Cerebellopontine angle syndrome


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]

Contents

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]

Radiographic Findings

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:

   * Elderly patients
   * Patients with small tumors, especially if their hearing is good
   * Patients with medical conditions that significantly increase the risk of operation
   * Patients who refuse treatment
   * Patients with a tumor on the side of an only hearing ear or only seeing eye
         o In a number of series reported to date, the individuals who are being observed ultimately require therapeutic intervention in between 15-40%.
         o During an observation period, most (70% or more) patients who are eligible for hearing conservation surgery initially lost their eligibility.
         o 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.

Stereotactic radiotherapy

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

   * Stereotactic radiation therapy makes use of one of several radiation sources and is administered using a variety of different machines with proprietary names (e.g., Gamma Knife, CyberKnife, BrainLAB).
   * Stereotactic therapy uses radiation delivered to a precise point or series of points to maximize the amount of radiation delivered to target tissues while minimizing the exposure of adjacent normal tissues. It can be delivered as a single dose or as multiple fractionated doses.
   * The effects of radiation delivered at the current low dose likely prevents further tumor growth by causing obliterative endarteritis of the vessels supplying the tumor. Radiosurgery may affect tumor cells undergoing mitosis by causing double strand DNA breaks. Hansen et al. demonstrated acoustic neuroma cells are radioresistant at the current low-dose radiation used with radiosurgery.3
   * Comparison of microsurgery and stereotactic radiation is difficult for the following reasons:
         o Tumor size is inconsistently reported in the literature.
         o Data using the lower radiation dosages are available for only the past 10 years.
         o Because the goal of radiotherapy is control of tumor growth, understanding whether posttreatment neuroimaging reflects adequate treatment or merely the natural history of vestibular schwannomas is difficult.
         o No data concerning the risk for secondary tumor induction by radiotherapy are available.
   * Advantages of radiation therapy include the following:
         o Decreased length of stay
         o Decreased cost
         o Rapid return to full employment
         o Lower immediate posttreatment morbidity and mortality
   * Disadvantages of stereotactic radiation include the following:
         o Necessity for regular monitoring and frequent rescanning (In the end, costs associated with long-term monitoring could exceed those of surgery.)
         o Does not eliminate the tumor and may fail to control tumor growth, sometimes requiring salvage surgery.
         o Higher incidence of trigeminal nerve injury.
         o Unknown long-term incidence of secondary malignancies. The best current estimates of developing a secondary malignancy from the radiosurgery are 1 in a 1000 patients over 30 years.
         o Does not address disequilibrium and may lead to long-term balance dysfunction.

Surgical Therapy

Surgical removal remains the treatment of choice for tumor eradication. Various surgical approaches can be used to remove acoustic tumors. Each approach is discussed in detail in the following sections. Preoperative Details

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

   * The retrosigmoid approach can be applied to all acoustic tumors and to many other histologic tumor types. It can be used for operations that sacrifice hearing and operations that attempt to conserve hearing. Its only limitation in this respect is its inapplicability for small tumors that occupy the far-lateral portions of the internal auditory canal.
   * The retrosigmoid approach provides the best wide-field visualization of the posterior fossa. The inferior portions of the cerebellopontine angle and the posterior surface of the temporal bone anterior to the porus acusticus are much more clearly observed than via the translabyrinthine approach. Panoramic visualization is especially helpful when displacement of nerves is not predictable, which occurs commonly with meningiomas.
   * Hearing conservation surgery can be attempted even for relatively large tumors via the retrosigmoid approach. Destruction of the labyrinth is not required as part of the retrosigmoid approach.

Disadvantages of the retrosigmoid approach

   * The retrosigmoid approach may require cerebellar retraction or resection. Manipulation of the cerebellum provides opportunities for postoperative edema, hematoma, infarction, and bleeding.
   * Increased incidence of cerebrospinal fluid leak occurred in some series.
   * The retrosigmoid approach is associated with greater likelihood of severe protracted postoperative headache.
   * The highest incidence of tumor recurrence or persistence occurs with retrosigmoid approaches.

Advantages of the translabyrinthine approach

   * The translabyrinthine approach provides the best view of the lateral brain stem facing the acoustic tumor.
   * Retraction of the cerebellum is almost never necessary.
   * The fundus and lateral end of the internal auditory canal are completely exposed; the facial nerve can be identified at a location where it is undistorted by tumor growth and compressed into the labyrinthine segment, decreasing the risk of delayed postoperative facial nerve palsy.
   * Incidence of cerebrospinal fluid leak is decreased in some series.
   * If the facial nerve has been divided or sacrificed, the translabyrinthine approach may allow restoration of the facial nerve continuity by rerouting the facial nerve and performing a primary anastomosis. Consequently, interposition graft can sometimes be avoided.
   * Facial function is more frequently preserved in some series.

Disadvantages of the translabyrinthine approach

   * Hearing sacrifice is complete and unavoidable.
   * The inferior portions of the cerebellopontine angle and cranial nerves are not as well visualized as they are in the retrosigmoid approach. The temporal bone anterior to the porus acusticus is also less well visualized.
   * A fat graft is required. Removal of fat from the abdomen creates opportunities for donor site complications, including hematoma, bleeding, and infection.
   * The sigmoid sinus is more vulnerable to injury. Bleeding from the sigmoid sinus can be difficult to control and can significantly increase operative blood loss. If a dominant sigmoid sinus is occluded during the operation, postoperative intracranial pressure elevation or venous infarct can occur.
   * A high jugular bulb or anteriorly placed sigmoid sinus can substantially compromise the space available for tumor removal. Occasionally, the space is so contracted that another approach has to be selected.

Advantages of the middle cranial fossa approach

   * It is the only procedure that fully exposes the lateral third of the internal auditory canal without sacrificing hearing.
   * It is extradural.

Disadvantages of the middle cranial fossa approach

   * The facial nerve generally courses across the anterior superior portion of the tumor. Consequently, it is in the way during tumor removal and is more vulnerable to injury. Although long-term facial nerve outcomes are as good with the middle cranial fossa approach as with other approaches, temporary postoperative paresis is more common.
   * The risk of dural laceration and avulsion becomes increasingly more likely as patients become older. The dura mater in elderly patients is more friable. This becomes especially noticeable during the sixth and seventh decades of life.
   * The approach provides only very limited exposure of the posterior fossa.
   * The operation is technically difficult and demanding.
   * Some patients incur postoperative trismus related to manipulation and/or injury to the temporalis muscle.
   * The temporal lobe must be retracted, presenting the opportunity for temporal lobe injury, usually in the form of a hematoma that is asymptomatic and, therefore, probably occurs more frequently than is realized. Scattered reports exist of seizure disorder following middle cranial fossa surgery, presumably due to temporal lobe injury.

Approach Selection

A variety of different considerations go into deciding which approach should be used for any individual patient. These variables are detailed below.

Preoperative hearing level

If the patient has no useful hearing, either the translabyrinthine or the retrosigmoid approach is selected, depending upon the experience and training of the surgeon. In most centers performing large numbers of surgeries for acoustic tumors, the translabyrinthine approach is preferred. Opinions vary considerably about what constitutes useful hearing. The 50/50 rule is frequently quoted. The rule suggests that individuals with a pure-tone average greater than 50 dB and speech discrimination less than 50% do not have useful or salvageable hearing. Other surgeons have stricter criteria and consider only individuals with better than a 30-dB pure-tone average and more than 70% discrimination for hearing conservation operations.

Auditory brainstem response

Normal preoperative ABR findings favor hearing conservation. Marked abnormalities of ABR wave morphology or increased wave I-III and I-V latencies make hearing conservation less feasible.

Electronystagmography

An abnormal caloric test on electronystagmography (ENG) increases the likelihood of successful hearing conservation surgery. The ENG tests the horizontal semicircular canal, which is innervated by the superior vestibular nerve. A normal ENG finding arguably demonstrates that the superior vestibular nerve is normal. Consequently, the acoustic tumor must have originated from the inferior vestibular nerve, which is directly adjacent to the cochlear nerve. Surgical removal, then, is more likely to directly injure the cochlear nerve or interfere with cochlear blood supply. Vestibular evoked myogenic potential (VEMP) testing is abnormal when the inferior vestibular nerve is affected. As a result, an abnormal VEMP with normal caloric testing on ENG strongly suggests an inferior vestibular nerve tumor with poorer hearing preservation.

Tumor size

Opportunities for hearing conservation decrease as tumors become larger. Hearing is much more difficult to conserve when tumors are 1.5-2.0 cm in diameter than if they are small intracanalicular tumors. Consequently, some surgeons limit hearing conservation surgery to smaller tumors, preferring to use a translabyrinthine approach to maximize the chance of facial nerve conservation for larger tumors.

Tumor position

If hearing conservation is to be attempted and the tumor lies within the lateral portions of the internal auditory canal, many surgeons prefer a middle fossa approach. The middle fossa approach permits direct exposure of the lateral end of the internal auditory canal without sacrificing hearing. The approach is frequently used for any tumor lying completely within the internal auditory canal, although tumors limited to the medial portions of the internal auditory canal can be managed using a retrosigmoid approach. Some surgeons extend the use of the middle fossa technique to include tumors that extend as much as 0.5-1.0 cm into the cerebellopontine angle. Division of the superior petrosal sinus may be required to gain sufficient access to the posterior fossa with larger tumors.

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.

Relevant anatomy

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

   * High-riding jugular bulb: In some individuals, the jugular bulb may actually ride up above the level of the inferior internal auditory canal.
   * Anteriorly placed sigmoid sinus: In such circumstances, the distance between the sigmoid sinus and the external auditory canal may be a few millimeters or less. Such a dramatic limitation of the space within which the surgeon has to operate not only makes a successful tumor extirpation much more difficult but puts the facial nerve and the displaced sinus itself at significantly increased risk of injury.
   * Contracted sclerotic mastoid: Such mastoid cavities provide little room for tumor removal. Moreover, they are often associated with suppurative otitis media, in itself a contraindication to the translabyrinthine approach.
   * Reduced or absent flow in the contralateral sinus: Previous operation, trauma, congenital anomalus development, and previous or concurrent disease can all result in markedly reduced or absent venous outflow through the contralateral sinus. In such cases, consideration may be given to a retrosigmoid approach merely because it reduces the risk of injury to the remaining sinus, occlusion of which would result in catastrophic venous infarction.

Surgeon preference

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 someone who is familiar with the appropriate approach.

Patient preference

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. 

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