Clinical and Imaging Manifestations of Trochlear Nerve Schwannomas-A Case Series

Clinical Correspondence Section Editors: Robert Avery, DO Karl C. Golnik, MD Caroline Froment, MD, PhD An-Gour Wang, MD Clinical and Imaging Manifestations of Trochlear Nerve Schwannomas—A Case Series Arvind Gupta, MBBS, FRCS (Ophthalmology), FRCOphth, Emma C. H. Lee, MBBS, Hillary R. Kelly, MD, Robert C. Chen, MD N euroimaging in patients with an isolated fourth cranial nerve (CN) palsy typically yields very little additional information because most cases are congenital, microvascular, or traumatic (1,2). Trochlear schwannomas causing fourth CN palsies are exceedingly rare. Of the 88 cases of trochlear schwannomas reported in the literature, only 34 cases have been pathologically proven, and therefore, their natural history and optimal treatment is not well established (1,3,4). Recommended treatment paradigms for nonvestibular schwannomas vastly differ and include surgery, radiosurgery, and watchful waiting; we wish to add our experience of these rare tumors to the literature (1,3,4). We present 5 patients without neurofibromatosis type 2, who presented with an isolated CN IV palsy and who had imaging features compatible with trochlear schwannomas (Table 1). Our series had a mean presentation age of 59 years with 80% of patients being men. All of our patients had lesions 0.5 cm or less in maximal dimension and all presented with clinical symptoms of diplopia. The median duration of follow-up was 2.5 years. More than half of our patients (3/5) showed adaptation with prisms, whereas 2 patients underwent strabismus surgery. One patient (Case 4) initially had adaptation with prisms, however, subsequently underwent strabismus surgery to relieve diplopia. Two of our patients (Cases 4 and 5) developed worsening of vertical diplopia after 54 and 18 months, respectively, while the remaining 3 patients remained clinically stable. Only one of our patients (Case 5) showed possible evidence of lesion size increase (Fig. 1). None showed significant mass effect on the brainstem. To the best of our knowledge, our collection is the first trochlear schwannoma case series detailing the presence of Department of Opthalmology, Counties Manukau DHB and Auckland DHB (AG), Auckland, New Zealand; Department of Diagnostic Radiology, SingHealth Institution (ELCH, RCC), Singapore; Massachusetts Eye and Ear (HRK), Boston, Massachusetts; Department of Diagnostic Radiology, Massachusetts General Hospital (HRK), Harvard Medical School, Boston, Massachusetts; and Duke Medical School (RCC), Singapore. The authors report no conflicts of interest. A. Gupta and E. L. C. Hwee contributed equally as first authors. Address correspondence to Robert C. Chen, MD, Department of Diagnostic Radiology, Singhealth Institution, Singapore General Hospital, Outram Road, Singapore 169608; E-mail: robert.chen@ singhealth.com.sg Gupta et al: J Neuro-Ophthalmol 2021; 41: e339-e342 superior oblique atrophy on imaging, which was found in 60% of cases (Table 1). Schwannomas are benign nerve sheath tumors of differentiated Schwann cells (WHO Grade 1), affecting both the cranial and peripheral nerves; when arising from a CN, they have a predilection for sensory or mixed sensory and motor nerves, such as the vestibulocochlear, trigeminal, and facial nerves (1). Schwannomas from pure motor CNs such as the trochlear nerve are infrequently seen, even in the context of a genetic predisposition to such tumors, such as in neurofibromatosis 2 (3). The trochlear nerve is the longest and thinnest CN and only CN to decussate across the midline and exit through the dorsal brainstem below the inferior colliculus (3). The fourth CN is categorized into cisternal, cisternocavernous, and cavernous segments; cisternal segment trochlear schwannomas account for 85% of cases (1). All 5 patients in our case series presented with cisternal segment lesions. The MRI characteristics of a trochlear schwannoma were first published in 1994 (5). Schwannomas appear heterogeneously hyperintense on T2 images with low or intermediate signal intensity on T1 images; avid contrast enhancement with or without a nonenhancing cystic component is typical (5,6). The lesions in our case series displayed typical imaging characteristics of schwannomas described in the literature and were found along the expected course of the fourth CN. Differential considerations for trochlear schwannomas include meningiomas, neurofibromas, leptomeningeal spread of tumor, and inflammatory conditions, such as sarcoid (6). Meningiomas tend to be darker on T2 images than schwannomas and careful radiologic inspection may reveal a dural tail (4,6). The remaining entities tend to be more multifocal, in distinction with the unifocal nature of schwannomas, and relevant clinical information of known or suspected systemic disease may aid in distinguishing such entities. Torun et al (3) reviewed the clinical presentation of 85 trochlear schwannomas reported until 2016. Three additional cases were reported by Peciu-Florianu et al (4) in 2017. Approximately 40% (34/88) of patients underwent surgical excision while the remainder received symptomatic management, defined by the administration of prism glasses or strabismus surgery to alleviate diplopia, radiosurgery, observation, or a combination thereof. Most of the patients who underwent surgical tumor excision had systemic constitutional symptoms suggestive of e339 Copyright © North American Neuro-Ophthalmology Society. Unauthorized reproduction of this article is prohibited. Clinical Correspondence TABLE 1. Clinical and imaging features of patients with presumed unilateral trochlear nerve schwannoma Case 1 2 3 4 Age (y) Sex Affected eye Duration of diplopia before presentation (mo) Other symptoms 74 Male Right 1 68 Female Right 12 65 Male Right 11 Nil Not available Past medical history Hyperlipidemia and chronic urticaria Hypertension, osteoarthritis, and chronic obstructive pulmonary disease MRI findings Nil Bilateral sensorineural Nil hearing loss, Meniere syndrome, and headache. Hyperlipidemia Hyperlipidemia, Hypertension, and ocular cervical benign prostate hypertension degenerative hyperplasia, spine disease, gastro-oesophageal and inguinal reflux disease hernia Irritable bowel syndrome, and inguinal hernia A small focus of Small Small extra-axial extra-axial enhancement in enhancing lesion the right ambient enhancing along the lateral cistern along the lesion along aspect of the upper course of the 4th the 4th pons along the cranial nerve cranial nerve expected course of course the 4th cranial nerve Small extra-axial Extra-axial enhancing nodule in homogenously the right enhancing solid lesion adjacent to the perimesenchephalic right hemipons along cistern, along the expected location of the expected course of the right 4th cranial the right 4th cranial nerve nerve Cisternal Cisternal Cisternal Segment of trochlear nerve affected Brainstem No compression Initial size of 5 schwannoma (mm) Size on follow- 5 up (mm) Duration of 2.5 follow-up (y) Adaptation with No prism Strabismus No surgery Improvement N/A after surgery Superior No oblique atrophy e340 45 Male Right 42 5 46 Male Right 1 Cisternal Cisternal No No No No 5 5 3 Not seen 5 5 3 3 3.5 2 4.5 1.5 Yes No Yes Yes No Right inferior oblique myomectomy N/A Yes Left inferior rectus No recession and right oblique myomectomy Unknown N/A No Yes Yes Yes Gupta et al: J Neuro-Ophthalmol 2021; 41: e339-e342 Copyright © North American Neuro-Ophthalmology Society. Unauthorized reproduction of this article is prohibited. Clinical Correspondence FIG. 1. MRI for Case 5. A. Initial noncontrast T1 and (B), T2 axial MR images using 5-mm cuts shows a possible lesion adjacent to the right hemipons. This is difficult to appreciate on these thick cut images without contrast and was only seen on retrospect. Follow-up MRI was performed 18 months later using 3-mm cuts. C. Precontrast T1 axial sequence shows an isointense extraaxial lesion adjacent to the right hemipons in the right ambient cistern. D. One millimeter cut T2 3D driven equilibrium (DRIVE) sequence further delineates the lesion. E. Postcontrast T1 axial sequence demonstrates enhancement of the lesion. F. T1 coronal image shows atrophy of the right superior oblique muscle on the ipsilateral side of the CN IV schwannoma, related to end organ atrophy. Despite the differences in technique, the lesion appears to be slightly thicker in the follow-up images. This highlights the importance of dedicated CN IV thin-cut imaging with administration of intravenous contrast. compression of local, regional brain structures (3,4,7). Signs or symptoms of raised intracranial pressure, paresis, sensory deficits, multiple cranial nerve palsies, ataxia, hiccups, and/ or dysarthria were present (3,4,7). The mean maximal tumoral diameter in the surgical excision group was large, measuring 3.04 cm (3). Conversely, asymptomatic patients or patients with minor symptoms such as diplopia and headache had smaller tumors with the mean maximal tumor diameter measuring 0.46 cm (3). This group was treated conservatively with symptomatic management. Based on the findings of a literature review of 85 reported cases, Torun et al proposed a pragmatic approach to the management of the trochlear schwannomas (3). They recommended surgical resection for larger lesions causing significant brainstem symptoms and sensory deficits. Smaller lesions causing mild headache and/or diplopia may be managed symptomatically with prisms, strabismus surgery, radiosurgery, or a combination of these modalities. The Gupta et al: J Neuro-Ophthalmol 2021; 41: e339-e342 experience from our case series is not at odds with this management algorithm. Based on the extensive literature review by Torun et al, in conjunction with the experience from our own small series, it seems that small tumors measuring 0.5 cm or less, without signs of brainstem compression, may do well with pure symptomatic management rather than surgical excision. Peciu-Florianu et al (4) reported their experience with stereotactic radiosurgery (SRS) in 3 patients with fourth CN schwannomas. They administered Gamma Knife surgery to small tumors with gross total volumes between 0.086 and 0.13 cm3 with marginal doses between 12 and 14 Gy. All patients were symptomatic with isolated diplopia before therapy; after SRS, patients showed resolution of their diplopia at the time of last follow-up. Furthermore, all tumors decreased in size on imaging (4). Other small case series have corroborated the ability of SRS to shrink fourth CN schwannomas and in some instances achieve complete e341 Copyright © North American Neuro-Ophthalmology Society. Unauthorized reproduction of this article is prohibited. Clinical Correspondence resolution of symptoms without the need for prisms, indicating some promise for this technique (8,9). In conclusion, we describe the clinical and radiological features of 5 patients consistent with CN IV schwannomas. They were symptomatically managed with Fresnel prisms or strabismus surgery, and most of them remained stable with observation. Although we believe that management should be individualized to each patient, for small (0.5 cm) schwannomas that do not cause mass effect on the brainstem structures, our series suggests conservative management such as observation and Fresnel Prisms is a viable alternative to surgical resection or radiotherapy to the primary tumor. Continued follow-up is recommended to exclude growth and the rare occurrence of malignant transformation. STATEMENT OF AUTHORSHIP Category 1: a. Conception and design: A. Gupta, R. C. Chen, and H. R. Kelly; b. Acquisition of data: A. Gupta, E. L. C. Hwee, R. C. Chen, and H. R. Kelly; c. Analysis and interpretation of data: A. Gupta, E. L. C. Hwee, and R. C. Chen. Category 2: a. Drafting the manuscript: A. Gupta, E. L. C. Hwee, and R. C. Chen; b. Revising it for intellectual content: R. C. Chen and H. R. Kelly. Category 3: a. Final approval of the completed manuscript: R. C. Chen. e342 REFERENCES 1. Elmalem VI, Younge BR, Biousse V, Leavitt JA, Moster ML, Warner J, Kupersmith MJ, Landau K, Brodsky MC, Frohman LP, May EF, Tomsak RL, Newman NJ. Clinical course and prognosis of trochlear nerve schwannomas. Ophthalmology. 2009;116:2011–2016. 2. Feinberg AS, Newman NJ. Schwannomas in patients with isolated unilateral trochlear nerve palsy. Am J Opthalmol. 1999;127:183–188. 3. Torun N, Laviv Y, Jazi KK, Mahadevan A, Bhadelia RA, Matthew A, Strominger M, Kasper EM. Schwannoma of the trochlear nerve-an illustrated case series and a systematic review of management. Neurosurg Rev. 2018;41:699–711. 4. Peciu-Florianu I, Tuleasca C, Comps JN, Schiappacasse L, Zeverino M, Daniel RT, Levivier M. Radiosurgery in trochlear and abducens nerve schwannomas: case series and systematic review. Acta Neurochir. 2017;159:2409–2418. 5. Jackowski A, Weiner G, O’Reilly G. Trochlear nerve schwannomas: a case report and literature review. Br J Neurosurg. 1994;8:219–223. 6. Skolnik AD, Loevner LA, Sampathu DM, Newman JG, Lee JY, Bagley LJ, Learned KO. Cranial nerve schwannomas: diagnostic imaging approach. Radiographics. 2016;36:1463–1477. 7. Santoreneos S, Hanieh A, Jorgensen RE. Trochlear nerve schwannomas occurring in patients without neurofibromatosis: case report and review of the literature. Neurosurgery. 1997;41:282–287. 8. Petrela E, Hodge CJ, Hahn SS, Chung CT, Mejico LJ. Stereotactic radiosurgery in two cases of presumed fourth cranial nerve schwannoma. J Neuroophthalmol. 2009;29:54–57. 9. Elflein HM, Thömke F, Müller-Forell W, Pitz S. Trochlear palsies caused by isolated trochlear schwannomas. Strabismus. 2010;18:83–86. Gupta et al: J Neuro-Ophthalmol 2021; 41: e339-e342 Copyright © North American Neuro-Ophthalmology Society. Unauthorized reproduction of this article is prohibited.