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Blood Supply of the Labyrinth, Referred Otalgia, and Recruitment

MS ENT University Examination Format

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1. BLOOD SUPPLY OF THE LABYRINTH

Arterial Supply

Main Artery: Internal Auditory (Labyrinthine) Artery

• Most commonly (45% of cases) from the anterior inferior cerebellar artery (AICA)
• May also arise directly from the basilar artery or, rarely, the superior cerebellar artery
• (Cummings Otolaryngology; Histology - Wheater/Ross)

Branches of the Labyrinthine Artery

Labyrinthine Artery
├── 1. Anterior Vestibular Artery
│       Supplies:
│       • Superior (anterior) semicircular canal ampulla
│       • Horizontal (lateral) semicircular canal ampulla
│       • Utricle (most of it)
│       • Small part of saccule
│
└── 2. Common Cochlear Artery
        ├── a. Proper Cochlear Artery (Spiral Modiolar Artery)
        │       Supplies the cochlea (turns 2.5–2.75)
        │
        └── b. Vestibulocochlear Artery
                ├── Cochlear ramus → cochlea
                └── Posterior Vestibular Artery
                        Supplies:
                        • Posterior semicircular canal ampulla
                        • Major part of saccule
                        • Parts of utricle body, horizontal & superior ampullae

Additional Artery: Subarcuate Artery

• Arises from the labyrinthine artery or AICA (or both as multiple branches)
• Passes within the arch of the superior semicircular canal
• Supplies bony and dural structures in that region

Venous Drainage

1. Vein of the Cochlear Aqueduct (main cochlear drainage):
   • Formed by the common modiolar vein (from anterior + posterior spiral modiolar veins) + vestibulocochlear vein
   • Drains into the inferior petrosal sinus
2. Vein of the Vestibular Aqueduct:
   • Drains vestibular labyrinth
   • Empties into the sigmoid sinus
3. Labyrinthine vein:
   • Follows the labyrinthine artery
   • Ultimately empties into the venous dural sinuses (inferior petrosal sinus or sigmoid sinus)

Clinical Significance

• Terminal artery = no collateral circulation = susceptibility to ischemic damage (sudden SNHL, vestibular loss)
• AICA infarction causes a characteristic syndrome: sudden SNHL, vertigo, facial numbness, ipsilateral Horner's - because the labyrinthine artery is usually the internal auditory artery arising from AICA
• Blood-labyrinth barrier analogous to blood-brain barrier - controls ionic exchange between blood and endolymph/perilymph

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2. REFERRED OTALGIA

Definition

Anatomical Basis - Nerves Mediating Referred Otalgia

Common Causes of Referred Otalgia (by nerve)

Via CN V3 (Trigeminal - Auriculotemporal nerve)

• Dental causes (most common cause overall)
  • Dental caries, pulpitis, periapical abscess
  • TMJ dysfunction / Costen's syndrome (otalgia + tinnitus + aural fullness + dizziness from TMJ)
  • Impacted wisdom tooth (pericoronitis)
• Infratemporal fossa tumors (adenoid cystic carcinoma most common)
• Trigeminal neuralgia

Via CN IX (Jacobson's nerve)

• Tonsillitis (acute follicular tonsillitis - classic, obvious case)
• Oropharyngeal carcinoma (tonsillar/tongue base - most important malignant cause)
• Peritonsillar abscess
• Styloid process elongation (Eagle's syndrome)

Via CN X (Arnold's nerve)

• Carcinoma of larynx (classic exam question - supraglottic > glottic)
• Carcinoma of hypopharynx (pyriform sinus)
• Oesophageal carcinoma
• Thyroid lesions

Via CN VII

• Herpes zoster oticus (Ramsay Hunt syndrome)
• Parotid pathology

Via C2/C3

• Cervical spondylosis/degenerative disc disease (Jaber et al. found 42% of referred otalgia was secondary to cervical disorders, predominantly CSDD; F:M ratio 4:1)
• Cervical lymphadenopathy
• Pain is typically retroauricular or infraauricular, related to neck movement

Malignancy - Important Red Flags

• Otalgia as sole presenting symptom in 14% of nasopharyngeal carcinomas
• Temporal bone metastases (breast, lung, kidney, stomach, prostate - via haematogenous spread)
• Triad of otalgia + periauricular swelling + facial nerve weakness = suspect temporal bone malignancy
• Infratemporal fossa malignancies (all 18 cases in one review had otalgia with normal otological findings)

Approach to Referred Otalgia

1. Full ENT examination (normal otological findings should prompt search for referred cause)
2. Oral cavity examination + flexible nasendoscopy (mandatory - pharynx/larynx)
3. Neck examination (nodes, cervical spine)
4. Dental/TMJ assessment
5. MRI with gadolinium - if more common causes excluded (to detect infratemporal fossa/skull base lesions)
6. CT temporal bone - if temporal bone malignancy suspected

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3. RECRUITMENT

Definition

Pathophysiology

Normal cochlear mechanism

• Outer hair cells (OHCs) have an active mechanism on the basilar membrane that provides cochlear amplification and compression
• This creates a wide dynamic range (~100 dB from threshold to discomfort)
• A 10 dB increase in sound level leads to only ~2.5 dB increase in basilar membrane response (compressive non-linearity)

In cochlear SNHL (OHC damage)

• Damage to OHCs removes the active mechanism and its compressive effect
• The cochlea loses its dynamic range compression
• Threshold is elevated (reduced sensitivity)
• BUT the upper limit of loudness tolerance (loudness discomfort level) is NOT elevated - it remains near normal
• Result: The dynamic range narrows dramatically

• Soft sounds are inaudible (raised threshold)
• Once audible, sounds become uncomfortably loud very rapidly
• "Can hear but cannot understand" - common complaint

Key Features of Recruitment

Tests for Recruitment

Recruitment vs. No Recruitment

Clinical Significance

1. Hearing aid fitting: Recruitment makes fitting difficult - amplified sounds may be too loud. Requires amplitude compression (non-linear amplification) in hearing aid design
2. Indicates cochlear (OHC) pathology - differentiates from retrocochlear lesion
3. Associated conditions: noise-induced hearing loss, Meniere's disease, presbycusis (all cause OHC damage)
4. "Hyperacusis" is sometimes confused with recruitment but refers to a different phenomenon - abnormal sensitivity to everyday sounds even in those with normal thresholds

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Summary for Quick Revision

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• Shambaugh Surgery of the Ear, 6th ed. - Labyrinthine Vessels, Sensory Nerves of Tympanomastoid Compartment, Sensorineural Hearing Loss
• Cummings Otolaryngology Head and Neck Surgery - Blood Supply to Vestibular End Organs (Ch. 129); Recruitment and Compression of Dynamic Range
• Scott-Brown's Otorhinolaryngology Head & Neck Surgery Vol. 2 - Referred Otalgia (Ch. 92); Sources of Distortion in Cochlear Hearing Loss; Psychoacoustics
• Histology: A Text and Atlas (Ross) - Blood Vessels of the Membranous Labyrinth
• Bailey and Love's Short Practice of Surgery - The Sensory Nerve Supply of the Ear