CONDUCTIVE HEARING LOSS

1. Obstruction of the external auditory canal

2. Tympanic membrane problems

3. Middle Ear problems

4. Congenital abnormalities

1. Obstruction

-Normal external auditory canal:

-average diameter of 6 mm & average length of 25 mm;

-resonance frequency of approximately 3500 Hz,

-its reverberatory contribution to sound transmission is approximately 10 dB near the resonance frequency. -Complete obstruction Þ CHL as much as 30 dB.

-Due to:

-obstructing process causes immobility of the tympanic membrane,

-direct occlusion of the canal.

-Causes:

-Infect: Acute and chronic otitis externa (bact,viral,fungal) & media Þ inflammatory tissue complete occlusion.

Furunculosis, Chondritis, Perichondritis, Relapsing polychondritis, Cellulitis

-Trauma: Laceration +/- clot, Burns, Cauliflower ear, Avulsion, Penetrating wounds, cleaning

-Inflam: eg Wegeners

-FB’s & cerumen impaction.

-NP’s (rarely)

-Benign:

-Keratosis obturans and external auditory meatus cholesteatoma (EAMC).

-desquamated keratin debris accumulates in the external auditory canal.

-Exostoses are the most common solid tumors of the external auditory canal.

-occur most commonly in men with a history of repeated swimming in cold water

-are multiple & bilat, appear as smooth, rounded nodules of bone covered by normal skin.

-Osteomas are less common pedunculated bony lesions, usually single & commonly occur along the external auditory canal suture lines.

-Adenoma and ceruminoma.

-Malignant: 1’ry(SCC, BCC) or 2’ry(eg melanoma), Sebaceous cell carcinoma

-Degen: Osteopetrosis, Paget's disease, Ostogenesis imperfecta

2. TM problems :

a. Perforation

Mechns Þ CHL:

-loss of airborne sound-receiving surface area

-sound distortion at its torn edge.

-Baffle effect

-Rebound effect

-Size, location, and nature of the perforation will affect the degree of hearing loss.

-HL associated with perforations, however, tends to be affected by low-frequency sound.

b. Others

3. Middle ear problems:

i. Effusion: serous, pus, blood, barotrauma,

ii. NP: Benign(eg cholesteatoma,adenoma), or Malig (Adenoid cystic adenocarcinoma)

iii. Ossic chain problems:

a. Ossicular fixation

-CHL is due to greater resistance to sound energy forces in both its compliance-dominated low frequencies and in its inertia-dominated high frequencies.

-Causes:

-otosclerosis,

-hyalinized connective tissue with tympanosclerosis,

-scar tissue after surgery or infection,

-passively restricted by pressure on parts of the ossicular chain from middle ear structures.

-Cholesteatomas, glomus tympanicum, occ glomus jug, hi jugular bulb

by direct pressure on ossic chain.......(Esp stapes mobility)

b. Ossicular discontinuity

-infection or trauma Þ great risk to devascularization of incudostapedial joint b/c it is suspended in the middle ear and relies on the passage of blood vessels along the incus long process and suprastructure of the stapes.

- Longitudinal fracture(more common), Transverse fracture

-If incudostapedial joint is separated completely:

-maximal CHL will occur when the tympanic membrane is intact.

-maximal conductive hearing loss will not be made manifest if a perforation exists b/c the phase difference between the oval and round windows may be preserved.

-tympanogram or pneumatic otomicroscopic examination:

-abnormally high compliance and excessive excursion.

-Nb: if incudostapedial joint space is maintained by soft tissue fibrous attachments,

Þ low-frequencies will be effectively transmitted, whereas high-frequency conduction may be lost.

iv. NP: Benign: Neurofibroma, Hemangioma, Hemangiopericytoma, Fibrous dysplasia

Malig: Rhabdomyosarcoma, , Lymphangioma, Leukemia, Multiple myeloma, Paraganglioma

v. Inflam: Wegener's, Polyarteritis nodosa, Eosinophilic granuloma: Hand-Christian-Schüller disease, Sarcoidosis, Mucopolysaccharidoses,

vi. Iatrogenic: Surgical failure, Keloid

vii. Negative pressure

-Goode (1980) Þ negative middle ear pressure (~100 mm H2O) Þ reduction of TM excursion

Þ an axiom "the worse the negative pressure, the worse the hearing."

4. Congenital malformation

-auricle, external auditory canal, and middle ear space with ossicles may each develop without full maturation of the other sites (Anson and Donaldson, 1981).

-maximum losses occurring in cases of bony atresias.

-Causes: Apert's disease, Godenhar's syndrome, Turner's syndrome, , Achondroplasia, Marfan's syndrome

Treacher Collins syndrome Mohr's syndrome Pierre Robin syndrome, Microtia

Otofacial cervical synd Otopalatal-distal syndr, Congenital-hereditary, Anotia Cup ear deformity

NATURE OF SOUND AND PHYSIOLOGY OF CONDUCTIVE SOUND TRANSFER

Wave character of sound transmission

-Sound is mechanical vibration within the range of approximately 20 Hz-20 kHz.

Acoustic impedance

-Air and perilymph have different acoustic impedances or cxtics with regard to the way they transmit sound.

-Whenever two media that have different acoustic impedances are in contact, there is both reflection and transmission as the sound energy wave passes from one to the other.

-If sound in air had to pass directly into fluid of perilymph, >99.9% of acoustic energy would be lost to reflctn.

Impedance-matching transformers and middle ear

-middle ear is an impedance-matching transformer.

-it permits propagation of energy from a medium with one cxtic impedance to a medium with another,

via large TM to capture soft disturbances of airborne sound & small oval window interfacing the perilymph.

-middle ear transformer has three components:

(1) area ratio of TM to the oval window,

(2) lever ratio formed by effective lengths of manubrium of the malleus and long process of the incus, and

(3) a contribution due to the changing of the surface shape of the tympanic membrane.

-impedance transformation ratios, is of the order of 185:1......... Pickles, 1982

-TM

-lies in an oblique plane, almost a continuation of posterior wall of EAM.

-remarkably compliant because of the curvature in its surface—the so-called catenary curves.

Þ permits membrane to undergo large excursions necessary to transmit low-frequency sound energy. -the effective area of TM is about 65% of its total anatomic area of 85 mm2.

-The ossicular chain:

-moves as rigid bodies, (b/c of physics, ie; if the solid body has dimensions, much shorter than wavelength of sound, the solid body will transmit mechanical motion as gross movement of the entire body -the rigid-body motions of ossicular chain are about diameter of hydrogen molecule at threshold of sound, and <0.1 mm—ie the thickness of a hair—at 120 dB.

-malleus & incus are rigid bodies whose primary motions are rotational.

-their peculiar shapes give them an inertia that favors particular rotations at particular frequencies. -Alterations of mass, & \ inertia, by disease or surgery, will affect sound transmission through them. and the effects will be different for different frequencies of sound.

-the axis of rotation extends approximately along a line extending from the tip of the short process of the incus forward through the anterior ligament of the malleus (Bàràny, 1938).

-The jt is arranged so that when the hammer is drawn inwards by handle, it bites the anvil firmly and carries it with it. Conversely when the drumskin, with the hammer, is driven outwards, the anvil is not obliged to follow it........preventing any possibility of stirrups being torn away from oval window

Understanding small

-Pressure disturbances of the loudest sounds are only ~30 Pa; normal atmospheric pressure >100,000 Pa. -Pressure disturbances of just audible sound at 1000 Hz are only 0.00003 Pa, and displacement amplitudes are 50 times less than the distance between atoms in metals (Sears et al., 1987).

-it is common (and good) practice on installation of ME prosthetics to "nudge" them very slightly as a test of the integrity of the conductive repair. The motion of that gentle nudge is several millions of times more excursion than the piece will see in the rest of its normal service.

Bibliography:

Cummings, C. Otolaryngology.  Chapter 175