Vestibular Schwannoma: Anatomy, Causes & Pathology Explained

Vestibular schwannoma, historically documented since 1777, saw key milestones, including clinical descriptions by Sir Charles Bell in 1830 and surgical attempts in 1903 by Krause. Approaches were further developed by Sir William House in 1960 and Rand and Kruze identified the suboccipital transmeatal route.

 Introduction 

Vestibular schwannoma, arising from proliferative schwann cells, is a common cerebellopontine angle tumor. The 1992 NIH Consensus Conference established the term. Incidence is 8-27 tumors/million, affecting those in their 4th-5th decade, more common in women, associated with genetic mutations, and possibly linked to radiation exposure. 

Anatomy 

The cerebellopontine angle is a triangular region, including structures like the lateral surface of the temporal bone, cerebellar hemisphere, pons, trigeminal nerve, and lower cranial nerves. Important anatomical relations involve the internal acoustic meatus contents: AICA, internal auditory artery, subarcuate arteries, facial nerve, and vestibulocochlear nerve. 

Pathology

Schwannomas mainly originate within the internal auditory meatus, with the superior vestibular nerve being a common site. The tumor causes progressive growth, leading to pressure effects on various nerves and structures, including the cochlear and vestibular nerves, trigeminal nerve, brainstem, and cerebellum.

Histological Appearance 

Vestibular schwannomas exhibit Antoni types A and B cells. Classification by size was established in a consensus meeting, providing a basis for reporting systems.

Clinical Presentation 

Involves 5 stages: Otological, Trigeminal, Brainstem and Cerebellar Involvement, Increasing Intracranial Pressure, and Terminal Stage. 

The progression of vestibular schwannoma through various stages presents a spectrum of symptoms and challenges for both patients and healthcare professionals. Let's delve deeper into the specific characteristics of each stage, examination procedures, and associated clinical manifestations.

1.Otological Stage 

Lesions: Intrameatal lesions and extrameatal tumors up to 2 centimeters. Common symptoms: Unilateral hearing loss, Tinnitus (non-pulsatile), gradual and progressive hearing loss with poor speech discrimination., Imbalance.   Nerve Involvement: obvious weakness is uncommon. Pain, pressure, or numbness around the ears. Altered lacrimation and defective nasolacrimal reflex (Nervus Intermedius involvement).

2.Trigeminal Nerve Involvement 

Sensory Changes: Earliest changes occur in the cornea when the tumor size is 2 to 2.5 centimeters. Tingling or numbness in the three divisions of the trigeminal nerve. One and a half to two years interval between the otological and trigeminal stages.

3& 4. Stage of Brainstem and Cerebellar Involvement:

Clinical manifestations include: Ataxia, Dysmetria, Dysdiadochokinesia, Gait disturbances, Nystagmus.  Increasing Intracranial Pressure Symptoms indicative of elevated intracranial pressure: Headache, Nausea, Vomiting, Neck pain, Falling vision due to papilloedema, Head posture and Titubation.

5.Terminal Stage

The terminal stage occurs due to the failure of vital centers in the brainstem.

Examination 

General Examination:

• Presence of cutaneous lesions associated with neurofibromatosis, known as Cafe au lait spots.

Ears:

• Presence of coexisting chronic middle ear diseases. Possibility of primary cholesteatoma or Glomus Jugulare tumor.

Trigeminal Nerve Examination:

• Corneal sensation, pin prick, and fine-touch assessment over the region of the tongue.

Facial Nerve Examination

• Difficulty in closing eyes.
• Presence of Hitzelberger’s sign.
• Conducting Schirmer’s test to check for lacrimation.
• Assessing the nasolacrimal reflex.
• Evaluating palatal or pharyngeal sensation or mobility.

In summary, a thorough understanding of the stages and associated symptoms of vestibular schwannoma is crucial for timely diagnosis and intervention. Comprehensive examinations, including trigeminal and facial nerve assessments, contribute to a holistic approach in managing this complex condition.

Investigations

The primary objective is to diagnose vestibular schwannoma at the otological stage when the tumor is confined to the internal acoustic meatus. A high degree of suspicion is crucial, and every case of unilateral sensorineural hearing loss, tinnitus, or vestibular hypofunction should be thoroughly investigated to exclude acoustic neuroma.

Audiological Investigations 

1. Pure Tone Threshold
   • No characteristic curve. Exhibits high frequency, mid frequency, and normal hearing. Presents an asymmetric sensorineural hearing loss.
2. Speech Discrimination
   • Disproportionately poor compared to the pure tone threshold. Lack of standardization poses limitations.

1. Performance Intensity Function for Phonetically Balanced Words (PIPB) Test
   • Evaluates speech discrimination at progressively higher sensation levels. Speech discrimination score should increase with the presentation level until a maximum score is reached. The rollover phenomenon, where discrimination decreases with additional increases in presentation level, indicates a retrocochlear lesion.
2. Loudness Recruitment
   • Abnormally steep growth of loudness with increasing intensity. Associated with sensorineural deafness due to cochlear pathology. Tests like the alternate binaural loudness balance and short increment sensitivity index (SISI) help differentiate neural from cochlear lesions. SISI test scores 0 to 20 percent in 70 to 90 percent of cases.
3. Auditory Adaptation
   • In cochlear pathologies, sound presented just above the threshold becomes inaudible after a short period (normal). In nuclear lesions, there is an increased speed of adaptation, resulting in a high number of false positives.

Stapedial Reflex Measurements 

Two parameters for stapedial reflex measurements: reflex threshold and rate of decay. Reflex threshold is elevated in retrocochlear lesions, with a significant elevation indicating abnormality. Reflex asymmetry, a difference of more than 15 Decibels between two ears, is considered abnormal.

Stapedial Reflex Decay 

Decline in amplitude of the reflex on prolonged stimulation. Increased in individuals with neural pathology. Pathological response amplitude declines by more than 50 percent in 5 seconds at 500 Hertz and 1000 Hertz. Impaired conduction in the cochlear nerve may result from demyelination, impaired blood supply, or abnormality of axoplasmic flow.

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Electric Response Audiometry 

Also known as BERA (Brainstem Evoked Response Audiometry). Electrocochleography measures fluid pressure or endolymphatic pressure in the inner ear. A ratio of summation potential to action potential less than 30 percent (0.3) suggests increased inner ear lymphatic pressure, indicative of Meniere’s disease. Auditory Brainstem Response (ABR) involves five waves, each originating from specific auditory structures.

Auditory Brainstem Response 

Single most reliable audiological indicator. Sound waves entering the cochlea are transduced into electrical potential transmitted via the eighth nerve to the brainstem, passing through relay stations. 90 percent sensitivity for diagnosing vestibular schwannoma. A latency difference greater than 0.2 meters per second or a prolonged interwave time interval suggests retrocochlear disorders. The complete absence of wave five indicates retrocochlear pathology, with a limitation that the patient must have no greater than a 70 Decibel threshold.

Vestibular Studies 

Caloric testing reveals decreased vestibular function ipsilateral to sensorineural loss. Bithermal caloric testing using the Hallpike and Cairns technique. Electronystagmography detects central vestibular dysfunction in the presence of a large tumor.

Radiological Investigations 

MRI scan with contrast is the investigation of choice, detecting intracanalicular lesions less than 2 mm in size. CT scan establishes limits of normal anatomy of the internal auditory canal. Abnormalities in the internal auditory canal include enlargement, shortening of the posterior wall, and displacement of the crista. MRI with Gadolinium contrast is the gold standard, providing advantages such as high intrinsic contrast, absence of bone artifacts, and avoidance of ionizing radiation. The characteristic "ice cream cone appearance" is observed in imaging.

Treatment 

Treatment options include observation, surgery (translabyrinthine, middle fossa, retrosigmoid approaches), and stereotactic radiosurgery (Gamma Knife). Radio surgery aims to stop tumor growth, offering a minimally invasive alternative with functional hearing preservation in many cases. 

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Complications 

Complications, both intraoperative and postoperative, include cranial nerve injury, bleeding, brain edema, venous air embolism, cardiac arrhythmias, hemorrhage, infarctions, cerebrospinal fluid leak, meningitis, and tension pneumocephalus. Non-Acoustic Tumors of Cerebellopontine Angle The section discusses non-acoustic tumors in the cerebellopontine angle, providing a broader context beyond vestibular schwannoma.

Hope you found this blog helpful for your ENT Residency Otology Preparation. For more informative and interesting posts like these, keep reading PrepLadder’s blogs. 

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