Susac syndrome (SS), also known as SICRET syndrome (small infarctions of cochlear, retinal and encephalic tissue), is a rare syndrome typically affecting young-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 afflicted 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, such as: 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.
The primary pathology is likely to be an autoimmune endothelialopathy affecting small vessels, with subsequent embolisation, of the brain, retina, and inner ear 1,3,9.
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, with relative sparing of the periphery 1,2,8,11
- lesions are small (3-7 mm) and resembling 'snowballs' or 'punched out' hole 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
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:
- 1. Susac JO. Susac's syndrome. AJNR Am J Neuroradiol. 2004;25 (3): 351-2. AJNR Am J Neuroradiol (full text) - Pubmed citation
- 2. Demir MK. Case 142: Susac syndrome. Radiology. 2009;250 (2): 598-602. doi:10.1148/radiol.2502051865 - Pubmed citation
- 3. Susac JO, Hardman JM, Selhorst JB. Microangiopathy of the brain and retina. Neurology. 1979;29 (3): 313-6. Neurology (abstract) - Pubmed citation
- 4. White ML, Zhang Y, Smoker WR. Evolution of lesions in Susac syndrome at serial MR imaging with diffusion-weighted imaging and apparent diffusion coefficient values. AJNR Am J Neuroradiol. 2004;25 (5): 706-13. AJNR Am J Neuroradiol (full text) - Pubmed citation
- 5. Do TH, Fisch C, Evoy F. Susac syndrome: report of four cases and review of the literature. AJNR Am J Neuroradiol. 2004;25 (3): 382-8. AJNR Am J Neuroradiol (full text) - Pubmed citation
- 6. Crawley BK, Close A, Canto C et-al. Susac's syndrome: intratympanic therapy for hearing loss and a review of the literature. Laryngoscope. 2009;119 (1): 141-4. doi:10.1002/lary.20040 - Pubmed citation
- 7. Gross M, Eliashar R. Update on Susac's syndrome. Curr. Opin. Neurol. 2005;18 (3): 311-4. Curr. Opin. Neurol. (link) - Pubmed citation
- 8. Saenz R, Quan AW, Magalhaes A et-al. MRI of Susac's syndrome. AJR Am J Roentgenol. 2005;184 (5): 1688-90. doi:10.2214/ajr.184.5.01841688 - Pubmed citation
- 9. Susac JO, Egan RA, Rennebohm RM, Lubow M. Susac's syndrome: 1975-2005 microangiopathy/autoimmune endotheliopathy. J Neurol Sci. 2007 Jun 15;257(1-2):270-2. Epub 2007 Feb 28. Pubmed citation
- 10. Magro CM, Poe JC, Lubow M, Susac JO. Susac syndrome: an organ-specific autoimmune endotheliopathy syndrome associated with anti-endothelial cell antibodies. American journal of clinical pathology. 136 (6): 903-12. doi:10.1309/AJCPERI7LC4VNFYK - Pubmed
- 11. Nidhi Garg, Stephen W Reddel, David H Miller, Jeremy Chataway, D Sean Riminton, Yael Barnett, Lynette Masters, Michael H Barnett, Todd A Hardy. The corpus callosum in the diagnosis of multiple sclerosis and other CNS demyelinating and inflammatory diseases. J Neurol Neurosurg Psychiatry. doi:10.1136/jnnp-2014-309649 - Pubmed
- 12. Oliveira P, Mayeux J. Teaching NeuroImages: Snowball-like lesions with sudden hearing loss. Neurology. 82 (12): e100. doi:10.1212/WNL.0000000000000244 - Pubmed
- 13. Rennebohm R, Susac JO, Egan RA, Daroff RB. Susac's Syndrome--update. Journal of the neurological sciences. 299 (1-2): 86-91. doi:10.1016/j.jns.2010.08.032 - Pubmed
- 14. Rennebohm RM, Egan RA, Susac JO. Treatment of Susac's Syndrome. Current treatment options in neurology. 10 (1): 67-74. Pubmed
- 15. Rennebohm R, Daroff RB. In memoriam. John O. Susac, MD (1940–2012). Neurology. 79 (3): 211-2. Pubmed