INTRODUCTION
Arachnoid cysts (ACs) are congenital developmental malformations.[1] They account for 1% of all intracranial space-occupying lesions.[2,3] Usually, asymptomatic and diagnosed incidentally when imaging is done for other causes, they can become symptomatic when they enlarge and compress neighboring neural structures or if there is intracystic bleeding.[1] Increase in size of AC may be due to the secretion of fluid by the lining membrane or accumulation of cerebrospinal fluid (CSF) driven inwards either by brain pulsation through a one way valve in the wall or inflow following an osmotic gradient.[4]
The posterior fossa is the second most common site for AC[2] and these are usually found in the cerebellopontine angle.[4] In the midline of posterior fossa (accounting for 10% of all AC), supravermian location is the most common followed by the retrocerebellar variant.[5] Enlargement of supracerebellar posterior fossa AC can cause cerebellar and brain stem compression as well as tonsillar descent and their presentation and management strategies are well described while those that are retrocerebellar essentially have been treated such as Chiari malformations with foramen magnum decompression, C1 laminectomy, and duraplasty alongwith with fenestration of the AC after treating any supratentorial hydrocephalus that may be present.[6]
Reports of symptomatic AC straddling the craniovertebral junction (CVJ) are very rare.[2,7] In a PubMed search using the terms “Craniovertebral” and “AC,” we obtained only 17 results while a search using the terms “Craniospinal” and “AC” gave 22 results. The relevant case reports with presentations and surgeries performed after discounting the ventrally located cysts, intramedullary cysts, postoperative cysts, and those that were purely in the posterior fossa or spine are summarized in Table 1.
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Table 1: Summary of cases of dorsally placed AC’s straddling the craniovertebral junction
CASE REPORT
A 38-year-old female presented with intermittent episodes of headache and neck pain that increased on coughing and straining for 2 years with gait ataxia for the past 6 months. On examination, she had no findings except positive Romberg’s sign. Magnetic resonance imaging (MRI) of the brain showed a large posterior fossa AC extending up to C1 arch effacing the 4th ventricle, lifting the cerebellar hemispheres and compressing the cervicomedullary junction. The CSF column in the front of the medulla was not visible [Figure 1a and b and Figure 2a]. She underwent C1 laminectomy and drainage of the AC by wide fenestration of the inferior wall of the cyst into the spinal subarachnoid space. Postoperative MRI done 6 months later showed regression of the AC, opening up of the 4th ventricle, absence of brain stem compression [Figure 1c and Figure 2b], and she was asymptomatic.
Figure 1: Preoperative T1 (a) and T2 (b) weighted sagittal MR images of the patient showing dorsally located cyst straddling the CVJ that is kinking and compressing the cervicomedullary junction pushing it anteriorly with effacement of the 4th ventricle and (c) postoperative T2 weighted sagittal image showing no residual compression and open 4th ventricle. Only C1 laminectomy has been done. CVJ = Craniovertebral junction
Figure 2: T2 axial MR images of the patient showing (a) preoperatively a midline cyst that is compressing the cerebellar hemispheres and brain stem and has effaced the CSF column anterior to the latter and (b) postoperative images showing normalization of CSF spaces and cerebellar hemispheres. No suboccipital craniotomy has been done. CSF = Cerebrospinal fluid
DISCUSSION
Dorsally placed AC of the CVJ produce symptoms either by compression of the lower medulla or upper cervical cord and may present with spastic quadriparesis, ataxia and lower cranial nerve dysfunction[2,3,7–16] or alternatively by causing alterations in CSF flow with resultant features of obstructive papilledema[2,11,12] hydrocephalus[11] or syringomyelia.[16] Our patient presented with ataxia alone. Headache and neck pain have also been frequently described in these patients[2,11–13,15] and was present in our case too.
Most of the cases described in literature had presented with a long history of symptoms-sometimes of several years[7,8,15,16]-with the exception of one case reported by Fukushima etal.[11] where papilledema, headache, and vomiting occasioned by raised intracranial pressure resulted in an early diagnosis (within 1 month). Our case was symptomatic for 2 years and had undergone computed tomography (CT) scan of the brain previously and this was reported as normal. Dorsally placed AC at the CVJ is often confused on CT scan with an enlarged cisterna magna or dismissed as “incidental findings.” A plain CT scan of brain showing just CSF spaces in the lowermost cuts may miss the diagnosis and is an inadequate diagnostic tool. The extent of medullary and cord compression, the lower limit of the cyst, delineation of additional pathologies such as signal changes and syrinx and differentiation from epidermoid mandate that MRI be the investigation of choice.
Some cases described in literature like ours had a large posterior fossa component of the cyst,[2,10,11] while in others,[16] the posterior fossa component was small. However, what is striking is the lack of reports of cerebellar symptoms in the literature.[2] The vertical extent of the lesion in the cervical spinal canal has also varied between C1 and C5. Numerous procedures have been described in literature including partial or complete excision, fenestration,[3,8–11,13,16] and cystoperitoneal shunting[15] with suboccipital craniotomy and cervical laminectomy of varying extents.
We felt extensive bony decompression with arachnoid dissection might cause arachnoidal scarring and alter the anatomy of the foramen magnum hampering the normal flow of CSF with respect to both direction and velocity. Higher velocities of craniocaudal flow of CSF would be maintained if there was no increase in effective foramen magnum diameter and this, along with the help of gravity in the erect position, would aid in keeping the residual cyst wall collapsed [Figures 3 and 4]. Hence, in our case, following C1 laminectomy, partial inferior cyst wall excision and fenestration of the cyst liberally into the spinal subarachnoid space was done. No attempt was made to separate the wall anteriorly from the lower medulla and upper cervical cord for fear of iatrogenic neural damage.
Figure 3: Schematic diagram showing that increase in effective diameter of foramen magnum that will result following a more extensive bony decompression leads to reduction of flow velocity [light blue arrows (b) as compared to the dark blue arrow (a)] according to the “continuity equation of steady flow” expressed by the formula v = q / A or v = 4q / πD 2 (continuity equation of steady flow-where v is the velocity of flow, q is the volumetric flow rate, A is the cross sectional area of the container in which flow is occurring and D is the internal diameter of the container). The light blue arrows denote decreased flow velocity while the dark blue arrows denote increased flow velocity as already mentioned
Figure 4: Schematic diagram before and after surgery showing that fenestration of lower cyst end will help gravity aid in emptying the contents of the cyst (blue arrow) in the standing position and also CSF flow (yellow arrow) with greater velocity via the normal pathway when there is no foramen magnum decompression will help in keeping remaining cyst wall collapsed. CSF = Cerebrospinal fluid
CONCLUSIONS
CSF collections in the posterior fossa seen on CT scans in lower cuts must not be dismissed as enlarged cisterna magna or “benign” ACs as they may enlarge in the future and may cause neural compression. Even large cysts seldom cause cerebellar symptoms and rather may mimic CVJ pathology. If there is clinical suspicion, further investigation with an MRI study that incorporates the upper cervical spine must be done and lower extent of the cyst delineated. Finally, if surgery is indicated, fenestration of the lowermost part of the cyst into the spinal subarachnoid space may suffice compared to more extensive procedures.
Declaration of patient consent
The authors certify that they have obtained all appropriate patient consent forms. In the form, the patient(s) has/have given his/her/their consent for his/her/their images and other clinical information to be reported in the journal. The patients understand that their names and initials will not be published and due efforts will be made to conceal their identity, but anonymity cannot be guaranteed.
Financial support and sponsorship
Nil.
Conflicts of interest
There are no conflicts of interest.
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