Cerebellopontine angle syndrome

Cerebellopontine angle syndrome
Classification and external resources
ICD-9 191.6 - Neoplasms of brain: Cerebellum NOS: Cerebellopontine angle

The cerebellopontine angle is the anatomic space between the cerebellum and the pons. This is a common site for the growth of acoustic neuromas or schwannomas. A distinct neurologic syndrome of deficits occurs due to the anatomic proximity of the cerebellopontine angle to specific cranial nerves.[1]

Anatomy

The cerebellopontine angle is a space filled with spinal fluid.

Signs and Symptoms

Lesions in the area of cerebellopontine angle cause signs and symptoms secondary to compression of nearby cranial nerves, including cranial nerve V, cranial nerve VII, and cranial nerve VIII.

For example, involvement of CN V from a cerebellopontine mass lesion often results in loss of the ipsilateral corneal reflex.

Patients with larger tumours can develop Bruns nystagmus due to compression of the flocculi.[2]

Causes

Cerebellopontine angle tumors

Treatment: Medical Therapy

Acoustic neuromas are managed in one of the following 3 ways: (1) surgical excision of the tumor, (2) arresting tumor growth using stereotactic radiation therapy, or (3) careful serial observation.

Observation

Simple observation without any therapeutic intervention has been used in the following groups of patients:

Telian has analyzed the important variables that should be evaluated when observation is considered, and these include the following:2

  1. preoperative hearing in both ears,
  2. the risk of immediate hearing loss as a consequence of surgery,
  3. the risk of facial nerve paralysis,
  4. the risk of other surgical complications and their seriousness,
  5. the patient's life expectancy,
  6. the size of the tumor,
  7. tumor growth rate, and
  8. patients with neurofibromatosis type 2 (NF2) or bilateral tumors.

Treatment: Stereotactic radiotherapy

Stereotactic radiotherapy has emerged within the last 20 years as an alternative to microsurgery for selected patients with acoustic neuroma.

Advantages of radiation therapy include decreased length of stay, decreased cost, rapid return to full employment, and lower immediate posttreatment morbidity and mortality.

Disadvantages of stereotactic radiation are:

Treatment: Surgical

Surgical removal remains the treatment of choice for tumor eradication. Three different approaches are used in the management of acoustic neuromas: the retrosigmoid, translabyrinthine, and middle fossa approaches. All have advantages and disadvantages as indicated below.

Advantages of the retrosigmoid approach

Disadvantages of the retrosigmoid approach

Advantages of the translabyrinthine approach

Disadvantages of the translabyrinthine approach

Advantages of the middle cranial fossa approach

Disadvantages of the middle cranial fossa approach

Deciding on a Surgical Approach

The following are important in deciding which approach should be used for any individual patient:

Generally, however, tumors that have significant volume medial to the plane of the porus acousticus are extirpated using a retrosigmoid approach if hearing is to be conserved. If hearing conservation is not an issue, the retrosigmoid approach is sometimes preferred for tumors with significant inferior extension since the lower cranial nerves are better visualized with a retrosigmoid approach. Occasionally, the retrosigmoid approach is combined with a translabyrinthine approach for such large acoustic neuromas.

The following anatomic variations can make the translabyrinthine approach much more difficult and at times impossible.

Some surgeons have more experience and are much more comfortable with one approach relative to another. Generally, such preferences should be followed. However, if hearing conservation is a realistic option using an approach unfamiliar to the primary surgeon, consideration should be given to referring the patient to another surgeon who is familiar with the appropriate approach.

Patient preferences should be carefully considered even when they do not conform to the surgeon's judgment. Some patients are adamant about going to any lengths for hearing conservation even when the treating physician is quite convinced that the patient's hearing is so poor as to be of little or no practical utility. Some patients willingly sacrifice even good hearing if doing so even slightly enhances the possibility of successful facial nerve preservation. Some patients have very clear-cut opinions about one type of incision versus another (sometimes based on cosmetic consideration). Intraoperative Details

Translabyrinthine approach

The translabyrinthine approach is the most versatile of the 3 common approaches to the cerebellopontine angle. The main disadvantage is profound deafness in the operated ear due to violation of the membranous labyrinth. In general, even the largest acoustic neuromas can be removed through a translabyrinthine craniotomy. In addition, the facial nerve is found at the fundus of the internal auditory canal where the vertical crest (Bill’s bar) provides a natural plane for facial nerve dissection from the superior vestibular nerve. At the author’s institution, the translabyrinthine approach is preferred with any acoustic neuroma over 2 cm or in an ear with poor hearing.

The patient is laid supine and a Mayfield head frame may be used. An incision is then made two finger-breadths from the postauricular sulcus. The temporalis muscle and mastoid periosteum are identified. The skin flap is then elevated anteriorly, leaving as much periosteum down as possible. The periosteum is then incised along the linea temporalis and then towards the mastoid tip in a T-shaped fashion. This will allow a water-tight second layer for closure to prevent postoperative cerebrospinal fluid leakage. The mastoid periosteum is then elevated from the underlying mastoid bone. Often, the emissary vein is encountered and this can be controlled with bipolar coagulation and/or bone wax.

A wide cortical mastoidectomy is performed. The middle and posterior fossa dura are identified as well as the sigmoid sinus. The bone is removed from these structures to allow retraction of the temporal lobe dura and sigmoid sinus. Next, the antrum, lateral semicircular canal, and vertical facial nerve are identified.

The incus is removed and a facial recess is performed. The in tensor tympani tendon is sectioned and the eustachian tube is packed with oxidized cellulose packing. The middle ear space is then packed with temporalis muscle.

A labyrinthectomy is performed and the jugular bulb is identified. The internal auditory canal is subsequently identified and troughs are developed both superiorly and inferiorly around the internal auditory canal until approximately 270° of internal auditory canal is exposed. The remaining bone is then removed from the internal auditory canal and the facial nerve is found as it turns into the labyrinthine segment. The superior vestibular nerve is then followed out to the ampullated end of the superior semicircular canal.

At this point, the transverse crest and vertical crest (Bill’s bar) are identified. The superior vestibular nerve is then reflected inferiorly from the ampullated end of the superior semicircular canal. The facial nerve can often be found superior medial to this and is confirmed using a facial nerve stimulator. At this point, the tumor is generally debulked and the facial nerve is located at the origin from the brain stem. Once the tumor is adequately debulked, the acoustic neuroma is then dissected from the facial nerve. Often, the facial nerve is very adherent to the acoustic neuroma around the porus of the internal auditory canal.

Once the tumor has been removed, the posterior fossa dura is then re-approximated. Fat is harvested from the abdomen and packed into the surgical defect. The periosteal and skin layers are closed in a water-tight fashion. The patient wears a pressure dressing for 3 days.

Retrosigmoid approach

The patient may be placed in the supine position on the operating table and with the head toward the contralateral shoulder. The true lateral or park-bench position is still used by some surgeons because it permits the occiput to be rotated a little bit more superiorly. This allows a slightly more direct view of the internal auditory canal.

The operation is performed through either a vertically oriented linear incision or an anteriorly based U-shaped flap. An occipital craniotomy is then performed. Any mastoid air cells are carefully waxed off to prevent postoperative cerebrospinal fluid leak. The dura is opened and the arachnoid incised. The cerebellum frequently falls away from the posterior surface of the temporal bone after the cisterna magna has been opened. Hyperventilation, steroids, and intraoperative diuretics (principally mannitol) are used to reduce intracranial pressure and to provide additional exposure with a limited amount of retraction. Nonetheless, gentle cerebellar retraction is occasionally required especially in larger tumors.

Once adequate exposure has been obtained, the tumor is clearly visualized along with the brain stem and lower cranial nerves. However, cranial nerves VII and VIII are rarely observed because they are almost always pushed forward and lie across the anterior surface of the tumor, which cannot be visualized. Debulking of the tumor is the next step and must be carefully performed so as to maintain the anterior portions of the capsule in order to prevent injury to cranial nerve VII and/or VIII. Once the tumor has been substantially debulked, the posterior wall of the internal auditory canal can be removed using a high-speed drill.

Great care must be taken to avoid injuring the labyrinth while removing the posterior wall of the internal auditory canal. Portions of the labyrinth quite commonly are medial to the lateral end of the internal auditory canal. Although no single anatomic landmark is completely reliable for prevention of injury to the labyrinth, the singular nerve and its canal, and the operculum of the vestibular aqueduct, are used as important surgical landmarks. Careful measurements taken from preoperative CT scans can provide useful information during drilling of the posterior wall of the internal auditory canal.

The length of the internal auditory canal varies considerably, and knowing exactly how much posterior canal wall needs to be removed to adequately expose the tumor can help limit inadvertent injury to the labyrinth. Blind extraction of tumor from the internal auditory canal without removing the posterior wall poses a significant risk to the facial and/or auditory nerve integrity and increases the chance of leaving tumor at the fundus. Use of intraoperative angled endoscopes has been reported as an adjunct in performing this phase of the operation.

Every effort should be made to prevent bone dust from entering the subarachnoid space during the intradural drilling of the internal auditory canal. One probable cause for severe and intractable postoperative headache is spillage of bone dust into the subarachnoid space during tumor removal. Surgicel, Gelfoam, Telfa pads, and/or cottonoid strips are placed around the operative site so that bone dust adheres to them and is removed as they are removed. Once the internal auditory canal is exposed, the dura is opened and the tumor is removed. Although never proven, dissection from medial to lateral is thought to be less traumatic to both the cochlear nerve and to the vascular supply of the inner ear. The vestibular nerves are generally sacrificed, and unless hearing is to be preserved, the cochlear nerve is sacrificed as well.

Eventually, the surgeon is left with the anterior portions of the capsule adhered to the brain stem and cranial nerve VII. As the tumor capsule is carefully removed from the brain stem, the root entry zone of cranial nerve VII can be identified. The capsule is then carefully removed from the facial nerve with as little trauma as possible.

The facial nerve monitor facilitates this portion of the dissection. A meaningful amount of data now shows that results are improved when facial nerve monitoring is employed. A variety of techniques have been used to monitor the cochlear nerve when hearing preservation is desired. The most commonly used method is intraoperative ABR, but it has a number of disadvantages. Most importantly, it requires summing a large number of repetitions in order to extract a response from background noise. Consequently, a delay occurs between surgical manipulations and ABR changes. Direct cochlear nerve monitoring offers the advantage of real-time feedback, but a fully satisfactory method of placing and securing the electrode still is lacking.

Once tumor removal is complete and hemostasis is absolute, the dura is closed and the craniotomy defect is repaired, either by replacing the original bone flap or with methylmethacrylate or hydroxyapatite.

Middle cranial fossa approach

Although some surgeons use an extended middle cranial fossa approach for tumors that extend a centimeter or more outside the porus acusticus into the cerebellopontine angle, the middle cranial fossa approach is most frequently used for intracanalicular tumors. It is, by consensus, the approach of choice for small tumors that lie within the lateral portions of the internal auditory canal when hearing conservation is desired.

The head must be in the true lateral position. In young individuals with a supple neck, this can often be accomplished by turning the head to the side with the patient in the supine position. But if neck mobility is limited or concern exists that forced head turning will limit posterior fossa circulation or aggravate cervical spine disorders, then a true lateral (park-bench) position should be used.

Exposure must be centered over a vertically oriented line that passes approximately 1 cm anterior to the external auditory meatus. This is most easily accomplished through a linear incision. A posteriorly based U-shaped or curvilinear S-shaped incision can be used if concern exists about scar contracture. Depending upon the incision used, the temporalis muscle is incised or reflected inferiorly. A temporal craniotomy (approximately 5 cm by 5 cm) is performed with its base at the root of the zygoma. The dura is elevated from the floor of the middle cranial fossa, and osmotic diuretics, head elevation, hyperventilation, and steroids are used to limit cerebral edema.

The dura of the temporal lobe is then elevated off the superior surface of the temporal bone. The anterior extent of such elevation is usually the foramen spinosum, but the middle meningeal artery can be divided between clips and elevation continued anteriorly to the foramen ovale if additional exposure is desired. Dural elevation should proceed from posterior to anterior to avoid injury to an exposed greater superficial petrosal nerve or geniculate ganglion. Bleeding from the veins associated with the middle meningeal artery is often quite brisk but can generally be controlled with oxidized cellulose packing. Medial dissection continues to the free edge of the temporal bone.

The superior petrosal sinus is attached to the posterior surface of the temporal bone but not always at its superior edge. Care must be taken to avoid injuring it. If inadvertent injury occurs, bleeding can generally be controlled with intraluminal oxidized cellulose packing, electrocautery, or hemoclips. When extended middle cranial fossa approaches are employed, the superior petrosal sinus is deliberately divided between clips.

When it can be identified easily, the arcuate eminence is an extremely helpful landmark. Careful drilling can often identify the blue line of the superior canal inferior to it. Because the most difficult exposure to achieve during middle fossa surgery is the lateral posterior end of the internal auditory canal, dissection is performed as close to the superior semicircular canal as possible. The greater superficial petrosal nerve is generally easy to visualize and can be followed retrograde to the geniculate ganglion. It lies approximately 1.0 cm directly medial to the foramen spinosum. Once the area of the geniculate is identified, small diamond burrs are used to completely expose it. If the greater superficial petrosal nerve cannot be located and no other landmarks are available, the middle ear space can be entered from above and the head of the malleus can be identified. The geniculate ganglion lies approximately 2–3 mm anterior and medial to the head of the malleus.

Once the geniculate ganglion has been completely exposed, the labyrinthine portion of the nerve can be identified and followed medially and inferiorly into the internal auditory canal. The labyrinthine portion of the nerve takes a markedly vertical and medial course as it moves from the lateral geniculate ganglion to the proximal fundus of the internal auditory canal, which lies 5 or more millimeters deep to the geniculate ganglion. Some surgeons prefer to identify the internal auditory canal medially. Once the medial end of the canal is completely identified, they follow the canal laterally to the fundus of the internal auditory canal.

The bone overlying the internal auditory canal should be removed until approximately 270 º of the internal auditory canal is exposed. The most difficult area to expose is the point at which the superior vestibular nerve penetrates the labyrinthine bone to innervate the ampulla; however, exposure in this area is critical if the anatomy of the lateral end of the internal auditory canal is to be well visualized. If the superior vestibular nerve channel is identified, tumor removal is generally successful and relatively straightforward.

Larger tumors frequently have the facial nerve splayed out over the anterior superior portions of the tumor. Tumor removal begins, as with other approaches, by careful debulking. Once the tumor is debulked, enough room is created within the internal auditory canal to carefully remove the tumor capsule from the inferior surface of the facial nerve. Again, care must be taken to avoid torsion or twisting of the nerve during tumor removal.

Once the tumor has been completely removed, the integrity of the facial nerve is tested using the intraoperative facial nerve monitor. Presumably, the monitor has been in use throughout the case. If the facial nerve can be stimulated with low stimulus intensities, chances of good postoperative facial nerve function increase. Fat is then packed into the internal auditory canal after using bone wax to fill obvious air cells to prevent postoperative cerebrospinal fluid leak. The facial nerve monitor generally alerts the physician if fat is being packed in too tightly that the integrity of the facial nerve is being compromised. Retractors are removed, and the temporal lobe dura is allowed to relax. The bone plate is replaced using miniplates, and the wound is closed in multiple layers.

See also

References

  1. ↑ Rolak LA. Neurology Secrets, 4th Ed. Chapter 10, "Cerebellar Disease." Elsevier.
  2. ↑ Nedzelski JM (October 1983). "Cerebellopontine angle tumors: bilateral flocculus compression as cause of associated oculomotor abnormalities". Laryngoscope 93 (10): 1251–60. PMID 6604857.