Endoscopic management of idiopathic foramen of Monro stenosis

INTRODUCTION

Idiopathic foramen of Monro stenosis (IFOMS) is a rare condition that results in hydrocephalus. The cause of stenosis or membrane formation is unknown. More enigmatic is the age and onset of presentation. The clinical presentation is often subacute and varied. It is unknown whether the stenosis is congenital or acquired. A classification was proposed based on imaging whether the stenosis is unilateral or bilateral and if there is a presence of membranes.^[1] Acquired causes could be due to brain tumors, infections, intraventricular hemorrhage (IVH), or iatrogenic (after intraventricular procedures).

Promptly identifying IFOMS is necessary for appropriate surgical management. Endoscopic septostomy and foraminoplasty or ventriculoperitoneal (VP) shunt placement have been described. Here, we present a series of patients with IFOMS. We also review the literature of all the reported cases of IFOMS and discuss management options.

METHODS

We conducted a retrospective study of all patients operated at our institute from 2000 to 2024. All patients with foramen of Monro stenosis were included. Foramen of Monro stenosis was identified based on radiological imaging showing unilateral asymmetric or bilateral lateral ventriculomegaly with normal appearing third and fourth ventricles [Figure 1]. Patients with a history of meningitis and IVH were excluded. A total of 11 patients were recognized, and data were collected from the hospital records and telephonic interviews. Descriptive statistics were done using Microsoft Excel version 2016.

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Figure 1:

(a) T2 fluid-attenuated inversion recovery axial magnetic resonance imaging (MRI) showing dilatation of bilateral lateral ventricles with transependymal effusion. (b) T2-weighted coronal MRI showing bilateral foramen of Monro stenosis with normal third ventricle. (c) T2-weighted midsagittal MRI.

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A search was conducted in PubMed, SCOPUS, and Google Scholar databases using key search words “Foramen of Monro stenosis” OR “Foramen of Monro occlusion.” Cross-referencing was done using relevant articles.

RESULTS

In this series of 11 patients, five were male, and six were female. The median age was 35 years, ranging from 13 to 53 years, and the mean duration of symptoms was 22 months (Range: 1 month–120 months). Most of the patients belonged to the third to fifth decade of life [Table 1]. The most common complaint was headache (n = 10, 90.9%). Eight of them had associated vomiting. Seven patients had papilledema on fundus examination. Six patients had at least one episode of loss of consciousness during a headache.

One patient was detected incidentally following regular imaging conducted for evaluation of a road traffic accident. One patient presented with complaints of memory and gait disturbances mimicking normal pressure hydrocephalus. None of the patients had a history suggestive of meningitis or IVH.

Preoperative magnetic resonance imaging (MRI) was done for all patients. MRI showed unilateral foraminal stenosis in four (36.4%) patients and bilateral foraminal stenosis in seven patients (63.6%). Figure 1 shows pre-operative images confirming the diagnosis of IFOMS with no other pathology causing obstruction. Aqueductal stenosis with Chiari malformation was seen in two patients [Figure 2].

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Figure 2:

(a) Midsagittal T2-weighted magnetic resonance imaging (MRI) showing aqueductal stenosis with tonsillar herniation. (b) Coronal T2-weighted MRI showing bilateral lateral ventriculomegaly with septum deviation toward left, stenosis of the bilateral foramen of Monro with normal third ventricle.

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All the patients in the series underwent endoscopic management. Endoscopic septostomy with foraminoplasty was the most commonly done procedure (n = 9, 81.8%). Endoscopic septostomy with placement of VP shunt was done in two patients. A membrane was observed above the foramen of Monro in all individuals. Of the two patients with concomitant aqueductal stenosis, endoscopic third ventriculostomy (ETV) was feasible in one patient. In the other patient, ETV was difficult due to challenges in maneuvering the endoscope within the slit-like third ventricle and the inability to visualize the floor. This patient subsequently required a VP shunt.

All the patients were symptom-free in the post-operative period. One patient developed IVH, which was managed conservatively. The median follow-up duration was 9 months (range: 2 months–13 years). Two patients were lost to follow-up. None of the patients had fresh memory disturbances at follow-up. One patient developed meningitis in the immediate post-operative period, which was managed successfully with intravenous antibiotics. Post-operative MRI at 6-month follow-up showed patent foramen of Monro with a reduction in hydrocephalus and improvement in Evans index [Figure 3]. One patient required VP shunt revision 20 months later due to tube exposure at the cranial end.

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Figure 3:

(a) Pre-operative coronal T2-weighted magnetic resonance imaging (MRI) showing bilateral foramen of Monro stenosis with membranes causing dilatation of bilateral lateral ventricles and slit-like third ventricle. (b) Post-operative MRI showing open left foramen of Monro 3 months following foraminoplasty. (c) Pre-operative axial T2-weighted MRI showing hydrocephalus with transependymal effusion, Evans index of 0.36. (d) Post-operative MRI showing a reduction in hydrocephalus, Evans index of 0.32. Change in the size and shape of the ventricles with the resolution of trans ependymal effusion can be noted in the post-operative MRI.

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DISCUSSION

IFOMS is a rare entity with only 23 published cases in the literature (excluding our patients in the series) [Table 2]. We believe that our series is the largest on IFOMS, comprising 11 patients. Foramen of Monro stenosis has varied presentation and can present in both pediatric and adult populations. Most of the patients with idiopathic bilateral stenosis in the literature are middle-aged and present with chronic headaches. It is also observed in our series, as well as other case reports, that older patients with IFOMS present with symptoms of normal pressure hydrocephalus, such as memory impairment and gait disturbances.^[2-4] Disproportionate cerebral atrophy compared to the age and elderly patients presenting with dementia suggests that the course of this disorder may not be benign, even in asymptomatic.

IFOMS can be due to true stenosis or the formation of membranes. Membrane formation and occlusion of the foramen of Monro can also be observed in post-meningitis or post-IVH patients and are only categorized as idiopathic if no cause is identified. Although true foramen of Monro stenosis (Atresia of the foramen of Monro) is considered a congenital disorder, its presentation in later decades cannot be explained. True atresia of the foramen of Monro is rare as compared to membrane occlusion. Its presentation in neonates and occasional association with aqueductal stenosis and Chiari may suggest a congenital pathology.^[5,6]

In our series, the mean age of presentation is 35 years, and the mean duration of symptoms is 22 months, ranging from 1 month to 120 months. One patient in our series had scalloping of the bone attributed to elevated intracranial pressure, indicative of chronic disease. This patient with bone alterations exhibited a subacute presentation, reporting headaches for 15 days, despite the bony abnormalities indicating a chronic disease. However, in chronic presentation of more than 1 year, there is no evidence of compensatory mechanisms (periventricular lucency) on MRI imaging in the majority of patients. It is unknown what mechanisms aid cerebrospinal fluid (CSF) absorption and what decompensatory processes contribute to such presentation.

We believe that an ambient factor must contribute to the development of membranes. An inadequately developed membrane may result in a dynamic blockage, leading to episodic symptoms of varying severity. When the membrane is fully formed, the patient exhibits persistent and escalating symptom severity.

Most of the patients in the literature were treated with endoscopic foraminoplasty with septostomy or endoscopic septostomy with VP shunt placement. Good symptomatic response was noted with both treatment modalities. Endoscopic foraminoplasty is challenging in true atresia of the foramen of Monro and carries a higher risk of fornicial injury.^[4,7] Septostomy with shunt placement is a good alternative to foraminoplasty in the presence of true bilateral stenosis. Even though memory disturbances and IVH are possible in foraminoplasty, their incidence is rare and is usually transitory. Endoscopic foraminoplasty avoids placement of the VP shunt and its associated complications. In patients with associated aqueductal stenosis, the addition of ETV can restore normal CSF dynamics. If ETV is not possible intraoperatively as exemplified by one of our cases, VP shunt can be considered as an alternative to ETV. Hence, careful evaluation of the pre-operative MRI with constructive interference in steady state (CISS) 3D imaging is required in suspected cases to make an informed decision regarding the best-suited procedure.

Although conservative management was previously recommended, we recommend treatment even in an asymptomatic patient, given the progression of the membrane formation and subsequent presentation.^[1,8]

CONCLUSION

IFOMS is a rare condition; however, it has favorable results with endoscopic surgery. Unilateral asymmetric or bilateral lateral ventriculomegaly may indicate foramen of Monro stenosis. Surgery is recommended upon diagnosis, even in asymptomatic patients. Further studies are necessary to understand the cause of this intriguing phenomenon better.