, these
travelling waves reach maximum at specific points along the basilar
membrane. The high frequencies are represented at the basal turn and
low frequencies at the apical portion.
THE VESTIBULAR SYSTEM: consists of
(A) Peripheral Vestibular System:
1. Three Semicircular Canals: lateral, anterior & posterior
Receptors: Crista
2. Utricle & Saccule
Receptors: Macula
3. Vestibular division of 8
th
Cr Nerve
(B) Central Vestibular Nuclei: Superior, Medial, Lateral & Inferior
PROJECTIONS OF VESTIBULAR NUCLEI: to
1. Cerebral cortex (Voluntory sensation)
2. Cr. Nerves 3
rd
, 4
th
& 6
th
(Vestibulo-Ocular Reflex)- eye movements
3. Spinal Cord (Vestibulo-Spinal Reflex)- maintenance of body tone
4. Cerebellum- coordination of body movements
5. Autonomic nervous system through 10
th
Cr. nerve (pallor, sweating,
nausia & vomiting).
- Reticular information & contralateral vestibular nuclei.
THE BALANCE MECHANISM:
The Balance of the body is maintained by co-ordination of information
from three sensory systems:
1. The Vestibular system
2. The Eyes or Visual system
3. The Proprioceptive system: sensation from muscles, joints,
tendons and ligaments.
THE SEMICIRCULAR CANALS (SCCs): are stimulated by angular
acceleration around an axis.
Lateral SCC: Ampulo-petal (utriculo-petal) movement of endolymph
causes increase in discharge rates in vestibular nerve while in
Anterior & Posterior SCCs, there is decrease in discharge
THE UTRICLES & SACCULES: are concerned with identification of head
position in relation to gravitational field and linear acceleration &
deceleration.
EXAMINATION OF THE EAR, HEARING AND BALANCE
EQUIPMENTS:
- Otoscope
- Seigle Speculum
- Tuning forks
- Headlight or Head Mirror and Stand Lamp
- Angled Tongue Depressor: for exam
of post-nasal space
EXAMINATION OF THE EAR:
Introduce yourself and position the patient
Ask patient and start by exam the better ear,
Ask patient if the ear is sore to touch,
Examine the pinna,
Examine the External Auditory Canal (EAC),
Examine the TM,
Pneumatic examination of TM,
Fistula test,
Free field voice tests,
Tuning fork tests,
Facial nerve examination,
Postnasal space exam
EXAMINATION OF HEARING:
Start whispered voice at 60 cm from the ear
Tuning fork tests: Preferably 512 Hz
-Rinne`s test
-Weber`s test
-Absolute Bone Conduction (ABC) test
Audiometry: the measurement of hearing threshold by Audiometer &
recorded on graph called "pure tone audiogram
(A) Subjective Audiometric Tests: such as
- Pure tone audiometry.
- Speech Audiometry.
- In children: Behavioural hearing tests,
visual response audiometry, and play audiometry
(B) Objective Audiometric Tests: such as
- Impedance audiometry: Tympanometry & Acoustic Reflex (AR)
- Oto-acoustic emissions (OAEs)
- Brainstem Evoked Response Audiometry (BERA).
EXAMINATION OF VESTIBULAR FUNCTION:
Romberg test
-
Unterberger test
-
Gait test
-
Fistula test or sign
-
Examination of Nystagmus
-
Positional Test (Hallpike manoeuvr)
-
Caloric Test (Bithermal CT- COWS)
-
Cold water 30
Hot water 44
Newer advances in vestibular assessment
Video head impulse test
Video nistagmography
VEMP (Vestibular Evoked Myogenic Potentials )