Schwannomas originating from cranial nerves account for 8% of intracranial neoplasms. The vast majority of these tumors arise from the sensory cranial nerves, most commonly, the vestibular or trigeminal nerve [
Tumors arising from the trochlear nerve can present a surgical challenge because they often involve the cisternal region near the brainstem [
A 64-year-old white male with an unremarkable past medical history presented with acute-onset diplopia. His symptoms began 2 months prior to his initial neurosurgical evaluation. There was no obvious history of trauma. The patient reported that his double vision was exacerbated by looking down and was improved when tilting his head to the left. He also was experiencing significant right frontal headaches that worsened throughout the day. On physical examination, he was found to have diplopia on downward gaze to the right, which suggested a right-sided trochlear nerve palsy with no other focal neurological deficits. The remainder of his neurological exam was unremarkable.
Magnetic resonance imaging (MRI) of the brain with and without contrast revealed a well-circumscribed, heterogeneously enhancing space-occupying lesion in the right ambient cistern. The mass measured 9 mm × 7 mm × 11 mm and compressed the adjacent anterior aspect of the upper pons. It appeared to have a broad base with a thin enhancing tail extending inferolaterally along the meninges (Figures
Preoperative MRI brain with contrast axial (a) and coronal T1-weighted (b) images reveals a well-circumscribed, heterogeneously enhancing lesion adjacent to the right side of the upper pons.
The patient was transferred to the operative table and placed in the lateral decubitus position. A lumbar drain was placed at this time. His head was placed in a Mayfield 3-point fixation system, which was aligned to maximally expose the right temporal region. The operative site was prepped and draped in the usual sterile fashion. A curvilinear incision was made overlying the right temporal region that began approximately 1.5 cm anterior to the external auditory canal and was carried superiorly to the superior temporal line. It was then carried posteriorly approximately 5 cm. The skin flap was rotated inferiorly and posteriorly and affixed into place with hooks. The temporalis muscle was mobilized anteriorly. This exposed the right temporal region. An electric drill was taken, and a right temporal craniotomy was elevated and the microscope was moved into the field.
Approximately 20 mL of CSF was removed from the lumbar drain at this time to promote brain relaxation. An extradural subtemporal dissection ensued. Dissection continued medially and the greater superficial petrosal nerve was identified. Dissection continued medially until Meckel’s cave was identified extradurally. Anteriorly, the middle meningeal artery was seen coursing through the foramen spinosum and posteriorly was the superior petrosal sinus.
A middle fossa retractor was inserted and extradural elevation of the right temporal lobe was performed. Careful attention was paid to avoid excessive retraction of the temporal lobe. An anterior petrosectomy was performed as originally described by Kawase et al. [
The trochlear nerve entering directly into the visualized tumor.
The patient had an uneventful postoperative course and was discharged to home on postoperative day 3. As expected, the patient continued to experience diplopia with downward gaze. At 1-month followup, the patient had recovered well from the surgery with no complaints. The right trochlear nerve palsy persisted as anticipated. Followup MRI of the brain performed 6 months postoperatively revealed no evidence of residual disease (Figures
Postoperative MRI brain with contrast axial (a) and coronal (b) T1-weighted images reveals complete resection of the trochlear schwannoma.
Nonvestibular cranial nerve schwannomas are extremely uncommon. Of these, trigeminal schwannomas are the most common but only account for up to 8% of intracranial schwannomas [
Prior Surgically-Defined Trochlear Schwannomas.
Author | Year | Age/Sex | Location | Approach |
---|---|---|---|---|
King [ |
1976 | 55/F | Ambient Cistern | Subtemporal transtentorial |
Boggan et al. [ |
1979 | 32/F | Ambient Cistern | Subtemporal transtentorial |
Leunda et al. [ |
1982 | 54/M | Ambient Cistern | Subtemporal transtentorial |
Leunda et al. [ |
1982 | 16/F | Cisternocavernous | Subtemporal transtentorial |
Yamamoto et al. [ |
1987 | 37/F | Ambient Cistern | Subtemporal transtentorial |
Garen et al. [ |
1987 | 18/F | Ambient Cistern | Subtemporal transtentorial |
Tokuriki et al. [ |
1988 | 43/M | Ambient Cistern | Subtemporal transtentorial |
Maurice-Williams [ |
1989 | 56/M | Ambient Cistern | Suboccipital, CP angle approach |
Samii et al. [ |
1989 | 53/F | Ambient Cistern | Pterional |
Celli et al. [ |
1992 | 51/M | Ambient Cistern | Subtemporal transtentorial |
Jackowski et al. [ |
1994 | 26/F | Ambient Cistern | Transtemporal with partial division of tentorium |
Abe et al. [ |
1994 | 60/M | Ambient Cistern | Lateral Suboccipital |
Abe et al. [ |
1994 | 57/M | Ambient Cistern | Subtemporal transtentorial |
Dolenc and Coscia [ |
1996 | 68/M | Ambient Cistern | Lateral suboccipital |
Beppu et al. [ |
1997 | 66/M | Ambient Cistern | Lateral suboccipital |
Santoreneos et al. [ |
1997 | 35/F | Ambient Cistern | Subtemporal with partial division of tentorium |
Nadkarni and Goel [ |
1999 | 48/F | Ambient Cistern | Subtemporal transtentorial |
Matsui et al. [ |
2002 | 61/M | Ambient Cistern | Presigmoid transpetrosal |
Veshchev et al. [ |
2002 | 26/F | Cavernous Sinus | Pterional |
Türe et al. [ |
2002 | 31/M | Ambient Cistern | Infratentorial, lateral supracerebellar |
Shenouda et al. [ |
2002 | 49/M | Cisternocavernous | Presigmoid combined transpetrosal |
Du et al. [ |
2003 | 17/F | Ambient Cistern | Orbitozygomatic pterional |
Shenoy and Raja [ |
2004 | 54/F | Ambient Cistern | Subtemporal transtentorial |
Ohba et al. [ |
2006 | 48/M | Ambient Cistern | Anterior transpetrosal |
Gerganov et al. [ |
2007 | 52/F | Ambient Cistern | Retrosigmoid |
Grigorian and Korobova [ |
2008 | 47/F | Ambient Cistern | Retromastoidal |
Grigorian and Korobova [ |
2008 | 44/F | Ambient Cistern | Paramedian subtentorial supracerebellar |
Kohama et al. [ |
2009 | 47/F | Ambient Cistern | Posterior transpetrosal |
Bartalena et al. [ |
2010 | 50/F | Ambient Cistern | Subtemporal transtentorial |
Younes et al. [ |
2012 | 65/F | Ambient Cistern | Pterional |
Boucher and Michael (current study) | 2013 | 64/M | Ambient Cistern | Anterior transpetrosal |
The approach for gross resection of trochlear schwannomas should be based on the size and location of the tumor. The size can vary, but symptomatic tumors that were previously resected ranged from 10 to 45 mm [
We utilized the extended middle fossa approach, also known as the anterior transpetrosal approach, in the gross total resection of the trochlear schwannoma. Kawase et al. first described this approach in 1985 for basilar artery aneurysms and later for the resection of sphenopetroclival meningiomas in 1991 [
While the subtemporal transtentorial approach also allows for adequate visualization of the surgical field, this approach potentially involves more temporal lobe retraction than the middle fossa approach. The increased traction increases the likelihood of morbidity due to damage of the temporal lobe. Another potential disadvantage of this approach is harm to the vein of Labbe with resultant venous infarction. The lateral suboccipital approach has also been used successfully, but this approach has disadvantages of inadequate exposure if there are significant parasellar portions of the tumor as well as increased risk of damage to the cranial nerves that surround the access route [
The main disadvantages of the middle fossa approach are possible inadequate exposure, if the petrosal bone resection is insufficient, possible unintentional entry into the middle ear, and risk to the geniculate ganglion, if there is excessive traction on the greater superficial petrosal nerve [
The anterior transpetrosal approach is an excellent approach for the resection of trochlear nerve schwannomas. It was used successfully in this patient to achieve gross total resection of this uncommon tumor. The patient retained his isolated right trochlear nerve palsy with no added morbidity. The anterior transpetrosal approach has advantages over other techniques for removal of these rare schwannomas. This approach offers optimal visualization of the petroclival region while introducing minimal morbidity and should be considered as a surgical corridor for removal of trochlear schwannomas with involvement of the ambient cistern.
The authors declare that there is no conflict of interests regarding the publication of this paper.
The authors received no financial support. The authors also wish to thank Andrew J. Gienapp for technical and copy editing, preparation of the paper and figures for publishing, and publication assistance with this paper.