unilateral ventriculomegaly is well - described in children secondary to neoplastic or infectious causes , and also due to congenital atresia , with or without the presence of a membrane . idiopathic occlusion of the foramen of monro in adulthood is exceedingly rare , with only 14 cases previously described . these patients present with clinical features of raised intracranial pressure ( icp ) necessitating early treatment . presentation as chronic hydrocephalus without clinical and radiological features of raised icp poses a dilemma regarding surgical treatment . there is a possibility that the patient may have a primary headache , and imaging findings are incidental . in such scenario we describe a case of unilateral hydrocephalus due to foramen of monro occlusion , presenting as chronic headache , successfully treated with neuroendoscopy . the headache used to be left - sided accompanied with vomiting , and intolerance to noise and light . she was diagnosed with migraine headaches , and took the following drugs for migraine : flunarizine , propranolol , tramadol , and naproxen . she had partial relief with flunarizine . however , she developed episodic retro - orbital pain with nausea . there was no history suggestive of neither prior central nervous system infection nor intraventricular hemorrhage . a combination of above - mentioned drugs was tried sequentially but did not relieve headache . she was evaluated with a magnetic resonance imaging ( mri ) of brain , which revealed the dilatation of left lateral ventricle , with no periventricular lucency . as she did not have any classical clinical features of raised icp , she underwent mri twice at the annual interval , with no change in her ventriculomegaly . the constructed interference in steady state three - dimensional sequence revealed a membrane blocking the foramen of monro but no lesion [ figure 1 ] . lumbar puncture was not done to find the etiology of the occlusion of foramen as the patient had unilateral ventriculomegaly , and it was deemed as a contraindication . a final diagnosis of idiopathic occlusion of the left foramen of monro was made . the proposed mechanism of headache was probably raised icp due to obstruction of the foramen of monro . as she did not have adequate relief of headache , a decision for surgery ( b ) t1-weighted axial acquisition she underwent left precoronal burr hole , and neuroendoscopy . the right side ventricle was small , and negotiation of endoscope in the right ventricle would have been technically difficult . moreover , the aim of surgery was also to inspect the occluded foramen and dilate it if possible , hence left side approach was used . after making the burr hole a ventricular cannula was inserted , and icp was measured by attaching it to a transducer that was connected to monitor . the opening pressure was 22 mm hg . the fenestration of the septum pellucidum was performed at the level of avascular zone , between anterior and posterior septal veins . , the endoscope was introduced into the stoma to inspect the contralateral ventricle and confirm the adequacy of septostomy . there were membranes at ipsilateral foramen of monro which were fenestrated [ figure 2 ] . the ventricular cerebrospinal fluid ( csf ) analysis was normal , hence infection was ruled out . she was discharged with instruction to take paracetamol for headache as and when required . at 3 months of follow - up , she was free from headache without medications . at follow - up of 15 months , she was still free from headaches . an mri done at 15 months showed septostomy defect ; however , the membranes at foramen monro were persistent [ figure 3 ] . ( b ) dilated foramen after formanioplasty magnetic resonance imaging at follow - up showing patency of septostomy though unilateral ventriculomegaly due to the occlusion of the foramen of monro is described in the literature , our case is unique in terms of clinical and radiological presentation . our patient did not have classical clinical as well as radiological features of raised icp leading to dilemma of surgical treatment . there were no classical symptoms of raised icp such as headache with vomiting and blurring of vision and papilledema in present case . though raised icp can present as chronic headache , it was difficult to ascribe headache to intracranial pathology in present case . the inherent risks of procedure including bleeding and subdural hygroma were kept in mind . hence a conservative treatment in our case as her headache was not responding to medications , the option of surgical treatment was considered . a septostomy was done to establish communication between ventricles so that csf can drain through opposite side normal ventricle . though there was no appreciable reduction in the size of ventricles at follow - up mri , the treatment was still considered successful as the patient had headache relief . one may argue that the cause of relief of headache is not surgery as there was no reduction in ventricular size . though comparison ventricular measurements are not available in cases of unilateral ventriculomegaly , it is known that in the cases of endoscopic third ventriculostomy , ventricular size is not always reduced . the proportion of patients who do not have ventricular size reduction ranges between 11% and 60% . the ventricular size reduction is easier to appreciate in patients with acute hydrocephalus , whereas in chronic hydrocephalus the ventricular size changes are subtler and in some cases are only perceived when detailed measurements or volumetric studies are performed . currently , the consensus is that ventricular size reduction does not seem to correlate with clinical outcome . in patients with chronic hydrocephalus , there may not be reduction in the size of ventricles after neuroendoscopic procedure , in such cases the success of treatment should be measured by clinical rather than radiological improvement . the neuroendoscopic options for unilateral occlusion of the foramen of monro are septostomy and/or foraminoplasty . though fenestration of membranes at the foramen of monro was done in our patient , the patency could not be established as evident by the persistence of membranes visible on mri at follow - up . however , the patency of septostomy helped the circulation of csf , and relief of headache . the early success rate of more than 95% and a long - term success rate of 92% has been described with septostomy . the septostomy allows the obstruction to be bypassed by creating a csf circulation between the obstructed ventricle and the opposite ventricle communicating with the third ventricle . the same aim can be achieved by performing a foraminoplasty , but it poses higher risk of injuring the adjacent fornix , veins , and hypothalamus . unilateral hydrocephalus due to the occlusion of foramen of monro can present with chronic headache .