Trochlear Nerve Palsy Caused by a Quadrigeminal Cistern Arachnoid Cyst Successfully Treated with a Hybrid Exoscope–endoscope Keyhole Approach: A Case Report

Mayuko KUNII; Toru UMEHARA; Takeshi MORIMOTO; Chisato YOKOTA; Ryuichi HIRAYAMA; Noriyuki KIJIMA; Satoru OSHINO; Haruhiko KISHIMA

doi:10.2176/jns-nmc.2026-0090

Abstract

Quadrigeminal cistern arachnoid cysts are rare intracranial lesions. While most intracranial arachnoid cysts in adults are incidentally detected and asymptomatic, quadrigeminal cistern arachnoid cysts, located adjacent to the dorsal midbrain and cerebral aqueduct, may become symptomatic, most often with obstructive hydrocephalus. Trochlear nerve palsy secondary to quadrigeminal cistern arachnoid cysts has been reported only rarely, and the pathophysiology and optimal management remain unclear. We report a 60-yr-old man who presented with progressive diplopia due to left trochlear nerve palsy. Magnetic resonance imaging revealed a 35-mm arachnoid cyst in the quadrigeminal cistern compressing the dorsal midbrain near the left trochlear nerve without hydrocephalus; no other causative lesion was identified. As the diplopia interfered with the patients daily activities, hybrid exoscope–endoscope–assisted keyhole cyst fenestration was performed 1 month after symptom onset. Communication between the cyst cavity and the ambient and superior cerebellar cisterns was successfully established to reduce the risk of cyst re-expansion. Postoperatively, the diplopia improved rapidly. Follow-up imaging demonstrated cyst shrinkage and decompression around the dorsal midbrain and left trochlear nerve. No perioperative complications were observed. This case supports a compressive mechanism for quadrigeminal cistern arachnoid cyst-related trochlear nerve palsy and suggests that timely surgical decompression may provide favorable outcomes when symptoms correlate with radiological compression. Hybrid exoscope–endoscope–assisted keyhole fenestration may be a useful surgical option for quadrigeminal cistern arachnoid cysts requiring surgical decompression.

Introduction

Arachnoid cysts are benign intracranial lesions lined by arachnoid membrane and filled with cerebrospinal fluid, with a reported prevalence of approximately 1.4% in adults.¹⁾ They most commonly arise in the middle cranial fossa and retrocerebellar region, whereas cysts originating in the quadrigeminal cistern account for only about 3% of cases.¹⁾ In adults, 92%-94% of arachnoid cysts remain asymptomatic and show little tendency to enlarge.²⁾ However, quadrigeminal cistern arachnoid cysts (QACs) are located dorsal to the midbrain and are therefore more likely to cause symptoms related to aqueductal compression and hydrocephalus.¹^-³⁾ The quadrigeminal cistern is a confined anatomical space bordered by the tectum, pineal gland, cerebellar vermis, and deep venous structures, and enlargement of a cyst in this region may compromise adjacent neural and vascular structures.⁴⁾ The trochlear nerve emerges from the dorsal surface of the inferior colliculus and courses along the lateral margin of the superior medullary velum,⁵⁾ rendering the nerve vulnerable to mass lesions in the quadrigeminal cistern. Despite this anatomical susceptibility, trochlear nerve palsy secondary to QACs has been reported only rarely,⁶^-⁸⁾ and both the underlying pathophysiology and optimal management strategy remain poorly defined.

Here, we describe a rare adult case of trochlear nerve palsy caused by a QAC that was successfully treated using a hybrid exoscope–endoscope keyhole approach. We also discuss the possible mechanisms of trochlear nerve involvement and considerations for surgical management in this uncommon condition.

Case Report

A 60-yr-old man presented to an ophthalmology clinic with diplopia and was diagnosed with left trochlear nerve palsy. Brain magnetic resonance imaging (MRI) revealed a QAC, and he was subsequently referred to our institution. He had no history of head trauma or vascular risk factors.

The diplopia was exacerbated in right gaze. The Bielschowsky head-tilt test was positive, and Hess chart examination demonstrated findings consistent with left superior oblique muscle palsy (Figure 1A). There were no abnormalities involving the oculomotor or abducens nerves. MRI demonstrated a 35-mm non-enhancing cystic lesion in the quadrigeminal cistern with signal intensity identical to cerebrospinal fluid (Figure 1B). Although hydrocephalus was not observed, the cyst compressed the superior medullary velum, the superior surface of the cerebellar vermis, and the left side of the midbrain tectum, suggesting compression near the left trochlear nerve (Figure 1C). No other lesions responsible for trochlear nerve palsy were identified. Accordingly, the QAC was considered the most likely etiology of the trochlear nerve palsy. As the diplopia progressively interfered with the patient's daily activities and work, surgical treatment was indicated, and cyst fenestration was performed 1 month after symptom onset.

Figure 1

Preoperative magnetic resonance imaging and Hess chart findings.

A: preoperative Hess chart showing underaction in right-down gaze and relative overaction in left-up gaze, consistent with left superior oblique muscle palsy.

B: contrast-enhanced sagittal MRI demonstrating a non-enhancing arachnoid cyst in the quadrigeminal cistern compressing the dorsal brainstem and cerebellar vermis.

C: CISS MRI showing compression near the left trochlear nerve (white arrowhead) by the arachnoid cyst. The trochlear nerve is seen coursing within the ambient cistern, dorsal to the inferior colliculus (white double arrows).

CISS: constructive interference in steady state; MRI: magnetic resonance imaging

Surgery was performed under general anesthesia with the patient in the right park-bench position. A 2.5-cm keyhole craniotomy was created medial to the left asterion. Using a 4K 3D exoscope (ORBEYE; Sony Olympus Medical Solutions Inc., Tokyo, Japan), the QAC was approached via a lateral infratentorial supracerebellar approach (Figure 2A). After opening the dorsolateral cyst wall, clear cerebrospinal fluid was released, resulting in cerebellar relaxation (Figure 2B). The procedure was subsequently continued under endoscopic visualization. A 4-mm-diameter rigid neuroendoscope with a 4K camera system (KARL STORZ SE & Co. KG, Tuttlingen, Germany) was inserted into the cyst, allowing visualization of the superior colliculi, vein of Galen, basal vein of Rosenthal, precentral cerebellar vein, and posterior cerebral arteries through the cyst wall (Figure 2C). Fenestration was performed inferior to the basal vein of Rosenthal to establish communication between the cyst cavity and the left ambient cistern (Figure 2D). An additional fenestration was created into the right superior cerebellar cistern. The trochlear nerve was not visualized as a discrete bundle and was not intentionally exposed or directly dissected. Adequate cystocisternostomy was achieved using the surrounding anatomical landmarks, and further manipulation near the nerve was avoided to minimize the risk of iatrogenic injury. A portion of the cyst wall was excised and submitted for histopathological examination, which confirmed the diagnosis of an arachnoid cyst.

Figure 2

Intraoperative findings.

A: exoscopic view via a lateral infratentorial supracerebellar approach, revealing the quadrigeminal cistern arachnoid cyst (arrow) in the deep operative field.

B: exoscopic view after opening the cyst.

C: endoscopic view within the cyst showing superior colliculi (asterisk), vein of Galen (white arrowhead), basal vein of Rosenthal (black arrowhead), precentral cerebellar vein (black double arrows), and posterior cerebral artery (white arrow) through the cyst wall. The trochlear nerve itself was not directly visualized.

D: Fenestration was performed inferior to the basal vein of Rosenthal to establish communication with the ambient cistern.

The patient noted subjective improvement in diplopia beginning on postoperative day 1. Follow-up MRI on postoperative day 4 demonstrated cyst shrinkage (Figure 3A) and decreased compression near the left trochlear nerve (Figure 3B and C). On postoperative day 6, the Hess chart examination showed improvement in left superior oblique dysfunction (Figure 3D). The postoperative course was uneventful, and the patient was discharged from the hospital on postoperative day 8. During the 4-month postoperative follow-up, the patient remained free of recurrent subjective diplopia, and follow-up imaging showed no cyst re-enlargement.

Figure 3

Postoperative MRI and Hess chart findings.

A: postoperative contrast-enhanced sagittal MRI demonstrating shrinkage of the quadrigeminal cistern arachnoid cyst and relief of compression of the cerebellar vermis.

B, C: postoperative CISS MRI showing adequate decompression of the left trochlear nerve (white arrowhead) and along its cisternal course within the ambient cistern (white double arrows).

D: Hess chart obtained on postoperative day 6 demonstrating improvement in left superior oblique function.

CISS: constructive interference in steady state; MRI: magnetic resonance imaging

Discussion

The present case represents a rare adult presentation of trochlear nerve palsy caused by a QAC and illustrates that QACs may become symptomatic through focal cranial nerve compression, even in the absence of hydrocephalus. In adults, the most common causes of isolated trochlear nerve palsy include presumed congenital, traumatic, and microvascular ischemic etiologies.⁹^,¹⁰⁾ Spontaneous recovery within 3 months is frequently observed in traumatic and microvascular ischemic palsies.¹¹^,¹²⁾ In contrast, compressive lesions such as cysts, tumors, or aneurysms are unlikely to resolve spontaneously unless the underlying pathology is addressed.¹⁰⁾ In the present case, the absence of trauma or vascular risk factors and MRI evidence of QAC-related compression near the left trochlear nerve favored a compressive etiology. The clinical manifestation of trochlear nerve dysfunction may vary depending on the degree and nature of nerve irritation or compression; superior oblique myokymia is generally attributed to pulsatile neurovascular compression causing focal hyperexcitability, whereas a cystic lesion such as a QAC may exert a sustained, nonpulsatile mass effect, leading to conduction block and palsy.¹³^,¹⁴⁾ Prolonged compression may progress from focal demyelination to axonal injury, potentially resulting in delayed or incomplete recovery.¹⁵^,¹⁶⁾ Given the progressive symptoms and concordant imaging findings, we proceeded with early cyst fenestration after discussing observation and symptomatic treatment as alternatives.

To date, 3 cases of QAC-associated trochlear nerve palsy have been reported in the literature.⁶^-⁸⁾ Including the present case, 4 cases are available for comparison (Table 1). Among these 4 cases, cyst decompression/fenestration resulted in improvement of trochlear nerve palsy in 2 patients, including ours. In contrast, cases treated solely with strabismus surgery without addressing the underlying cyst demonstrated persistent palsy. These limited reports suggest that when a QAC is identified as the causative lesion, cyst decompression may be more effective than symptomatic treatment alone in improving trochlear nerve palsy.

Table 1

Literature Review of Trochlear Nerve Palsy Caused by Quadrigeminal Arachnoid Cysts

Author	Year	Age/sex	Symptoms	Affected side	Symptom duration before diagnosis	Hydrocephalus	Treatment	Outcome
Abbreviations: AC, arachnoid cyst; F, female; M, male
Pagni et al.	1990	49/F	Diplopia, headache	Left	4 years	Yes	Open fenestration	Resolved
Ohtsuka et al.	1998	54/M	Torsional diplopia, truncal ataxia	Bilateral	2 months	Yes	Not reported	Not reported
Bison et al.	2024	62/M	Diplopia	Right	22 years	No	Prior strabismus surgery; prism after AC diagnosis	Transient improvement after surgery; recurrent diplopia
Present case	2025	60/M	Diplopia	Left	1 week	No	Hybrid exoscope/endoscope-assisted keyhole fenestration	Improved

Surgical intervention is generally recommended for symptomatic arachnoid cysts,¹³⁾ with the choice of approach determined by cyst location, size, and the presence of hydrocephalus.¹⁴⁾ In patients without hydrocephalus, cyst decompression via cystocisternostomy is commonly performed, whereas ventriculocystostomy and/or endoscopic third ventriculostomy may be considered when obstructive hydrocephalus is present. Although microsurgical fenestration via craniotomy has traditionally been employed, minimally invasive endoscopic techniques have been increasingly adopted in recent years.¹⁴^,¹⁷^,¹⁸⁾ Nevertheless, purely endoscopic surgery for deep-seated posterior fossa or pineal-region lesions, including QACs, remains technically demanding because of the narrow operative corridor and proximity to critical neurovascular structures. In this context, we used a hybrid exoscope–endoscope strategy to achieve cyst decompression through a limited 2.5-cm keyhole craniotomy, assigning each modality to the stage at which it provided the greatest technical advantage. Although the exoscope itself does not necessarily reduce surgical invasiveness and similar bimanual manipulation may be achieved with a conventional microscope or surgical loupes, it was useful during the initial exposure and cyst-wall opening, when the infratentorial supracerebellar corridor was narrow and cerebellar relaxation was limited. Under these conditions, the exoscope facilitated a shallow-angle, tangential line of sight along the infratentorial corridor while enabling monitor-based head-up surgery and shared visualization of the operative field.¹⁹⁾ After cyst opening and cerebrospinal fluid release, endoscopy was used for deep intracystic inspection and confirmation of sufficient cystocisternostomy into the ambient and superior cerebellar cisterns within the limited keyhole exposure, without requiring enlargement of the craniotomy, additional cerebellar retraction, or further dissection around adjacent neurovascular structures.

Alternative approaches, including the endoscopic occipital transtentorial approach, may also provide adequate access to the quadrigeminal cistern depending on the anatomy and surgical objective.²⁰⁾ However, in the present case, the goal was infratentorial cystocisternostomy into the ambient and superior cerebellar cisterns. We therefore selected the lateral infratentorial supracerebellar route, which provided a direct infratentorial corridor while avoiding a supratentorial occipital trajectory, tentorial incision, and early exposure of the Galenic venous complex.

The trochlear nerve may be difficult to visualize directly from within a QAC even when it is anatomically adjacent to the cyst wall. The quadrigeminal and ambient cisterns are compartmentalized by complex arachnoid structures, and Kurucz et al.²¹⁾ reported that the trochlear nerve, after a short free segment in the dorsal quadrigeminal cistern, is strongly attached to the inner surface of the outer arachnoid by numerous arachnoid trabeculae just below the tentorial edge. This anatomical configuration may explain why the nerve was not visible as a discrete bundle from within the cyst cavity. After adequate cystocisternostomy had been achieved, we did not use angled endoscopic views or pursue further exploration solely to identify the nerve in order to minimize the risk of iatrogenic trochlear nerve injury.

In conclusion, this report describes a rare adult case of trochlear nerve palsy caused by a QAC. The close anatomical correlation between the cyst and the trochlear nerve, together with prompt postoperative neurological improvement, supports a compressive mechanism. The hybrid exoscope–endoscope keyhole approach provided a useful means of achieving effective cyst decompression in this deep-seated quadrigeminal cistern lesion. Although uncommon, QACs should be recognized as a potential etiology of isolated trochlear nerve palsy, and timely surgical intervention should be considered when symptoms correlate with radiological evidence of focal compression.

Disclaimer

Author Haruhiko Kishima is one of the Editorial Board members of the Journal. This author was not involved in the peer-review or decision-making process for this paper.

Conflicts of Interest Disclosure

All authors have no conflict of interest.

References

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