A colloid cyst (CC) is a rare benign intraventricular tumor of neuroectodermal origin. It most commonly occurs near the foramen of Monro (interventricular foramen) in the anterior third ventricle.
Epidemiology: Incidence is up to 3 per million people. It accounts for 0.5–3% of primary brain tumors and 15–20% of intraventricular tumors. Some reports indicate 0.5–1% of all intracranial tumors1). Age at discovery ranges across all ages, but it is particularly common in the 30s to 50s1). Familial occurrence has been reported, but known genetic relationships are unclear.
Location: Most commonly at the height of the third ventricle and Monro foramen. Rarely occurs in the frontal lobe, cerebellum, and pons-midbrain.
Etiology: Thought to originate from abnormal invagination of primitive neuroepithelium (paraphysis component), but still under debate.
With an incidence of up to 3 per million people, it is a rare benign tumor accounting for 0.5–3% of primary brain tumors and 15–20% of intraventricular tumors. Most commonly found in the 30s–50s, but can occur at any age.
Most cases are asymptomatic and discovered incidentally on imaging 1). In a retrospective study of 162 cases, 60% were incidental findings.
In symptomatic cases, symptoms are mainly due to non-communicating hydrocephalus (increased intracranial pressure).
Ocular findings due to increased intracranial pressure are the main features, and the following three are important.
Papilledema
Bilateral papilledema (choked disc): The most important fundus finding due to increased intracranial pressure. Leads to visual impairment.
RAPD: Positive when optic nerve damage is asymmetric.
Fluorescein angiography: Peripapillary dye leakage is observed. OCT is useful as an adjunctive diagnostic tool.
Abducens Nerve Palsy
Abducens nerve (VI) palsy: Presents with diplopia as a non-localizing sign of increased intracranial pressure.
Bilateral abducens nerve palsy: Frequently occurs together; in children, it is often discovered as esotropia.
Dorsal Midbrain Syndrome
Vertical gaze palsy: Particularly impaired upward saccadic eye movements (Parinaud syndrome).
Convergence paralysis and convergence-retraction nystagmus: Caused by a lesion of the posterior commissure.
Lid retraction (Collier sign): Observed bilaterally. Accompanied by light-near dissociation (poor light reflex but preserved convergence reaction). When the cerebral aqueduct is obstructed, it may be complicated by papilledema.
Additionally, oscillopsia or a clinical picture similar to idiopathic intracranial hypertension (IIH) may occur.
Acute obstruction of the foramen of Monro can lead to acute hydrocephalus, brain herniation, and sudden death has been reported 1). Even in asymptomatic cases, deaths during follow-up have been reported, so patient education about symptoms of increased intracranial pressure is important.
Cyst enlargement near the foramen of Monro obstructs CSF flow, leading to non-communicating hydrocephalus and increased intracranial pressure. The cyst acts as a ball valve, causing intermittent obstruction and position-dependent intermittent symptoms.
Acute obstruction can lead to brain herniation and sudden death, which is the most clinically important risk 1).
The management risk score for asymptomatic CC proposed by Hussein et al. is shown below 1).
| Item | Score |
|---|---|
| Under 65 years old | 1 point |
| Cyst diameter 7 mm or more | 1 point |
| Anterior location (directly above the foramen of Monro) | 1 point |
| FLAIR/T2 hyperintensity | 1 point |
| Lesion-related headache | 1 point |
A total score of 3 or more points is considered high risk, and surgery is considered.
Factors that increase the risk of symptom onset include young age, cyst enlargement, ventricular dilation, and increased T2 signal intensity1).
CT and MRI each show characteristic findings. The main imaging findings are shown below.
| Test | Main Findings |
|---|---|
| CT | Unilocular, well-defined, round high-density area; calcification rare |
| MRI T1 | Approximately 50% show round high signal; cyst is non-enhancing with gadolinium |
| MRI T2 | Mostly low signal (highly viscous “motor oil”-like contents); uniform high signal pattern also possible |
| FLAIR | T2 hypointense CC resembles CSF and is difficult to recognize |
MRI is the optimal diagnostic method. Patterns of T1 isointensity and T2 hyperintensity (with internal hypointense foci) have also been reported 1).
In acute onset, emergency head CT should be performed to prioritize the presence of acute hydrocephalus.
The cyst wall is lined by a single layer of columnar epithelium. Peripheral anterior synechia staining, keratin, and EMA are positive. The contents are gelatinous material containing mucin, hemosiderin, cholesterol, and various ions.
Differential diagnosis includes subependymal giant cell astrocytoma (tuberous sclerosis), subependymoma, craniopharyngioma, ependymoma, germinoma, hemorrhage, lymphoma, and pituitary tumor.
In cases of acute hydrocephalus, secure the airway, breathing, and circulation (ABC) and emergently place an external ventricular drain (EVD).
Craniotomy
Transcallosal approach: Separate the left and right frontal lobes and reach the cyst from the rostral end of the genu of the corpus callosum.
Transcortical approach: Via the right middle frontal gyrus → lateral ventricle. Surgical risk is slightly higher, but recurrence rate is low.
Complete resection results in a low recurrence rate and is considered a curative treatment.
Minimally Invasive Surgery
Endoscopic resection: May have lower risk than craniotomy, but reoperation and recurrence rates are slightly higher.
Stereotactic aspiration: Can reduce cyst size and symptoms, but carries a risk of recurrence and may require retreatment.
Resection of the space-occupying lesion or ventriculoperitoneal shunt is the standard. Early reduction of intracranial pressure leads to rapid resolution of papilledema without visual impairment. Delayed treatment results in irreversible visual dysfunction.
For cysts less than 1 cm in size and centrally located, the risk of obstruction is low, and regular imaging follow-up is an option. However, for high-risk cases with a CCRS of 3 or more, surgery should be considered 1). There have been reports of sudden death during follow-up, so consultation with a neurosurgeon is always recommended.
When a cyst near the third ventricle or foramen of Monro enlarges, it obstructs the CSF pathway. Non-communicating hydrocephalus leads to increased intracranial pressure, causing the following neuro-ophthalmic complications.
The cyst moves like a pendulum, causing intermittent obstruction. Acute obstruction with positional changes leads to acute hydrocephalus and brain herniation, a mechanism of sudden death1).
It is caused by posterior commissure (PC) damage. Upward gaze signals reach both oculomotor nuclei via the PC from the rostral interstitial nucleus of the medial longitudinal fasciculus (riMLF). PC damage causes upward gaze palsy, convergence-retraction nystagmus, lid retraction (Collier sign), and tectal pupils. If midbrain aqueduct obstruction is present, papilledema also occurs (midbrain aqueduct syndrome).
The contents are a highly viscous (“motor oil”-like) gelatinous substance containing mucin, hemosiderin, cholesterol, and various ions. This high viscosity reflects low T2 signal on MRI and also affects the difficulty of stereotactic aspiration.
Spontaneous regression of CC without treatment has been reported, though rarely.
Lee et al. (2022) reported the 10th case of spontaneous regression of CC 1). A 49-year-old male, initial diameter 12 mm → reduced to 4 mm after 18 months, and maintained at 4 mm after 30 months. Summarizing the 10 cases in the literature, initial sizes ranged from 5 to 30 mm, and regression periods ranged from 15 months to 9 years. Five cases resulted in complete disappearance. According to CCRS risk classification, there were 5 moderate-risk, 3 low-risk, and 2 high-risk cases.
Hypothesis of regression mechanism: Unrecognized spontaneous cyst rupture → absorption of contents into the ventricle. The risk of aseptic meningitis is considered low 1). However, spontaneous regression is an extremely rare phenomenon and is currently insufficient to serve as a basis for treatment decisions.
A retrospective study of 162 cases by Pollock et al. (cited by Lee et al. 2022) demonstrated the validity of regular imaging follow-up in asymptomatic CC 1). However, reports of sudden death during follow-up also exist, highlighting the importance of risk stratification for asymptomatic cases.
Spontaneous regression has been reported in 10 cases in the literature, with reduction or disappearance observed in initial sizes of 5–30 mm over periods of 15 months to 9 years 1). The mechanism of regression is presumed to be cyst rupture followed by absorption of contents, but this is a very rare phenomenon and cannot be used as a basis for treatment decisions.
