Immittance

Description:

Immittance measures are test procedures that use measures of the tympanic membrane compliance to determine normalcy of structures medial to the eardrum.  The procedures are broken into three components: Tympanometry, Acoustic Reflex Thresholds, and Acoustic Reflex Decay.

  • Tympanometry is the measure of tympanic membrane compliance across a range of pressure.  It is diagnostic for middle ear effusion including negative middle ear pressure or fluid behind the eardrum, and is instrumental in the diagnosis of ossicular disruption such as disarticulation of the ossicles or otosclerosis.  The procedure involves placing a small rubber plug into the ear canal.  Once in place, a measure of canal volume is made while varying the pressure in the ear canal.
  • Acoustic Reflex Thresholds are used to measure the stapedious reflex.  This reflex arc is stimulated by presenting a moderately high intensity signal into the ear canal while recording the canal volume.  A reduction in volume, time linked to the stimulus presentation is considered a response.
  • Acoustic Reflex Decay is a screening procedure for retrocochlear involvement.  Once the reflex threshold has been established, the stimulus intensity is increased, and the reflex is measured for stability over a 10 second duration.

The entire process takes approximately 15 minutes.  The patient is not required to make any subjective judgments, and the pressure utilized does not create any patient discomfort (typical pressure range is -300 to +300 mm/H2O).

Indications:

  • Tympanometry — Patients of any age who are at risk for middle ear involvement.  This may include middle ear effusion, ossicular discontinuity, otosclerosis, tympanic membrane perforation.
  • Acoustic Reflex Threshold and Decay — Patients requiring differential diagnosis of cochlear vs. retrocochlear involvement, based on audiometric findings (see hearing testing).

Significant Findings:

  • Tympanometry
    • Type A: Normal middle ear pressure and tympanic membrane compliance.  This is typical of a normal ear.
    • Type AS: Normal middle ear pressure and low compliance.   This is typical of a stiff ossicular chain as in patients with otosclerosis or chronic middle ear disease.
    • Type B: Negative middle ear pressure with normal tympanic membrane compliance.  This is typical of an ear in transition (either improving or getting worse).  This ear should be watched.
    • Type C: Normal canal volume, but no compliance peak (a.k.a. flat tympanogram).  Typical of middle ear effusion.
    • Large Canal Volume: Typical of tympanic membrane perforation or patent P.E. Tube.
  • Acoustic Reflex Thresholds
    • Present at normal threshold and sensation levels — Normal hearing
    • Present at normal thresholds, but reduced sensation levels — cochlear hearing loss
    • Present at elevated thresholds and normal or elevated sensation levels — retrocochlear
    • Absent — May be consistent with retrocochlear or a severe (>60dBHL thresholds) cochlear hearing loss
  • Acoustic Reflex Decay
    • No decay — Normal hearing or cochlear hearing loss
    • Greater than 50% decay over 10 second recording — Retrocochlear
    • Negative On/Negative Off pattern with no evidence of decay — otosclerosis

Suggestions:

No special patient preparation is required. If you have any questions regarding these tests please call (425) 899-5050 or E-mail info@everhear.com.