CP angle Tumors

I. DEFINITION AND OVERVIEW

• Definition: The Cerebellopontine (CP) angle is a potential space in the posterior cranial fossa filled with cerebrospinal fluid (CSF), bounded by the pons, cerebellum, and the petrous bone.
• Common Pathologies: * Acoustic Neuroma (Vestibular Schwannoma): ~80-90%.
  • Meningioma: ~5-10%.
  • Epidermoid Cysts: ~5-7%.
  • Others: Facial nerve schwannomas, arachnoid cysts, glomus tumors, and metastatic lesions.
• Pediatric Relevance: Rare in children; when present, it is strongly associated with Neurofibromatosis Type 2 (NF-2), often manifesting as bilateral vestibular schwannomas.

II. ANATOMY OF THE CEREBELLOPONTINE ANGLE

Understanding the signs requires a detailed knowledge of the boundaries and contents of this space: * Anterolateral: Posterior surface of the petrous temporal bone. * Posteromedial: Anterolateral surface of the cerebellum and the pons. * Superior: Tentorium cerebelli. * Inferior: Lower cranial nerves and the jugular foramen. 2. Contents (Key Neurovascular Structures): * Cranial Nerves: V (Trigeminal), VI (Abducens), VII (Facial), VIII (Vestibulocochlear), and occasionally IX, X, XI in large tumors. * Vessels: Anterior Inferior Cerebellar Artery (AICA), Superior Cerebellar Artery (SCA), and the petrosal vein (Dandy’s vein). * Brainstem Components: Pons and Medulla. * Cerebellar Components: Flocculus and Cerebellar peduncles.

III. CLINICAL STAGING OF CP ANGLE TUMORS

Symptoms typically progress in a predictable sequence based on the “Centrifugal Growth” of the tumor:

1. Stage I (Intracanalicular Stage): Symptoms limited to CN VIII (hearing loss, tinnitus).
2. Stage II (Cisternal Stage): Tumor exits the internal auditory meatus; involvement of CN V and VII.
3. Stage III (Brainstem Compression Stage): Cerebellar signs and long tract signs.
4. Stage IV (Hydrocephalic Stage): Obstruction of the 4th ventricle leading to increased intracranial pressure (ICP).

IV. ANATOMICAL CORRELATION OF SIGNS AND SYMPTOMS

A. Vestibulocochlear Nerve (CN VIII) Involvement

• Cochlear Component:
  • Progressive Sensorineural Hearing Loss (SNHL): Most common presenting symptom (95%). Specifically, high-frequency loss with poor speech discrimination (Retrocochlear pattern).
  • Tinnitus: Often the earliest symptom; typically unilateral and high-pitched.
• Vestibular Component:
  • Disequilibrium/Unsteadiness: Slow-growing tumors allow for central compensation, so true “whirling” vertigo is rare.
  • Vertigo: More common in small, rapidly growing tumors.

B. Trigeminal Nerve (CN V) Involvement

• Anatomy: The nerve lies superior to the acoustic-facial bundle.
• Signs/Symptoms:
  • Loss of Corneal Reflex: One of the earliest and most reliable signs of CP angle extension (compression of the ophthalmic division).
  • Facial Numbness/Paresthesia: Typically in the V2 (maxillary) and V3 (mandibular) distribution.
  • Weakness of Mastication: Only in very large tumors (motor root involvement).

C. Facial Nerve (CN VII) Involvement

• Anatomy: CN VII is remarkably resistant to pressure despite being adjacent to CN VIII.
• Signs/Symptoms:
  • Hitselberger’s Sign: Hypesthesia of the posterior meatal wall (involvement of the sensory branch of CN VII).
  • Facial Nerve Paresis: LMN type facial weakness (late sign in schwannomas, earlier in meningiomas).
  • Taste Alteration: Involvement of chorda tympani leads to loss of taste on the anterior two-thirds of the tongue.
  • Reduced Lacrimation: Involvement of the greater superficial petrosal nerve (GSPN).

D. Cerebellar and Brainstem Involvement

• Cerebellar Signs: Due to compression of the flocculus or inferior cerebellar peduncle.
  • Ipsilateral ataxia, dysmetria (past-pointing), and dysdiadochokinesia.
  • Nystagmus: Usually “Bruns’ Nystagmus” (coarse, slow nystagmus when looking toward the lesion; rapid, fine nystagmus when looking away).
• Brainstem/Long Tract Signs:
  • Contralateral hemiparesis or hemisensory loss due to compression of the corticospinal or spinothalamic tracts in the pons/medulla.

E. Lower Cranial Nerve Involvement (CN IX, X, XI)

• Occurs with inferior expansion of the tumor.
• Symptoms: Dysphagia, hoarseness (vocal cord palsy), and diminished gag reflex.

F. Increased Intracranial Pressure (ICP)

• Mechanism: Dislocation of the brainstem causes compression of the Aqueduct of Sylvius or the 4th ventricle, leading to obstructive hydrocephalus.
• Triad: Headache (worse in the morning), nausea/vomiting, and papilledema.
• Abducens Nerve (CN VI): Diplopia due to lateral rectus palsy (often a “false localizing sign” from increased ICP).

V. INVESTIGATIVE CORRELATION

1. Audiometry: Pure Tone Audiometry (SNHL), Speech Audiometry (Roll-over phenomenon), and Brainstem Auditory Evoked Potentials (BAEP) showing increased I-V latency.
2. Vestibular Testing: Caloric testing shows canal paresis on the affected side.
3. Imaging (Gold Standard):
   • MRI with Gadolinium: “Ice-cream cone” appearance (Intracanalicular component is the cone, cisternal component is the ice cream).
   • CT Scan: Useful for viewing bone erosion of the internal auditory canal (IAC).

VI. MANAGEMENT PRINCIPLES

1. Observation: “Wait and scan” for small tumors in asymptomatic patients.
2. Stereotactic Radiosurgery (Gamma Knife): For tumors <3 cm or in patients unfit for surgery.
3. Surgical Excision:
   • Translabyrinthine Approach: Best for hearing preservation not possible; direct access to IAC.
   • Retrosigmoid (Suboccipital) Approach: For larger tumors; offers chance of hearing preservation.
   • Middle Cranial Fossa Approach: For small intracanalicular tumors where hearing preservation is priority.