Open Veterinary Journal, (2026), Vol. 16(4): 2366-2372

Case Report

10.5455/OVJ.2026.v16.i4.37

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Intracranial colloid cyst in the cerebellopontine angle of a dog: A novel case report and surgical outcome

Hyeji Jo, Kang-hyo Park and Donghwi Shin^*

Bundang Leaders Animal Medical Center, Bundang-gu, Seongnam, Gyeonggi-do 13558, Republic of Korea

*Corresponding Author: Donghwi Shin. Bundang Leaders Animal Medical Center, Bundang-gu, Seongnam,
Gyeonggi-do 13558, Republic of Korea. Email: hwi4010 [at] naver.com

Submitted: 04/08/2025 Revised: 27/02/2026 Accepted: 14/03/2026 Published: 30/04/2026

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ABSTRACT

Background: Colloid cysts are benign endodermal intracranial lesions that typically arise in the third ventricle of the human brain. Although uncommon, obstructing cerebrospinal fluid pathways may cause clinical signs. Although cases in atypical sites such as the cerebellum or pontomesencephalic region have occasionally been described, no reports exist of colloid cysts within the cerebellopontine angle (CPA) in either veterinary or human medicine.

Case Description: A 5-year-old castrated male Pomeranian presented with ataxia and excessive salivation. The neurological examination was largely unremarkable apart from gait disturbance. Magnetic resonance imaging revealed a well-defined cystic mass in the CPA measuring 12.7 × 12.7 × 10.8 mm, exerting compressive effects on the cerebellum and medulla. The lesion displayed gravity-dependent signal layering and peripheral rim enhancement, indicating a benign encapsulated cyst. Based on the location and imaging features, the differentials included epidermoid or dermoid cysts. Surgical removal was performed via dorsal craniectomy and durotomy to alleviate compression and establish a definitive diagnosis. Histopathology revealed a fibrous-walled cyst lined by ciliated cuboidal to columnar epithelium with papillary projections and no evidence of keratin, adnexal elements, or neoplasia. The findings were consistent with the presence of a colloid cyst. The patient made a complete recovery and remained clinically asymptomatic and neurologically intact during the 10-month follow-up.

Conclusion: This case represents the first known instance of a colloid cyst occurring in the CPA of a dog. The imaging and histologic features closely mirrored those described in human cases. The report highlights the need to consider colloid cysts among the differential diagnoses for CPA masses in dogs and underscores the importance of histopathological confirmation in cystic brain lesions presenting in atypical locations.

Keywords: Colloid cyst, Cerebellopontine angle, Dog, Intracranial cystic lesion, Surgical management.

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Introduction

Colloid cysts are benign, slow-growing intracranial lesions that most commonly occur in the third ventricle of the human brain, accounting for approximately 0.5%–1% of all intracranial tumors (Ho and Garcia, 1992; Annamalai et al., 2008; Roberts et al., 2021). They are believed to originate from the endodermal epithelium and are histologically lined by ciliated cuboidal or columnar cells, often resembling respiratory-type mucosa (Macaulay et al., 1997). Although most cases are discovered incidentally or present with signs of obstructive hydrocephalus, variable imaging characteristics reflecting the internal composition of the cyst are also frequently described (Armao et al., 2000; Khanpara et al., 2020; Sharif et al., 2023). Although the third ventricle is the predominant site of origin, rare cases have been reported in atypical locations, such as the pontomesencephalic region and cerebellum (Müller et al., 1999; Inci et al., 2001). To the best of the authors’ knowledge, however, colloid cysts located in the cerebellopontine angle (CPA) have not been previously reported in human or veterinary literature.

In dogs, cystic CPA lesions are most commonly attributed to epidermoid or dermoid cysts. While dermoid cysts have been histologically confirmed in veterinary cases (Howard-Martin and Bowles, 1988), human and radiologic literature has largely inferred the predominance of epidermoid cysts in this region (Mudgal, 2013). In contrast, colloid cysts arise from endodermal derivatives and exhibit distinct histologic features, including a lack of keratinized components or adnexal structures, which are typically observed in epidermoid or dermoid cysts (Cho and Lee, 2017).

This case report presents the first confirmed case of a colloid cyst in the CPA of a dog. Surgical excision successfully treated the lesion. The clinical, imaging, and histopathological features documented here not only expand the differential diagnosis for CPA lesions in veterinary medicine but also provide insight into an underrecognized and previously undocumented location for colloid cysts.

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Case Details

A 5-year-old castrated male Pomeranian dog, weighing 4.35 kg, was presented for hypersalivation and ataxia. The general physical examination was unremarkable, and the neurological assessment did not reveal any specific abnormalities. However, gait evaluation demonstrated ataxia. Laboratory investigations revealed a normal complete blood count and blood gas analysis. Serum chemistry was within the reference intervals, except for a mild decrease in the amylase level. Based on clinical findings, a lesion affecting the cerebellopontine angle was suspected.

For magnetic resonance imaging (MRI), 0.9% normal saline (0.9% Sodium Chloride Inj.™ HK inno. N Corp., Seoul, Republic of Korea) was administered intravenously (IV) at a rate of 5 ml/kg/hour. The premedication consisted of butorphanol (0.2 mg/kg IV; Butophan Inj., Myungmoon Pharm. Co., Ltd., Seoul, Republic of Korea) and midazolam (0.2 mg/kg IV, Midacum™ Injection, Myung Moon Pharmaceutical Co., Ltd., Seoul, Republic of Korea). Anesthesia was induced with propofol (6 mg/kg IV; Freepol-MCT™ Injection, Daewon Pharmaceutical Co., Ltd., Seoul, Republic of Korea) and maintained with 1.2%–1.3% isoflurane (Ifran™ Liquid for Inhalation, Hana Pharmaceutical Co., Ltd., Seoul, Republic of Korea) delivered in 100% oxygen. During anesthesia, positive pressure ventilation was applied to maintain end-tidal carbon dioxide (EtCO₂) between 30 and 35 mmHg. MRI (Signa™ HDxt 1.5T, GE Healthcare, Chicago, IL, USA) revealed an irregularly shaped intracranial cystic lesion measuring 12.7 × 12.7 × 10.8 mm, located in the CPA, accompanied by perilesional edema and regional inflammation. The lesion compressed the cerebellum and medulla oblongata and caused mass effect on the fourth ventricle (Fig. 1). The cystic lesion exhibited gravity-dependent layering. The non-dependent portion appeared hyperintense on T2-weighted (T2W) images, isointense on fluid-attenuated inversion recovery (FLAIR), and hypointense on diffusion-weighted imaging (DWI). In contrast, the dependent portion was isointense on T2W, FLAIR, and hyperintense on DWI. Fast Imaging Employing Steady-State Acquisition (FIESTA) images revealed a linear hypointense structure within the cystic lesion (Fig. 2). Intravenous administration of 0.1 mmol/kg of contrast agent (gadoterate meglumine, Clariscan™, GE Healthcare, Chicago, IL, USA) enhanced the rim along the peripheral margin of the cystic lesion. Additionally, mild to moderate lateral ventricle dilation and mild quadrigeminal cistern dilation were identified. Diminished occipital bone opacity at the level of the foramen magnum, along with a medullary kinking sign, suggested caudal occipital malformation syndrome.

Fig. 2. Transverse magnetic resonance images at the level of the cystic lesion showed four different sequences: T2-weighted (T2W) (A), fluid-attenuated inversion recovery (FLAIR) (B), diffusion-weighted imaging (DWI) (C), and Fast Imaging Employing Steady-State Acquisition (FIESTA) (D). The cyst demonstrated gravity-dependent layering. The non-dependent portion appeared hyperintense on T2W, isointense on FLAIR, and hypointense on DWI, indicating a relatively clear fluid, with elevated apparent diffusion coefficient (ADC) values (average, 2,996; range, 2,237–3,642). In contrast, the dependent portion was isointense on T2W and FLAIR and hyperintense on DWI, indicating a denser material with lower ADC values (average, 1,271; range, 1,048–1,633). A focal hyperintense area (white arrowhead) was observed in the medullary parenchyma ventral to the cyst on T2W and FLAIR, consistent with perilesional edema. A linear hypointense structure within the cyst (yellow arrowhead) was observed on FIESTA images, suggestive of internal septation (Scale bar=1 cm).

Fig. 1. Sagittal (A) and transverse (B) T2-weighted magnetic resonance images of the brain. A cystic lesion (*) was located between the cerebellum and the medulla oblongata. Mild dilation of the third and fourth ventricles (white arrowhead) and slight enlargement of the quadrigeminal cistern (yellow arrowhead) were also observed. The lesion caused compression of the cerebellum and medulla oblongata.

To relieve neurological symptoms and obtain a definitive diagnosis, surgical excision of the cyst was planned. On the following day, to reduce intracranial pressure (ICP) before surgery, mannitol (0.5 g/kg IV; Dia-Mannitol™ 20% Injection, Daehan Pharmaceutical Co., Ltd., Seoul, Republic of Korea) was administered. The dog was premedicated with cefazolin (30 mg/kg IV; Cefazolin Inj.™, Chong Kun Dang Pharmaceutical Co., Ltd., Seoul, Republic of Korea), methylprednisolone sodium succinate (15 mg/kg IV; Predisol™, Ilyang Pharmaceutical Co., Ltd., Seoul, Republic of Korea), and midazolam (0.2 mg/kg IV). Anesthesia was induced with propofol (6 mg/kg IV) and maintained with 1.2%–1.3% isoflurane in oxygen. Analgesia was achieved by a constant rate infusion (CRI) of fentanyl (loading dose 4 μg/kg and infusion rate 0.1 μg/kg/min IV; thereafter, Fentanyl Citrate Inj.™, Hana Pharmaceutical Co., Ltd., Seoul, Republic of Korea), prepared in 100 ml of normal saline. During anesthesia, positive pressure ventilation was applied to maintain EtCO₂ between 30 and 35 mmHg. The dog was positioned in sternal recumbency with its head in a flexed position. A dorsal midline skin incision was made from the external occipital protuberance to the level of the second cervical vertebra. The superficial dorsal cervical musculature was separated along the median raphe to expose the underlying biventer cervicis muscles. The paired biventer cervicis muscles were then separated at the midline, revealing the rectus capitis dorsalis muscles. The muscle overlying the dorsal arch of C1 was dissected, exposing the cerebellum. A durotomy was performed, exposing the cystic lesion. Dorsal laminectomy of C1 was performed using a Kerrison rongeur. Upon gentle retraction of the cerebellum, a cystic structure was visualized (Fig. 3A), and the cystic fluid was aspirated using a 26-gauge needle, followed by careful removal of the cyst using fine forceps (Fig. 3B). Decompression of the cerebellum was confirmed after excision. A titanium neuroplate (3D Dynamic Mesh, Jeil Medical Corporation, Seoul, Republic of Korea) was placed over the defect and secured using four 3-mm self-tapping titanium screws (Neuroplate-compatible 1.4 mm self-drilling cranial screw, Jeil Medical Corporation, Seoul, Republic of Korea). Polymethylmethacrylate (Exolent Spine^®, Elmdown Ltd., London, United Kingdom) was applied over the plate surface. The previously dissected musculature, subcutaneous tissues, and skin were closed in anatomical layers. The dog recovered uneventfully from anesthesia.

Fig. 3. Intraoperative images after durotomy and dorsal laminectomy of C1. Cystic structure (arrowhead) was visualized (A). After aspiration of the cystic fluid, the cyst was carefully removed using fine forceps (B).

Immediate postoperative care included regular noninvasive blood pressure monitoring, urinary catheter placement, and administration of intravenous fluid therapy. Postoperative analgesia was continued using fentanyl CRI (0.02 μg/kg/minute IV) until the remaining solution was fully administered. Levetiracetam (30 mg/kg PO; Keppra Tab.™, UCB Pharm Korea Co., Ltd., Seoul, Republic of Korea) was prescribed to prevent potential seizures associated with increased ICP due to cerebral edema. Prednisolone (0.4 mg/kg PO; Solondo Tab.™, Yuhan Corporation, Seoul, Republic of Korea) was administered for anti-inflammatory purposes. Prophylactic antibiotic therapy and strict cage rest were maintained during hospitalization for continued stabilization.

The patient was discharged on postoperative day 2 with marked clinical improvement and no abnormalities observed on physical examination, neurological evaluation, or laboratory testing. By postoperative day 8, gait and neurological function had nearly returned to normal. Gradual improvement continued thereafter, and no further hospital visits were required after 2 weeks postoperatively. During a 10-month follow-up period, no recurrence or postoperative complications were observed. The patient remained neurologically normal and clinically asymptomatic throughout the monitoring period.

Histopathologic evaluation revealed a large cystic cavity lined by a pseudostratified to cuboidal or columnar epithelium, with occasional cilia. The wall was fibrous, and focal papillary projections resembling the choroid plexus were present. Multifocal epithelial attenuation and focal inflammatory cell infiltration were also observed. No neoplastic features were identified, and the lesion did not extend to the section margins (Fig. 4). All of these findings were obtained from hematoxylin and eosin (H&E) staining, and no additional immunohistochemical (IHC) staining was performed.

Fig. 4. Histopathological examination of the cyst revealed a lining of pseudostratified epithelium, with cells ranging from cuboidal to columnar and occasional cilia. The cyst wall was fibrous and lacked keratinization or adnexal structures, findings consistent with a colloid cyst (hematoxylin and eosin stain; A: ×40, B: ×400 magnification; scale bar=100 μm).

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Discussion

This report describes the diagnosis and successful surgical excision of a colloid cyst located in the CPA of a dog. According to previous studies in humans, colloid cysts are often asymptomatic. However, when symptomatic, colloid cysts may cause headache, diplopia, gait disturbance, nausea, and decreased consciousness as a result of obstructive hydrocephalus (Müller et al., 1999; Inci et al., 2001; Ravnik et al., 2014). In the present case, central nervous system disease was suspected based on the clinical signs of ataxia and hypersalivation, with particular consideration given to a lesion affecting the cerebellum or the cerebellopontine angle.

Previous human literature has noted that the internal composition of colloid cysts can vary considerably. While some contain cholesterol or proteinaceous material, others may exhibit calcification and/or hemorrhage (Algin et al., 2013). Consequently, there is significant variability in MRI signal characteristics across different imaging sequences (Sharif et al., 2023). On T1-weighted images, approximately 50% of colloid cysts appear hyperintense, while others are iso- or hypointense. T2W sequences demonstrate even greater heterogeneity, often showing hypointense or mixed-signal lesions, a feature thought to reflect the motor-oil consistency of their dense and viscous contents (Khanpara et al., 2020). MRI demonstrated gravity-dependent layering within the cyst in the present case. The upper portion was hyperintense on T2W and isointense on FLAIR images, suggesting serous fluid (Fig. 2A and B). In contrast, the dependent portion was hyperintense on DWI and isointense on T2W and FLAIR sequences (Fig. 2A–C), likely reflecting denser proteinaceous material. The apparent diffusion coefficient values further supported this interpretation, with higher values in the non-dependent portion (average, 2,996; range, 2,237–3,642) and lower values in the dependent portion (average, 1,271; range, 1,048–1,633), indicating facilitated diffusion in the less viscous component and relatively restricted diffusion in the denser layer. A linear hypointense structure was noted on FIESTA images, raising suspicion for internal septation (Fig. 2D). Hyperintensity in the ventral medulla on T2W and FLAIR images indicated perilesional edema (Fig. 2A and B). Rim enhancement following contrast administration suggested a well-encapsulated lesion and was consistent with the benign, non-infiltrative nature of human colloid cysts (Armao et al., 2000; Khanpara et al., 2020; Sharif et al., 2023). In some cases, lesions may appear uniformly hyperintense on T2W images or show a peripheral hyperintense rim with a central hypointense core (Armao et al., 2000). T2W and FLAIR sequences also demonstrate variable intensity and patterns, depending on the characteristics of the cyst contents (Armao et al., 2000; Khanpara et al., 2020).

Colloid cysts typically exhibit hypointensity on DWI and show elevated apparent diffusion coefficient values (e.g., ≈1.58 × 10^-³ mm²/second), indicating free diffusion and facilitating differentiation from other lesions, such as epidermoid cysts or abscesses, which typically demonstrate restricted diffusion (Sener, 2007). As colloid cysts occurring in the CPA have not been reported in either veterinary or human medicine, an epidermoid cyst was initially considered the most likely diagnosis. This assessment was further supported by the fact that epidermoid cysts represent the most commonly described ectodermally derived epithelial cysts in the CPA of dogs (Mudgal, 2013). Although less commonly reported, a dermoid cyst is also considered a possible differential diagnosis (Howard-Martin and Bowles, 1988). However, the MRI findings observed in this case, including signal intensity and internal pattern, were consistent with those described in human cases of colloid cysts. Based on these similarities, a colloid cyst should be considered among the differential diagnoses for intracranial cystic lesions in veterinary patients, particularly when located in atypical regions. Accordingly, histopathological evaluation is essential for establishing a definitive diagnosis.

Histologically, colloid cysts in humans are typically lined by a single layer of ciliated cuboidal or columnar epithelium that may appear pseudostratified, resemble respiratory-type epithelium, and frequently contain mucin-producing cells (Macaulay et al., 1997). The cyst contents are usually amorphous, proteinaceous, and acellular gelatinous material (Khanpara et al., 2020). Although most colloid cysts are identifiable on routine H&E staining, IHC staining may provide a more definitive diagnosis. In one study, colloid cysts demonstrated AE1/AE3 and CK8 positivity together with GST-π positivity, while showing negativity for GST-μ and GFAP, supporting a non-neuroepithelial differentiation of the cyst lining (Tsuchida et al., 1992). More recently, another study reported that the cyst contents were strongly PAS-positive and the lining epithelium expressed CK7, further characterizing the epithelial nature of these lesions (Montana et al., 2024). In the present case, histopathological examination with H&E staining revealed a cyst wall lined by pseudostratified to columnar or cuboidal epithelium with occasional cilia and papillary projections resembling choroid plexus, supported by a fibrous wall. Multifocal attenuation of the epithelial lining and focal infiltration of inflammatory cells were also observed (Fig. 4). Because IHC staining was not performed, the lesion could not be fully characterized. However, the findings were consistent with those previously reported in human colloid cysts. Despite its intracranial location, the absence of neuroectodermal components and the presence of an endodermal-type epithelial lining supported its classification as a colloid cyst. On histopathology, lesions such as epidermoid and dermoid cysts demonstrate distinct structural features: epidermoid cysts are lined by stratified squamous epithelium containing cholesterol, keratinocytes, and keratinaceous debris (Kornegay and Gorgacz, 1982; Saeedi et al., 2017), whereas dermoid cysts contain adnexal structures such as hair follicles, sebaceous glands, and sweat glands (Balasundaram et al., 2019). The absence of these components ruled out the presence of epidermoid and dermoid cysts and supported the diagnosis of a colloid cyst.

Given the histologically benign nature of colloid cysts and the presence of neurologic symptoms, the goal of treatment in this case was complete surgical resection while avoiding injury to critical neurovascular structures. In human patients, seizures, memory impairment, hemiplegia, and supplementary motor area syndrome are reported postoperative complications following colloid cyst resection (Ajlan et al., 2023). In the present case, complete resection was achieved without any intra- or postoperative complications, and the patient exhibited full neurologic recovery.

In human medicine, recurrence of intracranial colloid cysts after surgical excision has been reported, particularly when the cyst capsule is incompletely removed (Vorbau et al., 2019). However, other studies have shown that complete capsule removal is not always required to achieve favorable long-term outcomes, as partial coagulation of the cyst wall can provide comparable recurrence rates (Isaacs et al., 2020). Gross total resection of both the cyst and capsule was successfully achieved in the present case, and no recurrence was observed during the 10-month follow-up period. Nevertheless, considering that delayed recurrence has been documented in human patients even years after complete excision, periodic postoperative follow-up imaging may still be warranted to ensure long-term disease-free status.

Anesthetic management plays a critical role in maintaining stable intracranial conditions and facilitating smooth recovery in patients undergoing diagnostic imaging and surgical intervention for intracranial disease. Opioids contribute to anesthetic stability during MRI acquisition and surgical procedures by providing adequate analgesia and reducing patient stress and movement. Although opioids themselves do not directly increase ICP, certain agents, such as morphine and hydromorphone, are not recommended in patients with intracranial disease because of their higher incidence of vomiting, which may secondarily elevate ICP (Leece, 2016). In the present case, butorphanol was used during MRI, whereas fentanyl was administered intraoperatively to provide consistent analgesia during surgical manipulation. Because opioid administration may predispose patients to hypoventilation, EtCO₂ was continuously monitored and maintained below 35 mmHg throughout anesthesia. Midazolam and propofol were selected for their favorable effects on ICP reduction and anticonvulsant properties (Leece, 2016). Isoflurane was used for anesthetic maintenance, with the concentration carefully maintained at approximately 1 minimum alveolar concentration (MAC). At concentrations less than 1.5 times the MAC, isoflurane exerts minimal or even reducing effects on ICP (Leece, 2016). In this case, the use of appropriate premedication allowed the maintenance of isoflurane at low concentrations while preserving adequate anesthetic depth.

In conclusion, this report represents the first documented case of a dog with a benign intracranial colloid cyst that was successfully managed with surgical resection. Given the imaging and histopathological features observed in this case, colloid cysts should be considered a differential diagnosis for intracranial cystic lesions in dogs, not only in the cerebellopontine angle but also in other intracranial locations. Further case reports and comparative studies are needed to determine whether CPA colloid cysts represent a true clinical rarity or have been underrecognized due to diagnostic limitations.

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Acknowledgments

The authors would like to express their gratitude to the clinical staff at Bundang Leaders Animal Medical Center for their invaluable assistance and support in this study. Owner consent was obtained for all diagnostic and therapeutic procedures described in this report.

Conflict of interest

The authors declare no conflict of interest.

Funding

No specific grant was received.

Authors’ contributions

Hyeji Jo: execution of the project, preparation of manuscript. Dr. Kang-Hyo Park: execution of the project, project design. Dr. Donghwi Shin: project design, supervision of the report, revision of the manuscript.

Data availability

All data supporting the findings of this study are available within the manuscript.

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