Susac Syndrome: Comprehensive Imaging Guide and Clinical Overview
Overview
Susac syndrome is a rare, presumably autoimmune disorder characterized by a clinical triad of encephalopathy, branch retinal artery occlusions, and sensorineural hearing loss. The syndrome is named after Dr. John Susac, who first described the constellation of symptoms. It typically presents in young adults and is considered self-limited but can lead to severe neurological, visual, and auditory impairment. The inheritance pattern is sporadic, with no clear genetic linkage established, suggesting an autoimmune microangiopathy affecting small blood vessels in the brain, retina, and inner ear.
Key Imaging Features
- MRI reveals multifocal, supratentorial white matter lesions with a hallmark involvement of the corpus callosum, showing characteristic central “punched-out” or “snowball” lesions primarily in the central fibers.
- Lesions frequently extend to the deep gray matter nuclei such as the basal ganglia and thalami, seen as hyperintense areas on T2-weighted sequences.
- Involvement of the posterior fossa, including the middle cerebellar peduncles, is often observed.
- Contrast-enhanced MRI may demonstrate patchy parenchymal enhancement and occasionally leptomeningeal enhancement, reflective of active inflammation.
- Diffusion-weighted imaging (DWI) can show transient restricted diffusion in acute lesions, depending on the time of imaging relative to symptom onset.
- The imaging appearance and distribution of corpus callosum lesions help differentiate Susac syndrome from demyelinating diseases, such as multiple sclerosis, where callosal lesions tend to be oriented peripherally rather than centrally.
Pathophysiology
Susac syndrome is believed to result from an autoimmune-mediated endotheliopathy causing microvascular occlusions in the arterioles supplying the brain, retina, and cochlea. This leads to ischemic injury manifesting as multifocal infarcts predominantly within the corpus callosum, deep gray nuclei, and retina. The characteristic central callosal lesions represent ischemic microinfarctions rather than demyelination, accounting for their distinct imaging morphology. The observed parenchymal and leptomeningeal enhancement corresponds to breakdown of the blood-brain barrier and inflammatory involvement of the small vessels.
Differential Diagnosis
- Multiple sclerosis: Predominantly affects periventricular white matter and corpus callosum with peripheral callosal lesions; lacks branch retinal artery occlusions and hearing loss; enhancing lesions are usually ovoid and oriented perpendicular to ventricles.
- CNS vasculitis: Can produce multifocal infarcts, but typically involves larger vessels with more diffuse vessel wall inflammation; MRI shows more mixed lesion patterns without isolated corpus callosum central lesions.
- CNS lymphoma: Enhancing mass-like lesions, often associated with edema rather than multifocal small infarcts.
- Mitochondrial encephalopathies (e.g., MELAS): Stroke-like lesions usually do not localize preferentially to the corpus callosum and are associated with metabolic abnormalities.
- Infectious or inflammatory encephalopathies: Usually have more diffuse or confluent lesions without the characteristic discrete corpus callosum involvement of Susac syndrome.
Imaging Protocols and Techniques
Optimal imaging of Susac syndrome requires a focused MRI protocol emphasizing detection of small infarcts and inflammatory changes, including the following sequences:
- T2-weighted and FLAIR sequences to identify white matter hyperintensities and assess lesion distribution within the corpus callosum, deep gray nuclei, and cerebellar peduncles.
- Diffusion-weighted imaging (DWI) and apparent diffusion coefficient (ADC) maps to detect acute ischemic changes and assess lesion evolution.
- Pre- and post-contrast T1-weighted imaging for evaluation of parenchymal and leptomeningeal enhancement indicative of active inflammation.
- High-resolution orbital and inner ear MRI may be considered when assessing retinal artery occlusions and cochlear involvement, respectively.
- Magnetic resonance angiography (MRA) is generally normal as the disease targets microvasculature rather than large vessels.
Serial imaging is valuable to monitor lesion progression or resolution and therapeutic response. Radiologists should be vigilant for the presence of central callosal lesions—this imaging pearl is crucial to avoid misdiagnosis as demyelinating disease.
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