Audiological Tests for Retrocochlear Pathology

Introduction

Classification of Audiological Tests for Retrocochlear Pathology

┌────────────────────────────────────────────────────┐
│       AUDIOLOGICAL TESTS FOR RETROCOCHLEAR         │
│                   PATHOLOGY                        │
└───────────────────┬────────────────────────────────┘
                    │
      ┌─────────────┼──────────────────┐
      ▼             ▼                  ▼
BEHAVIORAL     ELECTROPHYSIOLOGICAL  ACOUSTIC
  TESTS              TESTS           IMMITTANCE
      │               │              TESTS
      │               │                │
┌─────┴────┐   ┌──────┴──────┐   ┌────┴────────┐
│Pure Tone │   │  ABR/BAEP   │   │ Acoustic    │
│Audiometry│   │  (BERA)     │   │ Reflex      │
│          │   │             │   │ Decay Test  │
│Speech    │   │  ECochG     │   │             │
│Audiometry│   │             │   │ Acoustic    │
│(SDS,PI-PB│   │  ASSR       │   │ Reflex      │
│Rollover) │   │             │   │ Threshold   │
│          │   │  ACAEP/CAEP │   └─────────────┘
│Tone Decay│   │             │
│(ABLB/    │   └─────────────┘
│Carhart)  │
│          │
│SISI      │
│          │
│Bekesy    │
│Audiometry│
└──────────┘

A. BEHAVIORAL (PSYCHOACOUSTIC) TESTS

1. Pure Tone Audiometry (PTA)

• Sensorineural hearing loss (SNHL), usually unilateral
• Loss is more pronounced at high frequencies (sloping audiogram)
• Air-bone gap is absent (purely sensorineural)
• The degree of loss may appear mild to moderate, but speech discrimination is disproportionately poor

2. Speech Audiometry

• Defined as the lowest intensity at which 50% of spondee words are correctly repeated.
• In retrocochlear lesions: SRT-PTA gap may be wide (SRT worse than expected from PTA).

• Uses phonetically balanced (PB) word lists at comfortable loudness level.
• Retrocochlear pattern: SDS is disproportionately poor (often <50%) compared to the degree of pure tone loss.

• At increasing intensities, WRS initially rises then falls — this fall is called Rollover.

  WRS (%)
   100 ┤         ●●
    80 ┤       ●    ●
    60 ┤     ●        ●   ← Rollover (Retrocochlear)
    40 ┤   ●
    20 ┤ ●
     0 ┤────────────────────
       20  40  60  80  100
              Intensity (dB SL)

• Rollover Index (RI) = (Maximum PB score − Score at highest intensity) / Maximum PB score
• RI > 0.45 is significant for retrocochlear pathology (Jerger & Jerger, 1971)
• This is one of the most sensitive behavioral indicators of VIII nerve lesion (Scott-Brown, 8th Ed., Vol. 2)

3. Tone Decay Tests (Auditory Adaptation Tests)

A. Carhart's Tone Decay Test

• A tone is presented 5 dB above threshold.
• If heard for <60 seconds → intensity raised by 5 dB.
• Process repeated.
• Total decay > 30 dB = retrocochlear (Carhart, 1957)

B. Rosenberg Tone Decay Test

• Tone presented at threshold for 60 seconds.
• Noted how many dB must be added to maintain audibility.
• >30 dB decay in 60 seconds = positive for retrocochlear pathology.

C. Olsen-Noffsinger Tone Decay Test

• Threshold-level presentation for 60 seconds.
• Grading:
  • 0–5 dB: Normal
  • 5–10 dB: Slight
  • 15–20 dB: Moderate
  • 25–30 dB: Marked retrocochlear

FLOW CHART — Tone Decay Test (Carhart's)

[Present tone at threshold + 5 dB]
            ↓
  [Patient hears for 60 sec?]
     YES ↙           NO ↘
  [NORMAL]    [Increase intensity by 5 dB]
                        ↓
           [Repeat until heard for 60 sec]
                        ↓
      [Calculate total dB added above threshold]
                        ↓
         >30 dB added → RETROCOCHLEAR
         <15 dB → NORMAL/COCHLEAR

4. SISI Test (Short Increment Sensitivity Index)

• 20 increments are presented; each correct detection = 5% score.

• Retrocochlear lesions give LOW SISI scores (0–20%) because recruitment is absent.
• (Zakir Hussain, ENT Textbook)

5. Alternate Binaural Loudness Balance Test (ABLB)

• Applicable only in unilateral hearing loss.
• A tone is alternated between ears; patient adjusts intensity in affected ear to match loudness in normal ear.
• Laddering pattern absent in retrocochlear (no recruitment).
• Retrocochlear: No recruitment (loudness grows at normal or subnormal rate).

6. Bekesy Audiometry

• Types III and IV are pathognomonic of retrocochlear lesions, especially acoustic neuroma.
• (Scott-Brown, Otorhinolaryngology Head & Neck Surgery; Cummings, 6th Ed.)

Bekesy Audiogram — Retrocochlear (Type III)
dB HL
 0 ─────────────────── Interrupted (I)
10 ─────────
20 ──────
30 ────────────────────────── Continuous (C)
40 ───────────────────────────────────────
   250  500  1k   2k   4k   8k Hz
    ↑
Crossover at ~1000 Hz → TYPE III → RETROCOCHLEAR

B. ACOUSTIC IMMITTANCE TESTS

7. Acoustic Reflex (Stapedius Reflex) Testing

• Reflex present at normal thresholds but DECAYS on sustained stimulus (most important sign)
• Elevated or absent acoustic reflex threshold at ipsilateral and contralateral sides
• Retrocochlear lesions on the afferent arc (CN VIII) cause absent ipsilateral AND contralateral reflex when the affected ear is stimulated.

Acoustic Reflex Decay Test

ACOUSTIC REFLEX DECAY TEST PROTOCOL

[Present 500 Hz or 1000 Hz tone at 10 dB above reflex threshold]
                    ↓
    [Observe reflex response for 10 seconds]
                    ↓
  [Measure amplitude at 0 sec and 5 sec]
                    ↓
    Amplitude at 5 sec / Amplitude at 0 sec
                    ↓
    < 50% (i.e., decay >50%) = POSITIVE = RETROCOCHLEAR
    ≥ 50% remaining = NORMAL

• Positive Decay = amplitude falls to <50% of initial within 5 seconds at 500 Hz or 1000 Hz
• Sensitivity ~65–85% for acoustic neuroma (Cummings Otolaryngology, 6th Ed.)

Acoustic Reflex Pattern Summary

C. ELECTROPHYSIOLOGICAL TESTS

8. Auditory Brainstem Response (ABR) / Brainstem Evoked Response Audiometry (BERA) / BAEP

ABR Wave Generators (Neural Sources)

┌─────────────────────────────────────────────────────────┐
│               ABR WAVEFORM COMPONENTS                   │
├────────┬───────────────────────────────────────────────┤
│ Wave I  │ Distal VIII nerve (cochlear end)              │
│ Wave II │ Proximal VIII nerve / cochlear nucleus        │
│ Wave III│ Superior olivary complex (pons)               │
│ Wave IV │ Lateral lemniscus / Nuclei                    │
│ Wave V  │ Inferior colliculus (most robust, most used)  │
└────────┴───────────────────────────────────────────────┘

Normal ABR Absolute Latencies

Inter-peak Latencies (IPL) — Most Diagnostically Useful

1. Prolonged I–V inter-peak latency (most sensitive)
2. Prolonged III–V IPL (more specific for central lesion)
3. Absent or poorly formed waves beyond Wave I (Wave I present but no III/V)
4. Inter-aural latency difference for Wave V > 0.4 ms
5. Complete absence of all waves (profound involvement)
6. Increased I–III IPL with normal III–V (CN VIII/pontine junction lesion)

ABR in Retrocochlear vs. Cochlear Lesion

Normal ABR:
─────────────────────────────────────────
     I   II   III   IV   V
     ↑        ↑         ↑
   1.5ms    3.8ms    5.8ms

Cochlear SNHL (Threshold shift only):
─────────────────────────────────────────
All waves present, normal IPL, may have
elevated thresholds; wave V latency may
shift with hearing level

Retrocochlear ABR (Acoustic Neuroma):
─────────────────────────────────────────
     I                    (III absent/late)
     ↑          ← Absent/flat beyond →
   1.5ms        Waves III, IV, V absent
           OR prolonged I–V IPL > 4.6ms

ABR showing normal waveforms (Right) vs. absent/flat waveforms (Left) — indicating left retrocochlear pathology (e.g., vestibular schwannoma). Waves I, III, V are clearly identified on the right; the left shows absent peaks consistent with retrocochlear disruption.

9. Electrocochleography (ECochG)

• Measures: Cochlear microphonic (CM), Summating Potential (SP), and Action Potential (AP) of CN VIII
• In retrocochlear lesions: AP is absent or poorly formed; SP/AP ratio may be normal
• More useful to exclude cochlear pathology (endolymphatic hydrops) than to confirm retrocochlear
• Abnormally elevated SP/AP ratio (>0.4) = Menière's disease (cochlear), not retrocochlear
• Absence of AP with preserved CM = CN VIII lesion (retrocochlear)

10. Auditory Steady-State Response (ASSR)

• Frequency-specific electrophysiological threshold estimation
• Uses modulated tones (80–100 Hz carrier); responses detected by statistical analysis
• In retrocochlear: ASSR thresholds may be absent or highly elevated, frequency-dependent
• Useful as an objective audiogram, particularly when ABR is inconclusive
• Less specific for retrocochlear than ABR (Recent advances, Picton, 2011)

11. Middle Latency Response (MLR) and Cortical Auditory Evoked Potentials (CAEP)

• MLR assesses primary auditory cortex (waves Na, Pa, Nb at 15–50 ms)
• CAEP (P1, N1, P2, N2, P300) assesses higher cortical processing
• Used in central auditory processing disorders and cortical lesions
• Limited role in peripheral retrocochlear diagnosis

D. OTOACOUSTIC EMISSIONS (OAE) IN RETROCOCHLEAR DIAGNOSIS

• The combination of Present OAE + Absent/Abnormal ABR = "OAE/ABR Mismatch" is a hallmark of retrocochlear pathology (auditory neuropathy spectrum disorder included).
• (Scott-Brown, 8th Ed.; Cummings, 6th Ed.)

E. DIAGNOSTIC FLOWCHART FOR RETROCOCHLEAR PATHOLOGY

PATIENT PRESENTS WITH:
Unilateral SNHL / Asymmetric hearing loss / Poor speech discrimination /
Tinnitus / Vertigo

                         ↓
              ┌─────────────────────┐
              │   PURE TONE         │
              │   AUDIOMETRY (PTA)  │
              └──────────┬──────────┘
                         ↓
            Unilateral SNHL / Asymmetric PTA
                         ↓
         ┌───────────────┼────────────────┐
         ▼               ▼                ▼
   SPEECH          TONE DECAY        IMMITTANCE
   AUDIOMETRY      TEST              AUDIOMETRY
   (SDS/WRS)       (Carhart)         (SR + Decay)
         │               │                │
   SDS <50%         >30 dB decay     SR Decay >50%
   PI-PB Rollover   (positive)       in 5 seconds
   RI >0.45
         │               │                │
         └───────────────┼────────────────┘
                         ↓
              ┌─────────────────────┐
              │   BEKESY AUDIOMETRY │
              │   (Type III/IV)     │
              └──────────┬──────────┘
                         ↓
            Retrocochlear suspected
                         ↓
              ┌─────────────────────┐
              │  OAE (TEOAE/DPOAE)  │
              │  Present → OHC OK   │
              └──────────┬──────────┘
                         ↓
              ┌─────────────────────┐
              │   ABR / BERA        │
              │   (GOLD STANDARD)   │
              └──────────┬──────────┘
                         ↓
       Prolonged I-V IPL / Absent Waves III-V
       Inter-aural Wave V difference >0.4 ms
                         ↓
              ┌─────────────────────┐
              │ GADOLINIUM-ENHANCED │
              │    MRI IAC/CPA      │
              │   (Definitive)      │
              └──────────┬──────────┘
                         ↓
              ACOUSTIC NEUROMA / CPA TUMOR
              confirmed → Surgical/Radiological Rx

F. RECENT ADVANCES (Important for RGUHS)

1. High-Resolution MRI (Gd-DTPA Enhanced)

• Has largely replaced ABR as the definitive confirmatory test for acoustic neuromas.
• ABR sensitivity ~90–95% but MRI detects 100% of tumors, including intracanalicular ones <5 mm.
• ABR still cost-effective as a screening test before MRI.

2. Auditory Neuropathy Spectrum Disorder (ANSD)

• Characterized by: Present OAE + Absent/Abnormal ABR — the classic retrocochlear "mismatch."
• Lesion may be at inner hair cells, synapse, or CN VIII.
• First described by Starr et al. (1996).
• Management: Cochlear implants (CI) are often effective.

3. Chirp-Evoked ABR

• Uses frequency-swept chirp stimuli to compensate cochlear travel time, producing larger Wave V amplitude.
• Improves SNR and sensitivity of ABR in screening.
• Particularly useful in neonatal hearing screening programs.

4. Stacked ABR (Don et al., 2005)

• Summates energy from all frequency regions simultaneously.
• Increases sensitivity for small tumors (<1 cm) missed by conventional click-ABR.
• Reported sensitivity of 95% for small intracanalicular tumors.

5. ABR with Bone Conduction

• Useful when air-conduction thresholds are too high to generate clear waveforms.

6. VEMP (Vestibular Evoked Myogenic Potential)

• cVEMP (cervical): Saccular function via inferior vestibular nerve.
• oVEMP (ocular): Utricular function via superior vestibular nerve.
• In vestibular schwannoma: cVEMP is absent or elevated threshold on affected side.
• Increasingly used in the workup of CPA lesions (Recent literature, Murofushi et al.)

7. Auditory Change Complex (ACC)

• Can detect changes in ongoing sounds in real time.
• Future role in assessing central auditory processing in retrocochlear disorders.

G. COMPARATIVE SENSITIVITY/SPECIFICITY TABLE

H. SUMMARY: KEY RETROCOCHLEAR AUDIOLOGICAL FINGERPRINTS

┌──────────────────────────────────────────────────────────────┐
│           RETROCOCHLEAR AUDIOLOGICAL HALLMARKS               │
├──────────────────────────────────────────────────────────────┤
│ • Unilateral SNHL with POOR speech discrimination            │
│ • PI-PB Rollover Index > 0.45                                │
│ • Tone Decay > 30 dB (Carhart's Test)                        │
│ • SISI Score 0–20% (LOW — no recruitment)                    │
│ • Bekesy Type III or IV                                      │
│ • NO recruitment (ABLB negative)                             │
│ • Acoustic Reflex Decay (>50% in 5 seconds)                  │
│ • ABR: Prolonged I–V IPL (>4.6 ms) / Absent III–V           │
│ • Inter-aural Wave V latency difference > 0.4 ms             │
│ • OAE Present + ABR Absent = ANSD pattern                    │
└──────────────────────────────────────────────────────────────┘

Conclusion

References

1. Harrison's Principles of Internal Medicine, 21st Ed. (pp. 1037–1041)
2. Scott-Brown's Otorhinolaryngology, Head & Neck Surgery, 8th Ed., Vol. 2
3. Cummings Otolaryngology, Head & Neck Surgery, 6th Ed.
4. Dhingra PL — Diseases of Ear, Nose and Throat, 7th Ed.
5. Hazarika P — Textbook of ENT, Head & Neck Surgery
6. Stell & Maran's Head and Neck Surgery, 4th Ed.
7. Zakir Hussain — ENT Textbook
8. Jerger JF, Jerger S — Performance-Intensity Functions in Sensorineural Hearing Loss, JASA, 1971
9. Don M, Masuda A, Nelson R, Brackmann D — Stacked ABR, Am J Otol, 2005
10. Starr A et al. — Auditory Neuropathy, Brain, 1996
11. Murofushi T et al. — VEMP in vestibular schwannoma, Acta Otolaryngol, 2012