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Arachnoid Membranes of Unknown Origin Blocking the Foramen of Magendie: A Report of Two Cases and Review of Literature
Correspondence Address: Source of Support: None, Conflict of Interest: None DOI: 10.4103/0028-3886.271249
Sir, Fourth ventricular outlet obstruction (FVOO) is a rare cause of hydrocephalus. It is more commonly seen in children and less often in adults. When described in adults, there is usually a preceding history of intracranial infection including inflammation[1] (meningitis), intraventricular hemorrhage, or trauma.[2] We describe two adult patients of FVOO by arachnoid membranes across the foramen of Magendie. Neither of these had any previous history of central nervous system pathology. Our first patient was a 49-year-old male who presented with a history of progressively worsening gait ataxia for the last six months, such that he had become bed-ridden in the last 1 week before presentation to the hospital. He also complained of diplopia and headache of a one week duration. In addition, there was a past history of pulmonary tuberculosis 2 years previously for which he had completed nine months of antitubercular drugs. However, there was no previous history of meningitis. On examination, he had bilateral gaze evoked nystagmus and a positive Rhomberg's sign. Magnetic resonance imaging (MRI) showed a thin membrane at the lower end of the fourth ventricle near the foramen of Magendie and dilation of the fourth ventricle [Figure 1], left]. The membrane was not enhancing on contrast [Figure 1], centre], and the Foramina of Luschka More Details were patent on both sides [Figure 1], right].
At surgery, a suboccipital craniotomy was performed. On opening, the cisterna magna and seperating the tonsils, the foramen of Magendie was found to be completely obliterated by a thin transparent membrane, over which there was a leash of blood vessels. This membrane was bulging and it was fenestrated, following which the floor of the fourth ventricle could be seen. The patient had an uneventful recovery following surgery. Our second patient was a 22-year-old male presented with a history of holocranial headache and vomiting for last one month. On examination, he had papilledema in both eyes. An MRI of the brain showed a thin membrane obstructing the foramen of Magendie and tetraventricular hydrocephalus. At surgery, a suboccipital craniotomy was performed. On opening the cisterna magna and separating the tonsils, the foramen of Magendie was found to be completely obliterated by a thin transparent membrane. This patient, however, appeared to have no blood vessels over the membrane, though it was bulging and fenestrated. Following this, there was a gush of cerebrospinal fluid (CSF) which drained as if it had been under pressure within the fourth ventricle [Video 1]. The patient had an uneventful recovery following surgery. At nine months of follow-up, both patients have returned to work. FVOO is more often seen in children,[3] where it has been described in association with Dandy Walker malformation, Chiari malformation More Details,[4] cranio-vertebral junction anomalies, tuberous sclerosis, achondroplasia, spina bifida, etc.[2] In adults, inflammation following meningitis, head injury, shunt,[5] intraventricular hemorrhage, radiation or chemotherapy, subarachnoid hemorrhage, parasitic infections, and multiple sclerosis have been reported to cause FVOO.[2] However, obstruction to the flow of cerebrospinal fluid from the arachnoid membranes at the foramen of Magendie without a preceding history of central nervous system pathology, has not been commonly reported. On a review of the literature, we could find a few such reported cases in adults.[2],[3],[6],[7],[8],[9],[10],[11],[12],[13],[14],[15],[16],[17],[18] Most were case reports, with only one study having reported 10 patients in their series.[12] Adult patients usually present with features of chronic hydrocephalus until they decompensate and become overtly symptomatic.[10],[12],[17] At the beginning, patients complain of headache and gait ataxia for of a few weeks to months. Diplopia, visual obscuration and vertigo set in at a later stage along with worsening of headache. It is not clearly understood as to why most adults present with an insidious onset if the occlusion of the foramen of Magendie is complete. It is worth speculating that despite the obstruction in the midline cerebrospinal fluid is able to find its way out via the foramen of Lushcka until an event that triggers their obliteration or incompetence, resulting in the decompensation of the patient. Obviously, the chronicity of symptoms is easier to explain in patients who have a partially obliterating membrane[12] or where the foramina of Luschka are patent. One case of acute hydrocephalus has also been reported.[9] Normally, the foramen of Magendie and Luschka are known to be patent at birth. About 6% of patients may have a partial or complete obstruction of the foramen of Magendie and 20% have similar imperforate foramen of Luschka.[19] Similarly, descriptions of incomplete obstruction have been reported by Longatti et al. on transaqueductal endoscopic exploration of the fourth ventricle.[12] MRI is the investigation of choice for diagnosis. T2W sagittal CISS (counter interference in steady state) sequence can demonstrate the above membranes at the foramen of Magendie. The membranes do not take up contrast. CINE MRI sequence can show turbulence of CSF flow inside the fourth ventricle but may not have good diagnostic value.[7],[12] Various treatment options have been described for this condition. One of the earliest to have been mentioned in literature is suboccipital craniotomy and fenestration of the membrane obliterating the foramen of Magendie[2] with or without a ventriculo-cisternal shunt.[10] A number of publications have demonstrated the efficacy of endoscopic third ventriculostomy for this pathology. Endoscopic third ventriculostomy (with or without magendieplasty)[7],[12],[16] is an elegant solution to this problem, but may not be feasible when the ballooned fourth ventricle causes significant mass effect on the brain stem and obliterates the prepontine cistern or displaces the basilar artery upward and/or anteriorly.[17] Failure of ETV due to stenosis of the stoma requiring a second endoscopic procedure has also been reported.[11],[12] FVOO from membranes at the foramen of Magendie of unknown origin are rare to find. One must have a high index of suspicion to look for them. When diagnosed correctly and treated, it brings about gratifying results. Financial support and sponsorship Nil. Conflicts of interest There are no conflicts of interest.
[Figure 1]
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