mercoledì 26 aprile 2006

Superficial siderosis








Findings

Figure 1 and Figure 2: T2 and gradient echo hypointensity is demonstrated along the pial surfaces of the brain, and subarachnoid 7th and 8th cranial nerves. The hypointensity seen on the gradient echo images is more prominent than that on the T2/FLAIR sequences, consistent with blooming due to hemosiderin deposition.
Figures 3, Figure 4, and Figure 5: T2 and gradient echo hypointensity is demonstrated along the pial surfaces of the brain, spinal cord, and the subarachnoid courses of the visualized nerve roots. As in Figure 1 and Figure 2, the hypointensity seen on the gradient echo images is more prominent than that on the T2/FLAIR sequences, consistent with blooming due to hemosiderin deposition.


Diagnosis: Superficial siderosis


Superficial siderosis (SS) of the central nervous system is the sequela of chronic recurrent subarachnoid hemorrhage. Hemosiderin, a blood product of chronic hemorrhage, is deposited along the leptomeningeal surfaces of the brain and spinal cord, as well as the subarachnoid surfaces of the cranial and spinal nerves. The hemosiderin deposition is not benign. The leptomeninges become thickened and the affected nervous tissue can undergo neuronal loss, reactive gliosis, and demyelination. Classic locations for hemosiderin deposition in SS are the peripheral folia of the cerebellum. Hemosiderin deposited here is usually associated with Bergmann gliosis and death of Purkinje cells. This leads to cerebellar atrophy, especially involving the vermis.

The cranial nerves are often affected with preferential involvement of cranial nerves VIII and I. These nerves seem to be more frequently affected due to their relatively long courses through the subarachnoid space. As the hemosiderin can be neurotoxic, patients with SS can present with sensorineural hearing loss and anosmia. Symptomatic involvement of cranial nerves V and VII also occur with some frequency. SS is associated with hydrocephalus, as the subarachnoid granulations become impaired from the chronic subarachnoid hemorrhage.

The MR findings of SS are characteristic. Pial T2 and gradient echo (GRE) hypointensity is demonstrated along the leptomeningeal surfaces of the brain and spinal cord. The pons and cerebellum are two of the most frequently affected regions. T2 and GRE imaging also show hypointensity along the subarachnoid portions of the cranial and spinal nerves. The hypointensity demonstrated on the T2 “blooms” on GRE imaging due to hemosiderin MRI susceptibility.

The etiology of SS includes trauma, vascular anomalies, and neoplasm. Any process producing subarachnoid hemorrhage can result in SS. Between 80% and 90% of nontraumatic subarachnoid hemorrhage is the result of intracranial aneurysm rupture. Other vascular malformations that have significant frequencies of hemorrhage are AVMs and cavernous angiomas. Diagnosis of SS mandates an evaluation for the source of the hemorrhage and should include both intracranial MRI/MRA, as well as total spine MRI. Traditional cerebral - spinal angiography may be necessary to identify subtle vascular abnormalities. The etiology of SS is not identified in 25% to 46% of patients.

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