We selected all the 8 cases of confirmed Susac syndrome at our institution and retrospectively reviewed the brain MRI of each patient. 

Six patients were female and the median age at diagnosis was 33 years old (ranging from 28 to 55 years of age). 

Not all of them had the clinical triad from the onset of symptoms and in two of them, the diagnosis was suggested on MRI.

All patients were studied with at least one MRI study at our institution (either 1.5T or 3T scans with standard brain protocol). In 5 patients we had access to follow-up studies. 

The main finding was multifocal supratentorial white matter lesions, including in corpus callosum, due to arteriolar microinfarctions. T2 and T2 FLAIR are the most sensitive sequences, presenting high signal lesions in the periventricular, subcortical and deep white matter (Figure 1).

In a few patients, we could also see high T2 signal intensities in basal ganglia, thalami and internal and external capsule (Figures 2 and 3). Microinfarcts may extensively involve the internal capsule giving the “string of pearls” appearance on T2 in patients with signs of long tract involvement. The presence of a “string of pearls” appears as relatively specific for Susac syndrome in the proper clinical setting [8].

In our series corpus callosum was affected in 7 patients: the lesions were round resembling ‘snowballs’ and in the central part of corpus callosum (Figure 4-8), unlike the lesions of multiple sclerosis and ADEM, the main differential diagnosis (where demyelination often involves the undersurface of the corpus callosum – Figure 9).

In the literature, the corpus callosum ‘snowball lesions’ are thought to be pathognomonic for SS and in its absence, many authors suggest that the clinical diagnosis should be questioned. However, in the biggest review of all document cases of SS, the corpus callosum lesions were present only in 78% of cases, which proves that the absence of lesion in the corpus callosum should not be sufficient to exclude this diagnosis [8]. In fact, we had one patient that fulfilled all the criteria for SS but had no corpus callosum lesions (Figure 10).

It has also been described the presence of central callosal holes on T1 weighted images due to microinfarctions [4] as we could see in some of our patients (Figure 11).

Less frequently, the posterior fossa and the brainstem can also be affected [9].

In the acute setting brain, lesions can show restricted diffusion on DWI, suggesting active inflammation and demyelination (Figure 12-14).

The location of the brain lesions was accessed for each case (Figure 15) with corpus callosum lesions being the most frequents.

In two cases it was possible to identify parenchymal enhancement (Figure 16) and one them had also leptomeningeal enhancement (Figure 17).

There is few literature about brain atrophy, and typically it is said that it should not be a prominent feature in SS. However, like Susac [1] and Wuerfel [10] showed, corpus callosum atrophy can be marked in these patients. In our cases, almost all of them showed severe atrophy of the corpus callosum in a follow-up range of 3-9 years after the diagnosis (Figure 18).

There are no diagnostic criteria for this rare disease and the definite diagnosis is done by assumption.

It is important to keep in mind that the MRI findings are not specific and they must be interpreted in the right clinical scenario. Nevertheless, in the right clinical setting, the radiologic findings can help in the diagnosis of this rare disease.