Retrocochlear Causes of Hearing Loss and Tests Used to Evaluate Them

Definition

Causes of Retrocochlear Hearing Loss

I. Neoplastic (Most Important)

• The single most important retrocochlear cause; arises from Schwann cells of the superior vestibular division of CN VIII
• Presents with unilateral, progressive SNHL (usually high-frequency), tinnitus, and disequilibrium
• Can cause sudden SNHL in ~10% of cases
• CPA (cerebellopontine angle) is the most common site

• Second most common CPA tumor
• Arises from arachnoid villi; tends to be broader-based than schwannoma
• Causes hearing loss by compressing the VIII nerve

• Epidermoid cysts, facial nerve schwannomas, lipomas, glomus tumors, primary cholesteatoma of CPA
• Metastatic disease / carcinomatous meningitis - can infiltrate the VIII nerve

II. Inflammatory / Infectious

• Herpes zoster oticus (Ramsay Hunt syndrome): VIII nerve involvement with SNHL, vertigo, facial palsy, herpetic vesicles
• Herpes simplex, CMV: can cause auditory neuropathy pattern

• Pneumococcal, meningococcal, Haemophilus influenzae, tubercular meningitis, cryptococcosis
• Inflammation spreads along the VIII nerve and into the cochlear aqueduct
• Major cause of acquired SNHL in children

• Treponema pallidum can cause VIII nerve damage; both congenital and acquired forms
• Typically bilateral SNHL; may have Tullio phenomenon, Hennebert's sign

• VIII nerve neuropathy as part of Lyme neuroborreliosis

• Direct neural invasion, CNS opportunistic infections (cryptococcal meningitis, CMV radiculopathy)

III. Demyelinating Disease

• Demyelinating plaques in the VIII nerve or brainstem auditory pathways
• Produces abnormal ABR (prolonged I-V interpeak interval) even when audiogram appears near-normal
• Classic finding: auditory symptoms out of proportion to pure-tone loss

IV. Vascular

• Occlusion of the AICA or PICA affects brainstem auditory nuclei
• Sudden unilateral deafness can be the presenting feature of AICA territory infarct

• AICA vascular loop compressing the VIII nerve in the internal auditory canal can mimic schwannoma

V. Metabolic / Toxic

• Direct VIII nerve toxicity
• Uraemia - produces retrocochlear auditory neuropathy pattern

• Bony encroachment on the IAC (internal auditory canal) compresses the VIII nerve

• Iron deposition along neural structures

VI. Hereditary / Congenital (Auditory Neuropathy Spectrum Disorder - ANSD)

• Hearing loss caused by dysfunction of the VIII nerve or inner hair cells/synapses (dyssynchrony)
• OAEs are present (outer hair cell function intact) but ABR is absent or grossly abnormal
• Perinatal risk factors: hyperbilirubinaemia, hypoxia, prematurity
• Mutations in genes: OTOF (otoferlin), PJVK, ATP1A3

Tests to Evaluate Retrocochlear Hearing Loss

A. Behavioral / Audiological Tests

1. Pure-Tone Audiometry (PTA)

• Key finding: Asymmetric SNHL - any unexplained asymmetry of >15-20 dB at two or more frequencies, or >15 dB asymmetry in word recognition scores, raises suspicion for retrocochlear pathology
• Progressive high-frequency asymmetric loss: suggests VIII nerve disorder
• Low-frequency loss: associated with brainstem lesions
• Normal hearing does not exclude a retrocochlear lesion (especially small schwannomas)

2. Speech Audiometry (Word Recognition / Speech Discrimination Score - SDS)

• In cochlear HL: SDS is predictable from the degree of loss (PI-PB function is normal shape)
• Retrocochlear finding: SDS disproportionately poor relative to the PTA - "rollover" phenomenon
• Rollover Index: As stimulus intensity increases beyond maximum discrimination (PB max), the score paradoxically falls - a rollover index >0.45 is suspicious for VIII nerve lesion
• SDS < 30% is characteristic of retrocochlear (VIII nerve) lesion (Adams & Victor, p. 309)

3. Loudness Recruitment Testing

• Cochlear lesions show recruitment (abnormally rapid loudness growth) - tested with Alternate Binaural Loudness Balance (ABLB) test (Fowler's test)
• Retrocochlear lesions show decruitment (abnormally slow loudness growth) - the interaural loudness difference remains constant or widens at high intensities, unlike recruitment where it narrows

4. Tone Decay Test (TDT) / Auditory Adaptation Test

• Based on the phenomenon of excessive neural adaptation
• A pure tone is presented at 10 dB SL continuously for 60 seconds
• Normal: tone remains audible throughout
• Retrocochlear (positive): tone fades within 60 seconds; intensity must be raised several times by 5 dB increments to maintain audibility (Carhart: >30 dB decay = retrocochlear; Rosenberg modification)
• Marked tone decay (type IV-V by Green's classification) is strongly associated with VIII nerve pathology

5. Short Increment Sensitivity Index (SISI)

• Tests ability to detect 1 dB increments superimposed on a tone at 20 dB SL
• Cochlear: high SISI score (>70%) - intact recruitment
• Retrocochlear: low SISI score (<20%) - absence of recruitment
• Note: less used today due to lower specificity

6. Bekesy Audiometry

• Patient tracks threshold manually for both continuous and interrupted tones
• Type III: continuous tone tracing drops well below interrupted tone tracing - indicates pathological adaptation, highly suggestive of VIII nerve lesion
• Type IV: continuous tone tracing drops 25+ dB below interrupted tone at all frequencies - also retrocochlear
• Type I = normal; Type II = cochlear

B. Objective / Electrophysiological Tests

7. Acoustic Reflex Threshold and Decay (Impedance Audiometry)

• Metz test: Acoustic reflex threshold (ART) at 70-95 dB HL is normal; elevated ART (>105 dB) or absent reflex with pure-tone loss suggests retrocochlear
• Acoustic Reflex Decay Test: A tone at 10 dB above ART is sustained for 10 seconds; decay of >50% of initial amplitude within 5 seconds = positive (abnormal) = retrocochlear/VIII nerve lesion
• Sensitive (~85%) for large tumors; less sensitive for small intracanalicular tumors

8. Auditory Brainstem Response (ABR) / BAER

• Waves I (VIII nerve), III (cochlear nucleus/superior olivary complex), V (lateral lemniscus/inferior colliculus)
• Abnormal ABR criteria for retrocochlear disease:
  • Prolonged absolute wave V latency
  • Prolonged I-V interpeak interval (>4.4 ms; >99th percentile of adult population has I-V interval ≤4.6 ms)
  • Interaural latency difference (ILD) of wave V > 0.2-0.4 ms between ears
  • Abnormal V/I amplitude ratio (reduced wave V relative to wave I)
  • Absent or degraded waveform morphology - loss of late waves (III and V)
  • Absent ABR with normal OAEs = auditory neuropathy
• ABR sensitivity: ~95% for large tumors, ~80% for small intracanalicular schwannomas
• A normal ABR does not exclude a lesion, particularly if imaging shows structural change without functional consequence

9. Otoacoustic Emissions (OAEs) - TEOAEs / DPOAEs

• Reflect outer hair cell (cochlear) function
• Key diagnostic pattern in ANSD / retrocochlear disease:
  • Present OAEs + Absent/abnormal ABR = pathognomonic of auditory neuropathy / VIII nerve disorder (cochlea intact, neural pathway disrupted)
  • Absent OAEs do NOT exclude retrocochlear disease (tumor can secondarily affect cochlear blood supply)
• Contralateral OAE suppression: TEOAE amplitude is normally suppressed by contralateral noise (mediated by medial olivocochlear efferent system); absent suppression can indicate brainstem/VIII nerve pathology

10. Electrocochleography (ECochG)

• Records cochlear microphonics (CM), summating potential (SP), and compound action potential (AP/N1) from CN VIII
• Prolonged AP with normal CM confirms VIII nerve dysfunction while cochlea is intact
• Elevated SP/AP ratio (>0.4) more typical of Meniere's disease (cochlear)

C. Radiological Investigation

11. MRI with Gadolinium Enhancement (Investigation of Choice)

• Gadolinium-enhanced MRI of the IAC and CPA is the definitive investigation for retrocochlear pathology
• Sensitivity >99% for vestibular schwannoma; detects lesions as small as 1-2 mm
• Has largely superseded all audiological special tests in clinical practice as the final arbiter
• Characteristic findings:
  • Schwannoma: T1 iso/hypointense, T2 hyperintense, enhances with gadolinium; "ice-cream cone" appearance at IAC porus
  • Meningioma: enhances homogeneously; dural tail sign; more broad-based, eccentric to IAC

12. CT Scan (High-Resolution CT of Temporal Bone)

• Useful for bony abnormalities - widening of the IAC, erosion of petrous apex
• Shows bony remodeling in large tumors
• Not as sensitive as MRI for soft tissue lesions
• CT cisternography (older technique) with intrathecal contrast - largely replaced by MRI

Summary Table

Key Exam Points

• Rollover on PI-PB function and disproportionately poor SDS are hallmarks of VIII nerve lesions
• ABR with prolonged I-V interpeak interval is the most sensitive electrophysiological test; normal ABR does not exclude lesion
• Present OAEs + Absent ABR = diagnostic of auditory neuropathy spectrum disorder
• MRI with gadolinium is the investigation of choice and has superseded most behavioral special tests
• Retrocochlear lesions show decruitment (not recruitment) and marked tone decay - the opposite of cochlear findings
• Any unexplained unilateral or asymmetric SNHL mandates an MRI to exclude a retrocochlear lesion