Course of arteries and nerves in internal auditory meatus

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Internal Auditory Meatus (Internal Auditory Canal) - Arteries and Nerves

Overview

The internal auditory meatus (IAM) is a short canal approximately 1 cm in length, lined with dura mater, running laterally from the cerebellopontine angle (CPA) into the petrous part of the temporal bone. It transmits the facial nerve (VII), vestibulocochlear nerve (VIII), internal auditory artery, and internal auditory vein.

The lateral end (fundus) is closed by a perforated bony plate. A horizontal shelf on the inner surface of this plate - the crista falciformis (transverse crest) - divides the fundus into upper and lower compartments. A vertical ridge called Bill's bar further subdivides the upper compartment.

Nerves in the IAM

Endoscopic view of the left internal auditory meatus fundus showing the arrangement of nerves

Endoscopic view of the left IAM apex showing: transverse crest, Bill's bar, facial nerve region, superior vestibular nerve region, inferior vestibular nerve region, cochlear nerve region, and the singular foramen.

1. Facial Nerve (CN VII)

Emerges from the ventrolateral aspect of the lower pons, then travels laterally through the CPA together with CN VIII.
On entering the meatus, the motor division lies on the superoanterior surface of the vestibulocochlear nerve (CVN). The nervus intermedius lies between the motor division and CN VIII.
At the fundus, it occupies the anterosuperior quadrant, separated from the superior vestibular nerve region by Bill's bar.
No major branches arise within the meatal segment.
At the lateral end, it enters the facial (fallopian) canal to begin the labyrinthine segment - the narrowest and shortest segment - which runs anterolaterally to the geniculate ganglion.

2. Vestibulocochlear Nerve (CN VIII)

CN VIII occupies the inferior and posterior portions of the IAM and splits into cochlear and vestibular divisions at or near the fundus.

Above the transverse crest:

Division	Location at fundus	Distribution
Facial nerve	Anterosuperior (separated by Bill's bar)	Facial muscles, parasympathetics, taste
Superior vestibular nerve	Posterosuperior (multiple foramina)	Superior + lateral semicircular canals, utricle, part of saccule

Below the transverse crest:

Division	Location at fundus	Distribution
Cochlear nerve	Anteroinferior (spiral foramina + central canal = cochlear area)	Organ of Corti
Inferior vestibular nerve	Posteroinferior (one or two foramina)	Saccule
Singular nerve	Foramen singulare (behind + below inferior vestibular foramen)	Posterior semicircular canal ampulla

The singular nerve runs obliquely through the petrous bone close to the round window to supply the sensory epithelium of the posterior semicircular canal ampulla.

Arteries in the IAM

Internal Auditory Artery (Labyrinthine Artery)

A branch of the anterior inferior cerebellar artery (AICA) in ~80% of cases; less commonly arises directly from the basilar artery.
Enters the IAM alongside the facial and vestibulocochlear nerves.
Gives a branch to CN VIII at the CPA before entering the meatus.
Within the IAM it supplies ganglion cells, nerves, dura, and arachnoid, then splits into two main branches:

Branch	Supplies
Anterior vestibular artery	Anterior and lateral semicircular canals, utricular macula
Common cochlear artery (splits further)	→ Cochlear artery (cochlea proper) and posterior vestibular artery (posterior semicircular canal, saccular macula, part of cochlea)

Because the labyrinthine artery is an end artery with no collateral supply, occlusion produces sudden sensorineural hearing loss and/or vertigo.

The labyrinthine artery also supplies the facial nerve in its cisternal, meatal, and labyrinthine segments (the more distal tympanic and mastoid segments are supplied by the petrosal branch of the middle meningeal artery and the stylomastoid artery from the posterior auricular artery).

Internal Auditory Vein

Drains the labyrinth and accompanies the artery, emptying into the inferior petrosal sinus or the sigmoid sinus.

Summary Diagram (Quadrant Map of the Fundus)

         ANTERIOR          POSTERIOR
         ─────────────────────────────
SUPERIOR │  Facial n. (VII) │ Superior vestibular n.  │
         │                  │ (separated by Bill's bar)│
         ─────── Transverse crest (crista falciformis) ──
INFERIOR │  Cochlear n.     │ Inferior vestibular n.  │
         │                  │ (+ singular n. below)   │
         ─────────────────────────────

Sources:

Scott-Brown's Otorhinolaryngology Head & Neck Surgery, Vol. 2 - "The internal auditory meatus"
Localization in Clinical Neurology, 8e - "Anatomy of the Peripheral Course of the Facial Nerve"; "Meatal Segment"; blood supply of cochlea
Cummings Otolaryngology Head and Neck Surgery - blood supply of the labyrinth

O Steiner redlich zone

ObSteiner redlich zone

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Obersteiner-Redlich Zone

Definition

The Obersteiner-Redlich (O-R) zone marks the morphological junction between the central nervous system (CNS) and the peripheral nervous system (PNS) along a nerve root or cranial nerve. It is the point where:

Central myelin (produced by oligodendrocytes) ends
Peripheral myelin (produced by Schwann cells) begins

The zone is named after Heinrich Obersteiner and Emil Redlich, who described it in the context of posterior nerve roots in the late 19th century.

Structural Basis

Feature	CNS side (central to O-R zone)	PNS side (peripheral to O-R zone)
Myelinating cell	Oligodendrocyte	Schwann cell
One cell wraps	Multiple axons	One axon (if myelinated)
Connective tissue	Absent (only delicate arachnoid)	Endoneurium, perineurium, epineurium
Vulnerability	Less to trauma, more to demyelinating disease	More mechanically robust

At the O-R zone itself, myelin is so thin that the fibers appear almost unmyelinated - creating a structurally weak transition point.

Location on Different Nerves

Nerve	Location of O-R zone
Spinal nerve roots	Just past the emergence of the posterior root from the spinal cord
CN VII (Facial)	A few mm from the brainstem surface, within the cisternal/CPA segment; distinct from but near the root entry/exit zone (REZ)
CN VIII (Vestibulocochlear)	Within the internal auditory canal (IAC), near the fundus
CN V (Trigeminal)	Near the porus trigeminus (Gasserian ganglion region)

For the facial nerve, both the REZ and the more distal O-R zone are surgical targets in microvascular decompression for hemifacial spasm.

Clinical Significance

1. Site of Vestibular Schwannoma Origin (Classical Teaching)

The classical teaching holds that vestibular schwannomas arise at the O-R zone of the vestibular nerve, where Schwann cells accumulate at the glial-Schwann junction. "Whorl-like" Schwann cell nests, eosinophilic bodies, and ganglion cells have been described here histologically (Prisig et al.), and these nests were proposed as forerunners of vestibular schwannomas.

However, this is now contested: imaging and surgical observations of early tumors show they more frequently originate near the fundus of the IAC, lateral to the O-R zone (i.e., in the more peripheral part of the IAC). The inferior vestibular nerve is now recognized as the predominant site of origin (up to 94% in some reports), not necessarily the O-R zone itself.

2. Hemifacial Spasm

Vascular compression of CN VII at or near the REZ/O-R zone (the transition between central and peripheral myelination) is the most common cause of hemifacial spasm. The central myelin at this region is less resistant to vascular pulsation, making it particularly vulnerable.

3. Demyelinating and Infectious Disease

The O-R zone in posterior spinal roots is a site of predilection for immunologic diseases - for example, the immune-mediated demyelination seen in the late stages of syphilis (tabes dorsalis) targets this thin-myelin transition zone.

4. Surgical Relevance

In CPA surgery (e.g., vestibular schwannoma removal via retrosigmoid approach), the CPA portion of CN VII and CN VIII is covered only by delicate arachnoid with no robust connective tissue sheath - rendering the nerves extremely vulnerable to surgical trauma and difficult to identify during tumor dissection.

Sources:

THIEME Atlas of Anatomy - General Anatomy and Musculoskeletal System - "Obersteiner-Redlich zone in a posterior root"
Scott-Brown's Otorhinolaryngology, Vol. 2 - "Intraparenchymal elements, organization and course" (facial nerve chapter)
Shambaugh Surgery of the Ear - "Pathology" (vestibular schwannoma chapter)
Localization in Clinical Neurology, 8e - "Peripheral nerve lesions and the cerebellopontine angle syndrome"; "Hemifacial spasm"

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