Small Cyst, Cautious Approach: Risk-Stratified Conservative Management of a Third Ventricle Colloid Cyst

A colloid cyst is a rare, congenital, typically slow-growing brain tumour that is predominantly located within the third ventricle next to the foramen of Monro.It is imperative to recognise that these cysts in the third ventricle, albeit small on presentation, may grow in size and, due to their close proximity to the foramen of Monro, can block the flow of cerebrospinal fluid between the lateral and third ventricles, leading to acute obstructive hydrocephalus and its potential life-threatening sequelae.We report a case of a 30-year-old woman who presented with several weeks' history of headache and blurring of vision (after having finished treatment with antibiotics for sinusitis).Imaging identified a small (5.6 × 5.6 mm) third ventricle colloid cyst without hydrocephalus.Despite its size, application of the Colloid Cyst Risk Score identified her as moderate-risk, warranting urgent MRI and ophthalmology assessment.She was ultimately managed conservatively with structured interval imaging rather than surgical intervention.This case highlights that cyst diameter alone is an unreliable indicator of risk and that risk-stratified conservative management can be an appropriate pathway once hydrocephalus has been excluded.