B. Keegan Markhardt, MD and PL Westesson MD, DDS, PhD
Clinical
Presentation: A 69-year-old female with history of Marfan's syndrome, aortic valve replacement in 1992, Bentall procedure secondary to annuloaortic ectasia in 1993, subdural hematoma secondary to supratherapeutic anticoagulation, and dural ectasia with root avulsions, now presents with gait abnormality.
Radiological Findings: CT of the head shows a hyperdense rim along the surface of the midbrain (Fig. 1). MR of the brain demonstrates a low T2WI signal rim (Figs. 2, 4, 6) and faintly high T1WI signal rim (Figs. 3, 5) around the basal cisterns, vermis, brainstem and spinal cord.
Figure 1: Axial CT at level of the midbrain. There is hyperdensity along the surface of the midbrain from increased iron content.
Figure 2: Sagittal T2WI MR image midline. Hypointense hemosiderin staining is seen along the brainstem, pons and cervical spinal cord.
Figure 3: Axial T1WI MR image at level of the midbrain. Hyperintense signal from hemosiderin is along the surface of the midbrain.
Figure 4: Axial T2WI MR image at level of the midbrain. Hemosiderin staining of the surface of the midbrain and cerebellum is illustrated by hypointense staining along the surface of the midbrain and in the folia of the vermis.
Figure 5 : Axial T1WI MR image at level of quadrigeminal cistern.
Figure 6 : Axial T2WI MR image at level of quadrageminal cistern. Hypointense hemosiderin staining is seen around the quadrigeminal cistern.
Differential diagnosis: Neurocutaneous melanosis and meningioangiomatosis
Diagnosis: Superficial siderosis of the central nervous system
Clinical Discussion: Superficial siderosis of the central nervous system is a rare condition characterized by deposition of hemosiderin on the cerebellum (especially the vermis), basal frontal lobe and olfactory bulbs, temporal cortex, brainstem and cranial nerves (especially VIII), spinal cord and nerve roots [1]. Superficial siderosis is characterized by sensorineural deafness (95%), cerebellar ataxia (88%), pyramidal signs (76%) and a mired of less common neurological defects. Bilateral deafness progresses over a course of 1-15 years. The age of onsethas been described between 14 to 77 years. Males are more often affected than females (3:1). CSF examination reveals high protein in 100% and xanthochromia in 75% of cases [2].
The deposition of hemosiderin is due to repeated chronic subarachnoid or intraventricular bleeding most commonly secondary to repeated hemorrhages from tumors (especially ependymomas), vascular malformations and subdural hematomas. In 46% of cases no source of bleeding is found [2]. Macroscopically, there is a brownish discoloration of the leptomeninges and of the adjacent CNS parenchyma up to a depth of 3 mm [1]. Microscopically, there is hemosiderin deposition, neuronal loss, reactive gliosis, and demyelination. The vulnerability of cranial nerve VIII likely results from its long glial segment exposed to CSF and course through the pontine cistern, which contains a large pool and high flow of CSF. This results in a greater length of hemosiderin deposition on the nerve and, consequently a greater chance of axonal damage.
CT may occasionally reveal atrophy of the cerebellar vermis or a hyperdense rim over the brain surface, corresponding to hemosiderin deposition. MRI is the gold standard for diagnosis. Gradient-echo T2-weighted images are sensitive to the presence of hemosiderin. On T2WI, the pathognomonic features of superficial siderosis are a rim of hypointensity around the brainstem and cerebellum (greatest at the vermis). Similar changes may be seen at the VIIth nerve and periphery of the spinal cord [2]. On T1WI, a hyperintense signal may be seen on CNS surfaces. No enhancement is seen. Vermal atrophy and the underlying bleeding source occasionally may be identified.
MRI of the spine should be included for investigation of a bleeding source. Spinal cord sources include tumors, arteriovenous malformations and root avulsions. If there is a high clinical suspicion for aneurysm angiography may be pursued.
The differential diagnosis of superficial siderosis includes neurocutaneous melanosis and meningioangiomatosis [2]. Neurocutaneous melanosis is a congenital syndrome consisting of benign pigment cell tumors which may involve the leptomeninges to produce a similar high T1WI, low T2WI changes [3]. However, neurocutaneous melanosis is identified by presence of large congenital melanocytic nevus and may be distinguished by this physical finding. Meningioangiomatosis is a rare, benign, focal lesion of the leptomeninges and underlying cerebral cortex characterized by leptomeningeal and meningovascular proliferation and fibrosis [4]. Mixed T1 and T2 signal and, occasionally, enhancement may be seen in these lesions. The locality of these lesions, mixed signal characteristics and occasional enhancement of these lesions should distinguish them from the diffuse character of superficial siderosis.
Treatment of superficial siderosis is focused around treating the source of bleeding.
References:
Fearnley JM, et al. Superficial siderosis of the central nervous system. Brain 1995; 118:1051-1066. [Medline]
Osborn AG, et al. Diagnostic Imaging: Brain. Amirsys 2004.
DeDavid M et al. Neurocutaneous melanosis: clinical features of large congenital melanocytic nevi in patients with manifest central nervous system melanosis. Am Acad Dermatol. 1996 Oct; 35(4):529-38. [Medline]
Wiebe S, Munoz DG, Smith S, Lee DH. Meningioangiomatosis: A comprehensive analysis of clinical and laboratory features Brain 1999 Apr;122 (Pt 4):709-26. [Medline]
