Acoustic Neuroma

Acoustic Neuroma

Definition

• A benign, slow growing, well encapsulated tumor that arises from the vestibular portion of the VIIIth cranial nerve and usually expands the internal acoustic meatus and spills into the cerebellopontine angle.

• 1910, Henschen's mortality rate was 85%. In 1932, Cushing's mortality was 4%.

Epidemiology

• Women:Men :: 3:2. Usually occur from ages 35-55 years.

• 8-10% of tumors in most series. Incidence of 1/100,000 per year. Result in 2280 new cases per year in US. 95% are unilateral.

Pathology

Genetics

• Part of Neurofibromatosis II (bilateral acoustic neuromas). Cytologically identical to the sporadic cases, although may be somewhat more infiltrative into the nerve fibers.

Pathophysiology

• A predilection for sensory nerves, especially the VIIIth cranial nerve (superior vestibular division). Less commonly, the Vth > VIIth >>> IXth, Xth, and IXth cranial nerves. Extremely rarely, schwannomas arise from within the parenchyma of the cerebrum, cerebellum, or brainstem presumably originating from small perivascular nerves.

• The human acoustic nerve is about 18mm long. Its proximal 8-12mm are endowed with glia. The distal portion has an endoneurium, epineurium and perineurium like a regular peripheral nerve. Only upon entry through the porus acousticus does the nerve acquire a vestment of Schwann cells. It is at the interface between stroma of oligodendroglia and Schwann cells that neoplastic transformation occurs. Characteristically, acoustic schwannomas arise in the vestibular division of VIII within the porous acousticus. Early enlargement compresses the cochlear nerve and labyrinthine blood vessels which supply the organ of Corti. The neoplasm expands and erodes the porus acousticus. The mass then intrudes from its bony canal into the CP angle.

• The pattern of growth is characteristic. The facial nerve tends to be stretched over the top of the tumor while the trigeminal nerve is compressed as it exits from the lateral pons. Caudal extension brings it into contact with the IX-XIth nerves and medial growth may contact the VIth nerve. Cerebellar herniation follows pronounced growth; hydrocephalus can follow.

Gross

• A lobulated mass on whose surface the parent nerve is stretched. The neoplasm is gray-yellow with flecks of xanthomatous degeneration, areas rich in lipid. It may be 3 - 6 cm in size before detection. Often has an investment of arachnoid around it, appearing encapsulated and giving rise to cysts around the tumor.

Microscopic

• Two characteristic patterns of growth.

1. Antoni A. Aligned, compact, variable prominent interwoven fascicles of Schwann cells

2. Antoni B. Porous and less structured; its cells are clustered about blood vessels, microcysts, xanthomatous cells, and sites of antecedent hemorrhage.

• Cells align into rows and groups creating formations called Verocay bodies. Hyalinized vessels are seen.

• Malignant transformation is rare, perhaps nonexistant. Occasionally necrosis on the basis of meager blood supply.

• Melanotic variants have been described.

• Major differential diagnosis is with meningioma. Differentiation is on the basis of immunohistochemistry. Schwannomas are S-100 positive and EMA negative while meningiomas are EMA positive and 20% are S-100 positive.

• Basement membrane of the schwann cells can be seen. Sometimes will see Luce bodies (racoon tails), consisting of long-spaced collagen. Differentiate from mening on basis of Intracytoplasmic intermediate filaments, desmosomes, hemidesmosomes, gap junctions, which are classic for all meningiomas and absent in schwannomas.

Clinical Features

Symptoms

• Are closely correlated with tumor size. Virtually all tumors eventually present with otologic symptoms. As the tumor enlarges (>2cm), patients develop Vth nerve findings. Larger tumors will produce brainstem compression.

Tinnitus is the most common initial symptom (70%).

Vertigo (67%).

Unilateral hearing loss, progressive (>90 %). Most common complaint is difficulties with speech discrimination (phone conversations).

Facial nerve palsy is seen late (10%).

Trigeminal nerve involvement similarly late; rarely cause tic, but check for corneal reflex! facial numbness in 30%.

Cerebellar signs late.

Papilledema, hydrocephalus late.

A tumor that gets large enough can cause spontaneous thrombosis if the internal auditory artery, which causes sudden, complete loss of auditory and vestibular function.

Signs

• 66% of patients have no abnormal findings except hearing loss. Rinne is normal (AC/BC). Weber lateralizes to contralateral ear.

• Abnormal corneal reflex (33%), nystagmus 25%, facial numbness 25%, facial weakness 10%, papilledema 10%, Babinski 5%.

• Meningiomas usually arise from the anterior-superior edge of the IAC, and thus, clinically, may have slightly different presentation, particularly, with earlier VIIth nerve signs, later hearing loss, more frequent tic-like pain (Vth nerve involvement), calcification, and bony hypertrophy on imaging.

Laboratory and Imaging

Auditory

Neurotologic work-up for retrocochlear lesion (i.e acoustic neuroma).

Pure-tone air conduction Elevated thresholds.
Pure-tone bone conduction elevated, equal to pure-tone air.
Speech discrimination moderate to severe loss, out of proportion to pure-tone loss.
Tympanometry normal.
Static compliance normal.
Acoustic reflex (stapes contract) absent or decayed.
Brain stem evoked response delayed - best test.

• The usual neuroma would present as unilateral sensorineural hearing loss with discrimination impariment out of proportion to pure-tone thresholds, normal tympanometry and compliance, absent acoustic reflex or with decay, and positive (delayed) BSAER (brainstem auditory evoked response).

• Most common BERA finding are prolonged I-III and I-V interpeak latencies. <5% false negatives. 85% specificity in distinguishing acoustic neuroma (retrolabyrinthine) from Meniere's disease (labyrinthine) which has normal latencies.

• Note that audiometric studies can only be performed if the affected ear has useful hearing!

• Tumors enhance homogeneously with contrast. Calcification is rare. One third can ring enhancing. 3-5% of tumors will not enlarge the IAC (too small).

• Gas CT cisternography, affected side up, 5 ml air injected, gas will not enter the porous in cases where there is a small tumor.

• Metrizamide CT cisternography useful in analyzing the relationships between large CP angle masses and the brain stem (intra- vs extra-axial lesions).

MRI

• MRI with gadolinium is the most effective way to diagnose these lesions. Procedure of choice. 98% sensitivity and close to 0% false positive rate.

Angiography

• For vascular neoplasms only. Jugular venography if the diagnosis of a glomus jugulare tumor is suspected. Examine for invasion of the jugular vein.

Vestibular. Electronystagmography. Abnormal, not as reliable

Plain films. Widening, so-called trumpeting of the IAC (frontal, Towne's, Stenver's, submentovertex).

Tomograms. Subtle erosion of IAC.

Treatment and Results

Natural History

• These are very slow-growing tumors. Many stay the same size for many years. 6% are said to shrink with time following diagnosis. The intracranial schwannomas never turn malignant (no case reports in literature).

Treatment Options

1. Nothing

• Observation may be indicated in a number of situations

1. Elderly and/or medically unfit patients without progressive neurological symptoms or long-tract/brainstem/cerebellar signs.
2. Patients with bilateral acoustics with residual hearing on side of interest.
3. Any patients with deafness in contralateral ear.
4. Patients refusing treatment.

• Should be followed with gadolinium MRI, initially every six months.

2. VP Shunt

• Argument for shunting without operating on tumor may be made with any of the reasons given above.

3. Surgical Goals

1. Reduce or prevent neurological morbidity.
2. Preserve cranial nerves.
3. Preserve hearing in select cases.

4. Radiosurgery

• Radiosurgery was initially believed to be restricted to non-surgical candidates but with longer term results now available, its efficacy is widely accepted. Certainly, the results with smaller tumors and the lower doses (14-16 Gy), the tumor control rate exceeds 90%, usually with few cranial nerve complications. In general, the tumor must be <3cm.

• In Kondziolka's paper (NEJM 339:1426, 1998), the rate of tumor control was 98%, with 62% decreasing in size, 33% staying the same, and 9% increasing in size following radiosurgery. Normal facial nerve function was preserved in 79%, normal trigeminal nerve function in 73%. Hearing preservation occured in 47% of cases (15 of 32) who had useful hearing pre-treatment.

• Long term complications such as second malignancy remain poorly defined.

5. Combination of Above

Results

House-Brackmann Classification of VIIth Nerve Function

Grade	Description
1	Normal function.
2	Mild dysfunction. Complete eye closure. Normal symmetry at rest.
3	Moderate dysfunction. Complete eye closure. Noticeable asymmetry at rest.
4	Moderate-to-severe dysfunction. Incomplete eye closure, oabvious asymmetry.
5	Severe dysfunction. Incomplete eye closure, only twitch of gross motor movement.
6	Total paralysis.

• Up to grade 3 is considered acceptable. If VIIth nerve is left in continuity, it will recover within a year in majority of cases. If it isn't, it should be immediately repaired (nerve graft).

• VIIth nerve is pushed anteriorly in 75% of cases, but it can be anywhere.

• Aids to intraoperative VIIth nerve preservation include continuous nerve EMG recordings.

Gardener-Robertson Hearing Classification for VIIIth Nerve Function

Class	Description	Pure-tone Audiogram (dB)	Speech Discrim Score
I	good
II	serviceable	<50dB	>50%
III	nonserviceable
IV	poor
V	none

• Class I can use phone on affected side. Class II can localize sounds. Hearing class II and above is considered useful hearing. This constitutes only about 10-15% of patients in large series. Hearing can only be preserved if tumors are <1-1.5 cm

• Cochlear nerve is incorporated into the tumor mass in 90% of tumors.

Cranial Nerve Preservation Following Surgery

Tumor Size	VIIth Nerve	VIIIth Nerve
< 1 cm	95-100%	57%
1-2 cm	80-90%	33%
>2 cm	50-75%	6%

Operative Complications

1. Immediate

Anaesthetic. Air embolization.
Positional. C-spine injury, peripheral n. injuries.
Surgical. Hemorrhage, cranial nerve injury, vascular injury (injury to AICA, aka artery of death, may result in pontine infarct). Cerebellar swelling, retraction injury, occult hemorrhage (e.g., supratentorial, subdural).

2. Early

Post-op hemorrrhage, acute hydrocephalus, bacterial meningitis, CSF leak, aspiration (lower cranial nerve dysfunction).

3. Delayed

Hydrocephalus, meningitis, CSF leak, corneal injury, delayed VIIth nerve palsy, stroke, pseudomeningocoele, tumor recurrence.

Surgical Approaches

Retrosigmoid

• The 'standard' neurosurgical approach. The cerebellum is retracted to allow exposure of the posterolateral aspect of the tumor. The major theoretical advantage of this approach over the translabyrinthine approach is preservation of hearing.

Translabyrinthine

• Described by House in 1964 to expose the posterior fossa dura in the retromeatal trigone bounded by the sigmoid sinus, the jugular bulb and the superior petrosal sinus. Sacrifices hearing.

• Mastoidectomy and labyrinthectomy performed with high-speed drills. Facial nerve identified and preserved. Dura is opened, and tumor is removed piecemeal. Muscle, fat graft for closure.

• Pros: See VIIth nerve early with less risk to cerebellum, brainstem. Patients are less ill post-op.

• Cons: Hearing is lost. The exposure is limited, the procedure takes longer, and there is a greater chance of CSF leak.

• Described by House in 1961.

• It is an extradural subtemporal approach with microsurgical unroofing of the internal auditory canal. Limited to small intracanalicular tumors that have not gone beyond porus. It may spare hearing. Possible to follow the facial nerve back to the IAC. Bill's bar is the vertical crest of bone separating the facial nerve from the superior vestibular nerve.

Posterior Fossa Transmeatal
Suboccipital

References

Gardner G, Robertson JH. Hearing preservation in acoustic neuroma surgery. Ann Otol Rhinol Laryngol 97:55, 1988.

House JW, Brackmann DE. Facial nerve grading system. Otolaryngol Head Neck Surg 93:146, 1985.

Kondziolka D, Lunsford LD, McLaughlin MR, Flickinger JC. Long term outcomes after radiosurgery for acoustic neuromas. NEJM 339:1426, 1998. (A retrospective review of the Pittsburg group's experience with radiosurgery from 1987 to 1992. The follow up is greater than five years. The study results are impressive but the authors technique was changed about half way through this time period.)

Updated 05/02/2001