Impedance Audiometry: Complete Textbook Account

1. Principles and Terminology

• Stiffness component - determined by the tympanic membrane, ossicular ligaments, and middle ear air-column compression. Stiffness limits passage of low-frequency sounds.
• Mass component - governed by the ossicles. Mass limits passage of high-frequency sounds.
• The greatest transfer function of the middle ear occurs at its resonant frequency between 1 and 3 kHz.

1. Tympanometry (static compliance + middle ear pressure)
2. Static acoustic compliance
3. Acoustic (stapedial) reflex testing

2. Tympanometry

Principle

Measurements Obtained

Jerger Classification of Tympanograms

• Type B with large canal volume (>2.0 cm³): indicates TM perforation or patent grommet
• Type B with normal canal volume: indicates fluid in middle ear
• Type C: Eustachian tube dysfunction; negative pressure reflects inability to equilibrate middle ear pressure

3. Static Acoustic Compliance

• Reduced compliance (Type As): ossicular fixation (otosclerosis, tympanosclerosis)
• Increased compliance (Type Ad): ossicular chain discontinuity, hypermobile TM
• Compliance is measured as: Total compliance = Peak compliance - Tail compliance

4. Acoustic (Stapedial) Reflex Testing

Anatomy of the Reflex Arc

1. Afferent limb: CN VIII (cochlear nerve)
2. Cochlear nucleus (brainstem)
3. Superior olivary complex (bilateral)
4. Efferent limb: CN VII (facial nerve) to stapedius muscle
5. Stapedius contracts → stiffens ossicular chain → measurable change in compliance

Reflex Thresholds

• Normal acoustic reflex threshold (ART): 70-100 dB HL at 500, 1000, 2000, and 4000 Hz
• Both ipsilateral and contralateral reflexes are measured

Interpretation

Diphasic (On-Off) Reflex

Reflex Decay (Tone Decay Test)

• Retrocochlear pathology (CN VIII lesion, acoustic neuroma)
• Positive decay at 500 Hz is most sensitive for retrocochlear disease

5. Clinical Applications of Impedance Audiometry

Ideal Meatoplasty

Definition and Purpose

• Canal-wall-down (CWD) mastoidectomy / modified radical mastoidectomy
• Canalplasty procedures
• Revision surgery for meatal stenosis

1. Adequate visualization of the cavity/drum
2. Self-cleansing (epithelial migration)
3. Aeration of the mastoid cavity
4. Fitting of a speculum holder for transmeatal surgery (ideally a 10-12 mm diameter speculum for CWD cases)

Principles of an Ideal Meatoplasty

Key Anatomical Alignment

Auricular Repositioning Techniques (when needed)

• Superior elevation: sharply releasing skin and subcutaneous tissue over the parotid fascia anteriorly and over the sternocleidomastoid muscle inferiorly. Care must be taken to stay superficial and not enter the parotid gland substance (risk of salivary fistula).
• Posterior repositioning: excising a strip of skin from the postauricular incision and suturing the auricle to the new postauricular skin edge.

Surgical Technique of Meatoplasty

Step 1: Infiltration

Step 2: Exposure

Step 3: Cartilage Excision (Körner / Conchal Cartilage Removal)

Step 4: Creation of the U-Shaped Tragal Skin Flap

• A crescentic incision is made in the skin of the conchal bowl
• Skin is sharply elevated off the underlying cartilage and hinged at the tragus
• The skin flap is thinned and reflected anteriorly
• Underlying cartilage and soft tissue are cored out with a #11 blade

Step 5: Flap Inset and Suturing

• The skin flap is brought through the new meatus medially down to the cuff of TMJ periosteum
• Sutured with 2-3 buried 4-0 undyed Vicryl sutures to create the lateral anterior canal wall
• The postauricular incision is tacked with interrupted 3-0 undyed Vicryl sutures
• The skin graft is delivered through the meatus and sutured to native skin at the conchal bowl edge with interrupted 5-0 fast-absorbing gut sutures

Step 6: Packing

Critical Points

• A hastily performed meatoplasty ruins an otherwise excellent operation - it results in meatal stenosis
• Even a carefully constructed meatus can stenose; care reduces but does not eliminate risk
• An ideal CWD meatoplasty admits a 10-12 mm diameter speculum that can be stabilized with a holder

Reconstruction of the Posterior Canal (Meatal) Wall

Context: Canal-Wall-Up (CWU) vs Canal-Wall-Down (CWD)

• CWU (intact canal wall): posterior bony EAC wall is preserved; normal anatomy maintained
• CWD (open cavity): posterior and superior bony canal wall is removed, creating an open mastoid bowl

1. Converting a CWD cavity back to a CWU anatomy (obliteration/reconstruction)
2. Managing ossicular chain reconstruction within a CWD cavity

Reconstruction in Canal-Wall-Down Mastoidectomy

Ossicular Chain Assessment

• Presence/absence of stapes superstructure
• Relationship of stapes to the level of the horizontal facial nerve

Staging

Transmeatal Approach (When Meatoplasty is Adequate)

1. Incision parallel to the course of the facial nerve creates an anteriorly based tympanomastoid flap
2. The incision begins 3-5 mm superior to the fallopian canal, passes posterior-superior to the horizontal semicircular canal, extending inferiorly 3-5 mm posterior to the facial ridge
3. Flap is raised by pushing tissue anteriorly; inspect carefully for dehiscent facial nerve
4. Sharp lysis of adhesions (do NOT slide a sharp knife along bone - risk of injury to dehiscent nerve)
5. Once beyond facial nerve, flap elevated to expose middle ear
6. PORP or TORP placed depending on defect type (usually PDRP/TORP)

Postauricular Approach (When Meatus is Too Small)

• If the meatus is inadequate for transmeatal exposure
• Enlarging the meatus should be considered simultaneously

Körner Flap for Posterior Wall Coverage

Creation of the Körner Flap

1. An inferior incision at 6 o'clock in the inferior canal, carried into the conchal bowl, curved around the inferior margin
2. A superior incision at 12 o'clock carried between the tragus and anterior helix
3. These incisions create a long, vascularized flap based in the posterosuperior conchal bowl
4. This flap constitutes the back wall of the mastoid cavity

Grafting Sequence

• Auricle and flap retracted anteriorly to expose mastoid and middle ear
• Epinephrine-soaked gelatin sponge removed; middle ear and Eustachian tube packed with saline-moistened gelatin sponge to the level of the anterior annulus
• Fascia graft placed medial to the anterior annulus and drum remnant, extending over the stapes to the facial ridge into the mastoid

Factors Affecting Hearing Reconstruction in CWD vs CWU

• CWU preserves normal EAC anatomy and attempts reconstruction within near-normal middle ear volume
• In CWD mastoidectomy, the facial ridge is lowered variably; middle ear volume around the ossicles can be critically reduced
• Middle ear volume (as shown by Merchant et al.) is a critical factor in hearing reconstruction outcomes
• Variable middle ear volume near the minimal critical range for adequate acoustics makes CWD ossicular reconstruction more challenging

Summary Comparison

Key References (Textbooks Cited)

• Shambaugh Surgery of the Ear - Chapters 24 (Meatoplasty in reconstruction) and 31 (Canal-wall-down mastoidectomy), Chapters 29 (Ossicular reconstruction in CWD mastoidectomy)
• Cummings Otolaryngology Head and Neck Surgery - Chapters 134 (Audiometry / immittance testing) and 146 (Otosclerosis / acoustic reflex interpretation)
• Scott-Brown's Otorhinolaryngology Head & Neck Surgery (Vol. 2) - Chapter 48 (Middle ear function, impedance audiometry, tympanometry)
• K.J. Lee's Essential Otolaryngology - Test battery approach; meatoplasty and tympanoplasty