Superficial siderosis of the central nervous system

Superficial siderosis (SS) is a rare condition which results from deposition of haemosiderin along the leptomeninges, with eventual neurological dysfunction.

The literature is divided as to whether the term superficial siderosis should be confined to cases where there is no history of symptomatic subarachnoid haemorrhage, or whether it is a blanket term referring to the superficial deposition of hemosiderin, irrespective of cause. 

For the purpose of this article, we take the later definition. 

Epidemiology

As there are many causes of recurrent or extensive subarachnoid haemorrhage, the demographics are ill-defined and represent those of the underlying cause. Cases have been reported between 14 and 77 years of age ⁵. Causes include ^1-6:

• spinal dural defects
  • traumatic cervical nerve root avulsion
  • post-operative pseudomeningocoele
• intracranial neoplasms
  • ependymoma
  • oligodendroglioma
  • astrocytoma
• vascular abnormalities
  • arteriovenous malformation (AVM)
  • aneurysm
  • fragile capillary regrowth after brain surgery ²
• cerebral amyloid angiopathy: seen in 60% of patients⁷
• idiopathic: up to 46% of the time ²

Overall there is a male predilection (M:F 3:1) ^2,5.

Clinical presentation

Symptoms can vary depending on the distribution of haemosiderin deposition. Common symptoms include: ^2-5:

• sensorineural hearing loss
  • most common, found in ~ 95~95% of patients
  • bilateral and gradual
• cerebellar dysfunction (ataxia): ~ 88~88%
• pyramidal signs: ~ 76~76%
• other less common findings include:
  • dementia
  • bladder incontinence
  • other cranial nerve dysfunction
  • sensory deficits

It is important to realize that the degree of imaging abnormality does not always correlate with the degree of clinical impairment ⁴. 

Pathology

Superficial siderosis is thought to result from recurrent occult subarachnoid bleeds although the source of bleeding is not usually identified in imaging ¹. Although it is common to see a small amount of hemosiderin deposition at the margins of a previous haemorrhage or surgical resection margin, a single episode of subarachnoid haemorrhage is usually not sufficient to result in this condition ². 

Postulated etiologies include:

• occult small ependymoma
• micro-arteriovenous malformation

Vestibulocochlear nerve (CN VIII) dysfuction, resulting in sensorineural hearing loss is believed to be due to the combination of a long cisternal course (thus with ample exposure to the subarachnoid space) and the susceptibility of micorglial cells (providing the myelin for the nerve) to damage by iron compounds ⁴. 

Radiographic features

MRI

MRI is the modality of choice for assessment and diagnosis of superficial siderosis. The findings are characteristic, with all pial and ependymal surfaces (particularly of the brainstem and cerebellum -: vermis and folia of the cerebellum are excellent locations to identify subtle deposits) coated with low signal haemosiderin. In long standing cases, cerebellar atrophy may also be present. 

• T1: - low low signal
• T2: - low low signal
• GE (gradient echo):- low low signal with blooming
• SWI:- low low signal with blooming

As part of the work up for superficial siderosis, if no lesion is identified in the intracranial compartment, then imaging of the whole spinal canal should be performed (e.g. myxopapillary ependymoma) ⁵. 

Angiography

Usually unrewarding and will not demonstrate a point of bleeding ¹.

Treatment and prognosis

Unfortunately no proven direct treatment exist for established siderosis, and workup is focused on identifying the causative lesion, although often even this is not possible. 

Iron chelating agents have been tried with limited anecdotal success ⁶. 

When no correctable cause is identified, signs and symptoms are slowly progressive.