Intracranial fluid filled collection and superficial siderosis

Superficial siderosis (SS) of the CNS is caused by repeated haemorrhage into the subarachnoid space with resultant haemosiderin deposition in the subpial layers of the brain and spinal cord.¹,²,³,⁴ The commonest neurological manifestation is that of slowly progressive gait ataxia and impaired hearing. A remote history of injury is common. A fluid‐filled intraspinal cavity of variable dimension is a frequently noted finding on neuroimaging.³,⁴,⁵,⁶ A high prevalence of CSF containing cysts in SS supports the hypothesis of bleeding from a dural defect as the origin of SS.⁷ Removal of the likely bleeding source is a logical therapeutic strategy.

Case report

A 58‐year‐old man was evaluated for a 10 year history of progressive decline in hearing and imbalance. Additional symptoms included a 5 year history of slurred speech and a 1 year history of increasing falls. His past history was remarkable for a motor vehicle accident at 7 years of age. This was followed by a prolonged hospitalisation but no evident neurological sequelae. His examination was remarkable for decreased hearing, an ataxic dysarthria, a wide based ataxic gait with increased lower limb tone and hyperreflexia. Rapid alternating movements with the hands and feet were irregular and the heel–shin and finger–nose tests were positive. T2 weighted head MRI showed typical MRI changes of superficial siderosis with the characteristic hypointensity involving the cerebellum (fig 1A), brainstem (fig 1A) and the frontotemporal regions. Also present was post‐traumatic encephalomalacia involving the anterior frontal lobes. A CSF study was remarkable for increase in red blood cells to 380 cells/mm³ and a protein concentration of 94 mg/dl. No abnormality was noted on head and neck magnetic resonance angiography. A small cystic lesion was noted behind the cerebellum on T2 weighted axial (fig 1A) and sagittal head MRI (fig 1B), and on axial head CT (fig 1C). This was felt to be of no consequence until a review of the head CT (fig 1C) with careful attention to the bone sequence (fig 1D) revealed discontinuity of the inner skull table underlying the cyst (fig 1D). A CT myelogram of the spine, performed to look for a bleeding source, showed extravasation of the dye from the fourth ventricle into the cystic structure (fig 1E1 and E2).

Figure 1 A T2 weighted axial head MRI shows haemosiderin deposition along the cerebellar folia and around the brain stem (single arrows). Also shown is the posteriorly located cystic structure (double arrows). (B) T2 weighted sagittal head MRI shows the posteriorly located cystic structure. (C) Axial head CT showing the cystic structure (single arrow) and fracture of the inner table (double arrow). (D) Axial head CT (bony window) showing the discontinuity in the inner skull table. (E1, E2) CT myelogram showing extravasation of contrast from the fourth ventricle (E1) into the cystic structure (E2). (F1) Dural defect noted at the time of the suboccipital craniotomy. This was leading to the cyst through the defect in the inner table and was overlying the fourth ventricle from which CSF was flowing into the cyst. (F2) Site after repair of dural defect.

Discussion

Recent reports have noted that intraspinal fluid filled collections are frequently seen in SS.³,⁷ In some cases the associated dural defect has been repaired surgically, and decrease or resolution of the cavity and CSF red blood cells has been documented.³,⁵,⁶ This experience prompted us to surgically explore the identified intracranial cystic structure. A burr hole was opened in the superolateral portion of the right suboccipital region. A CSF filled cavity was encountered within the bone. The outer table was removed, a defect in the inner table was identified and was noted to overlie a large dural defect (fig 1F1). CSF from the fourth ventricle was flowing directly out of the defect into the cavity. Nervous tissue discoloured by haemosiderin deposition was noted. A fascia lata graft was harvested from the right thigh and was used to patch the dural defect (fig 1F2). Macerated muscle was placed over the repaired dura which was then buttressed with Gelfoam.

A fluid filled collection in the posterior fossa is often interpreted as an incidental cyst or prominent cisterna magna. The overlying fracture of the inner table and the perioperative finding of a large dural defect communicating with the cyst suggests that the cause was likely post‐traumatic. Intraspinal fluid filled cavities associated with SS are a recently recognised finding. This report suggests that an intracranial dural defect may also be the underlying cause of SS. Bleeding from friable vessels in the cyst wall of intraspinal fluid filled cavities has been considered to be a likely bleeding source.³ Our patient had no demonstrable cyst wall. Friable vessels in the diploic space were the likely bleeding source. It is speculative whether exposure of CSF to extradural tissue may lead to SS by an unidentified mechanism. Given the long natural history of SS, the available follow‐up duration of 3 months is too brief to assess response to the intervention. Intradiploic CSF fistulas are exceedingly rare.⁸ They are generally benign and are often related to trauma or neurosurgical procedures. No case with associated SS has been reported.