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|Year : 2015 | Volume
| Issue : 6 | Page : 915-917
Neurocognitive impairment in Susac syndrome: Magnetic resonance imaging and Tc-99m hexamethylpropyleneamine oxime single photon emission computed tomography correlation
Dalveer Singh, Charlie Chia-Tsong Hsu
Department of Medical Imaging, The Townsville Hospital, Douglas, Queensland, Australia
|Date of Web Publication||20-Nov-2015|
Department of Medical Imaging, The Townsville Hospital, Douglas, Queensland 4102
Source of Support: None, Conflict of Interest: None
Susac syndrome is a clinical triad of branch retinal artery occlusions, sensorineural hearing loss, and encephalopathy. The characteristic central corpus callosum involvement in Susac syndrome is readily recognizable on conventional magnetic resonance imaging (MRI); however, the neurocognitive effect of these lesions is not well-understood. We present a case of Susac syndrome with typical MRI findings of central callosal lesions at diagnosis. The patient had a protracted clinical course and did not respond well to immunosuppression therapy. Follow-up brain single photon emission computed tomography with Tc-99m hexamethylpropyleneamine oxime revealed marked unilateral frontoparietal and temporal lobe hypoperfusion. Our case highlights the utility of functional neuroimaging to uncover the possible underlying white matter dysfunction, which is not otherwise detectable with conventional MRI techniques.
Keywords: Corpus callosum; single photon emission computed tomography; Susac syndrome; Tc-99m hexamethylpropyleneamine oxime
|How to cite this article:|
Singh D, Hsu CC. Neurocognitive impairment in Susac syndrome: Magnetic resonance imaging and Tc-99m hexamethylpropyleneamine oxime single photon emission computed tomography correlation. Neurol India 2015;63:915-7
|How to cite this URL:|
Singh D, Hsu CC. Neurocognitive impairment in Susac syndrome: Magnetic resonance imaging and Tc-99m hexamethylpropyleneamine oxime single photon emission computed tomography correlation. Neurol India [serial online] 2015 [cited 2019 Dec 11];63:915-7. Available from: http://www.neurologyindia.com/text.asp?2015/63/6/915/170076
| » Case Report|| |
A 45-year-old female patient presented to the emergency department in an acute state of confusion on a background of major depression diagnosed 16 months ago. On assessment, she was profoundly disorientated, showed expressive dysphagia, and family members reported her declining short-term memory over several months. Neurological examination was pertinent for reduced visual acuity on the right to 20/70 compared to 20/20 on the left. Normal tone, reflexes, and power were observed in the upper and lower limbs. Sensation and proprioception were also intact. Magnetic resonance imaging (MRI) of the brain showed multiple rounded T2 hyperintense lesions in the body and splenium of the corpus callosum [Figure 1]a and [Figure 1]b. On the T1 sagittal image, these lesions were hypointense and situated within the central layer of the corpus callosum [Figure 1]c. The differential considerations at this point included primary demyelination, vasculitis, and, in particular, the central location of lesions within the callosal fibers raised the possibility of Susac syndrome. This prompted audiometric evaluation, which revealed the bilateral low-frequency sensorineural hearing loss. Retinal fluorescein angiography showed peripheral retinal arteriolar branch occlusions in the right eye. Despite the partial response to corticosteroid therapy, there were residual cognitive impairments affecting memory and concentration. Follow-up MRI at 6 months showed no interval change in the morphology of the existing lesion and no new lesion was detected. Given the persistent cognitive impairment, single photon emission computed tomography (SPECT) scan of the brain with Tc-99m hexamethylpropyleneamine oxime [HMPAO] (CERETEC ®) SPECT was performed. It showed a marked reduction in the cerebral blood flow in the right frontoparietal and temporal lobes despite the presence of a symmetric brain volume [Figure 2].
|Figure 1: Magnetic resonance imaging brain axial T2-weighted images (a and b) showing multiple rounded hyperintense lesions ranging in size from 5 mm to 10 mm in the body and splenium of corpus callosum (white arrows). Sagittal T1-weighted image (c) showing discrete hypointense lesions within the central layer of the corpus callosum|
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|Figure 2: Serial axial Tc-99-labeled hexamethylpropyleneamine oxime single photon emission computed tomography images showing a marked reduction in the cerebral blood flow in the right frontotemporal and parietal lobes despite the presence of symmetric brain volume. There is a symmetric tracer uptake in the cerebellum|
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| » Discussion|| |
Susac syndrome is the clinical triad of retinal arteriolar branch occlusions, sensorineural hearing loss, and subacute encephalopathy, with predominantly cognitive impairment, memory loss, and psychiatric symptoms.,, This rare condition predominantly affects females in the third to fifth decades. The pathophysiological basis is hypothesized to be an autoimmune-mediated occlusive endotheliopathy involving arterioles (<100 µm) in the retina, cochlea, and brain causing microinfarcts. Patients do not always present with the classical triad, and the degree of neurological disturbance can be variable., Brain MRI findings in Susac syndrome can show both gray and white matter lesions, with basal ganglia and thalamic lesions seen in up to 70% of cases. Parenchymal lesions in the acute phase can demonstrate gadolinium enhancement, while leptomeningeal enhancement is less frequently observed. White matter lesions in Susac syndrome can resemble multiple sclerosis (MS) or acute disseminated encephalomyelitis. A distinctive feature of Susac syndrome, however, is that it almost always involves the central callosal fibers, which are predisposed to arterial microinfarcts. On the contrary, plaques in MS are seen on the ependymal under-surface of the corpus callosum along venous drainage pathways. A recent 7T MRI study comparing MS and Susac syndrome has found key differences in lesion morphology on T2*-weighted sequences, namely the presence of a hypointense rim at the edges of MS plaques, which is not seen in Susac syndrome. The significance of these callosal lesions in Susac syndrome and their relation to the observed neurocognitive impairment is not well-understood. A recent diffusion tensor imaging study in Susac syndrome revealed damage to the callosal fiber integrity and prefrontal white matter, which correlated much better with the clinical severity of encephalopathy than the conventional MRI findings. Brain perfusion and metabolism alterations are yet to be fully investigated in Susac syndrome, with only one reported case showing unilateral decreased cerebral glucose metabolism utilizing fluorine 18-fluorodeoxyglucose positron emission tomography, which resolved with treatment. Interestingly, our case showed markedly reduced unilateral cerebral hemispheric perfusion with Tc-99 HMPAO SPECT. The significance of this finding is not entirely clear; however, the reduced cerebral perfusion in the right hemisphere may be secondary to callosal fiber disruption. This is supported by similar findings of reduced cerebral perfusion and glucose metabolism in other pathologies affecting the corpus callosum such as large anterior corpus callosal infarct, callosal hematoma, and Marchiafava-Bignami disease.,,, The neurocognitive disturbance reported in Susac syndrome, in particular the frontotemporal executive functions, may be due to macroscopic axonal disruption of the corpus callosum as described above, or due in part to microscopic arteriolar endotheliopathy beneath current imaging resolution. Long-term outcomes in Susac syndrome are not well known and rely upon small case series with variable follow-up. The spectrum of reported neurological deficits ranges from behavioral or personality changes, memory, concentration or mild cognitive disturbance, ataxia, and weakness. The largest long-term follow-up case series (mean follow-up 6.4 years) in 9 patients found no correlation between the initial severity of encephalopathy and the long-term neurocognitive decline, which was seen in half the patients at final follow-up.
Callosal lesions in Susac syndrome may well represent the "the tip of iceberg" for greater underlying white matter dysfunction. Further functional neuroimaging studies may improve understanding of the neurocognitive impairment in Susac syndrome and potentially aid in the assessment of response to immunosuppressive therapy.
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[Figure 1], [Figure 2]