Susac syndrome (SS), also known as SICRET syndrome (small infarctions of cochlear, retinal and encephalic tissue), is a rare syndrome typically affecting young to middle-age women that is clinically characterised by the triad of acute or subacute encephalopathy, bilateral sensorineural hearing loss, and branch retinal arterial occlusions.
The condition more commonly affects females around the 2nd to 4th decades 1. Approximately three females are affected for every male case 1.
It consists of a clinical triad of:
- acute or subacute encephalopathy which could manifest in a broad spectrum of symptoms, e.g. memory impairment, confusion, behavioral disturbances, ataxia, dysarthria, paranoid psychosis, and headaches 1-8
- sensorineural hearing loss at low and medium frequencies 1-8
- branch retinal artery occlusions leading to scotomata and vision distortion 1-8
All clinical features may not be present at the same time, and may in fact fluctuate over a 1-2 year period, which often leads to delay in diagnosis 1.
Branch retinal artery occlusions (BRAO) should be sought by fluorescein angiogram as it may help differentiating from multiple sclerosis 16.
The primary pathology is likely to be an autoimmune endothelialopathy (microangiopathy) affecting small vessels, with subsequent embolisation, of the brain, retina, and inner ear 1,3,9. Hence, microinfarcts associated with endothelial proliferation and hypertrophy, basement membrane thickening, and thickening of precapillary arteriolar adventitia with loss of internal elastic lamina 16. Demyelination is not a pathological hallmark of Susac syndrome, in contrast with multiple sclerosis 16.
Anti-endothelial cell antibodies (AECAs) have been reported as being a promising marker in some studies, but these are not yet considered to be specific to the syndrome 10.
Characteristic radiographic features are present even if all components of the clinical triad have not yet manifested 11. There tend to be multiple, small white matter lesions which have a predilection for the corpus callosum 1,2,8,11.
Callosal lesions are considered almost pathognomonic and have many important characteristic features:
- typically involve the central fibers of the callosal body and splenium without abuting the callosal undersurface (with relative sparing of the periphery) 1-2,8,11,16
- lesions are small (3-7 mm) and resembling 16:
- 'snowballs' on T2 andFLAIR when acute
- 'punched out' holes on T1 when chronic
- lesions, best appreciated on sagittal views, depending on the sequence used 11,12
- the roof of the corpus callosum is also frequently involved, rather than the callososeptal interface (which is more typical of multiple sclerosis), resulting in 'icicle' or linear 'spoke' lesions that look to hang from the roof of the corpus callosum 11
In addition to the corpus callosum, lesions can also involve the periventricular white matter, centrum semiovale, cerebellum, brainstem, and middle cerebellar peduncles 13. A 'string of pearls' appearance due to punctate microinfarcts involving the internal capsule has been described 17.
Signal characteristics of all of these lesions include:
- T1: lesions are low signal, especially in the chronic stage (see T1 black holes) 1,2,8,11
- T1 C+ (Gd): lesions frequently enhance during the acute stage 1,2,8,11
- T2/FLAIR: lesions are high signal 1,2,8,11
- DWI/ADC: normal, although T2 shine-through may be appreciated on DWI 1,11
Treatment and prognosis
Although Susac syndrome generally has a monophasic and self-limiting course, case series-level evidence suggests management of the acute episode with immunosppressants such as prednisolone 14. Intratympanic injection of dexamethasone has also shown to help with hearing loss, although in severe cases cochlear implants should be considered 6,14. Some patients may have residual neurological sequelae, especially related to retinal infarction 14.
History and etymology
Initially described by John O. Susac (1940-2012), an American neurologist and neuro-ophthalmologist, and his colleagues in their 1979 seminal paper describing two patients with the classic clinical triad 3. The eponym was famously first used in 1986, when a patient was presented in a neuro-ophthalmology conference with the same triad that Susac et al. had described, prompting one member of the conference to exclaim “this is Susac’s syndrome” 15.
Considerations on the basis of pure radiographic (MRI) appearances include:
- demyelinating conditions
- multiple infarcts caused by
- transient lesion of splenium of corpus callosum: shows restricted diffusion and no enhancement.
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