Schwannoma of the 6th nerve: case report and review of the literature

Intracranial schwannomas account for 6–8 % of all primary brain tumors as a kind of
benign tumor 1], 3]. They most commonly arise from the vestibular cranial nerves followed by the trigeminal
nerve, facial nerve, and lower cranial nerves; they rarely affect pure motor CN (3rd,
4th, 6th) 1]. Schwannomas of the 6th cranial nerve are exceptionally rare. In the English literature,
22 cases have been reported of intracranial schwannomas of the 6th cranial nerve pathologically
confirmed 4]–23] (Table 1), except 4 cases reported by Hayashi et al. 24] who performed only stereotactic radiosurgery. We report a patient with a small schwannoma
of the abducens nerve performed by surgical treatment based on a review of the literature.

Table 1. Cases of intracranial 6th nerve schwannomas published in the literature

Classification

According to the site where the schwannoma arises and its predominant location along
the course of the 6th nerve, Tung 8] and Celli 25] have proposed two kinds of classification. Celli 25] divided the tumor into three types concerned for the 3rd to the 5th nerve: type I
(cisternal, CI), 8 cases 4], 11]–14], 17], 21] (present case) out of 22; type II (cavernous, CA), 5 cases 9], 10], 15], 18], 19] out of 22; and type III (cisternocavernous, CI-CA), 8 cases 5]–8], 16], 20], 22], 23] out of 22. Tung 8] classified them into only two types: type I, schwannoma arises from the cavernous
sinus, including the symptoms of the 6th nerve palsy and with or without mild headache;
and type II, the tumor located in the prepontine area and perhaps accompanied by obstructive
hydrocephalus, raised intracranial pressure, the 6th nerve palsy, and with or without
other cranial nerve palsy. By contrast, Celli’s classification which emphasizes on
the anatomical position, without symptomatology, is more accurate and simple. But
Tung’s classification seems to be more comprehensive and stresses on symptomatology.
The big tumors belonging to the CI-CA type of Celli’s classification are difficult
to be labeled using Tung’s classification. Maybe it is only classified on the basis
of the principal part or origination of the tumor using Tung’s system.

Clinical presentation

It reveals an average age of 43.41 years old, from 10 to 68 years old. Eighteen out
of 22 cases were over 30 years old (82 %) and only 2 cases 5] (present case) were under 14 years old (0.09 %). The male/female ratio is about 1:1.
A typical 6th nerve palsy was present in all except 3 of the reported patients 17], 21], 23]. Six cases 6], 8], 16], 22] (present case) presented isolated 6th nerve palsy. This indicates that a patient
presenting an isolated 6th nerve palsy, with a specific mass in CI or CA found in
neuroimaging, could be likely to diagnose a schwannoma of the 6th cranial nerve preliminarily.
Twelve cases included signs of other cranial nerves, 9 cases 5], 7], 9]–14], 18] involved trigeminal nerves, 3 cases 9], 10], 15] involved oculomotor nerves, and 7 cases 7], 12], 14], 17], 19], 21], 23] involved vestibulocochlear and facial nerves. Headaches were present in 11 cases.
Hydrocephalus was present in 4 cases 4], 5], 7], 12]. About the present patient, 3 preoperative pictures at 10, 12, and 14 years old descript
the symptoms and course of the 6th nerve palsy accurately.

Diagnosis

Almost all cases of abducens nerve schwannoma were misdiagnosed preoperatively. It
is high possibility that the tumors were initially misdiagnosed as trigeminal or vestibular
schwannomas. The trigeminal schwannomas could show isolated 6th nerve palsy, and the
6th nerve schwannoma could present without any 6th nerve palsy. In addition, the low
incidence is the significant cause for most surgeons. The location and the attachment
to the 6th nerve is the key for diagnosis. Some cases 5], 7], 10], 11] were not revealed during surgery that it comes from the 6th nerve. Therefore, the
6th nerve schwannoma diagnosis could only come from the pre- and postoperative neurological
deficit. In the present case, the definite diagnosis of the 6th nerve schwannoma was
established from intraoperative observation of the tumor location and the attachment
to the 6th nerve, as well as from the neuroimaging and the obvious isolated 6th nerve
palsy pre- and postoperatively. It should be emphasized that ampliative Dorello’s
canal demonstrated the tumor origination from the 6th nerve, for only the 6th nerve
enters the cavernous sinus through Dorello’s canal, which is located between the petrosphenoidal
ligament and petrous apex.

Histological images of our case revealed a cellular schwannoma with spindle cells
in a collagenous background (Fig. 4a, b). Cellular schwannomas are a variant of standard schwannomas. They show a benign
clinical behavior relative to standard schwannomas, but they are more likely to local
recurrence 2].

Operation

Based on the location of the tumor, most cases are treated by a suboccipital 4], 12]–14], 17], 20], 22], 23] approach, then subtemporal 5], 15], 16] (present case) and frontotemporal 8]–10] approaches, and anterior transpetrosal 18], 21], orbitozygomatic 19], and transcondylar 11] approaches for a few patients. Schwannomas located partially or entirely in the cavernous
sinus are a completely special surgical group. Frontotemporal 8]–10] or subtemporal 5], 15] approaches or their variations 18] are used. One-half cases of the CI types and one-half cases of the CI-CA type schwannomas
obtain a radical resection, respectively. Tumor residue was left on the brainstem
5] (present case) or the 6th nerve 13]. In our present case, the tumor was adherent to the prepontine cisternal part of
the pons and the basilar artery. Comparatively speaking, 2 10], 18] out of 5 cases belonging to CA types obtain total resection (Table 2). Most of this type was not totally removed because of invasion of the cavernous
sinus or firm adherence to the nerves.

Table 2. Result of resection of the 6th schwannoma in different locations

Because of the frangible nature, the 6th nerve function seldom completely recovered
postoperatively. A complete recovery was obtained in only 4 cases 17], 18], 21], 22]. The case of CA type presented by Nakagawa 18] is out of the ordinary. It showed transient diplopia preoperatively and complete
recovery of the 6th nerve function postoperatively. Reference 22] is the second case whom there was diplopia preoperatively and complete postoperative
recovery of the 6th nerve palsy. The tumor was cystic and located in the CI-CA. An
anastomosis was performed following total resection of the tumor. The other two cases
reported by Nakamura 17] and Vachata and Sames 21] are both belonging to CI type and showed no diplopia preoperatively and complete
recovery of the 6th nerve function postoperatively. These may be due to tumor growth
that has been functionally compensated by the rest of intact nerve fibers. Above all,
it seems easier to recover the function of the 6th nerve in cases of schwannomas in
the CI-type group. The prognosis for the 6th nerve function was not in a good status
in the present case, perhaps because the tumor had invaded into the cavernous sinus
through Dorello’s canal and was adherent to the 6th nerve, the pons, and the basilar
artery.

Stereotactic radiosurgery

Regarding an alternative or adjuvant therapy, stereotactic radiosurgery has been reported
26], 27] for controlling nonvestibular schwannomas. In 2010, Hayashi et al. 24] performed stereotactic radiosurgery in 4 intracavernous 6th nerve schwannomas. The
tumors are small in size and controlled to grow without acute complications or adverse
effect. It could be an alternative measure for a small one. All in all, the direction
of further research is to improve the diagnosis and therapy for the 6th schwannomas
to have better postoperative recovery.