|RANZCR ASM 2011 / R-0109|
|Diffusion Tensor Imaging and Fiber Tractography in Joubert Syndrome|
|This poster was originally presented at the RANZCR Annual Scientific Meeting 2011, October 6-9, in Melbourne/AU.|
In all three JS patients, conventional MR imaging showed the classic molar tooth sign with a deep interpeduncular fossa, thickened superior cerebellar peduncles, a widened fourth ventricle and hypoplasia of the cerebellar vermis (Fig 1 and Fig 2). All patients demonstrated widening of the dilatation fourth ventricle with bat’s-wing shape on axial view and increased convexity of the roof of the fourth ventricle on sagittal view (Fig 3 and Fig 4). All patients showed an elongated mesencephalon, a narrowed isthmus, and a normal quadrigeminal plate and pons. No patients showed supratentorial anomalies or polymicrogyria.
In two JSRD patients with DTI acquisition, the fibers in the superior cerebellar peduncles were oriented horizontally as represented by a green color coding on the FA-maps, in contrast to the slight vertical orientation blue color coding in healthy control subjects (Fig 5).
Furthermore, in both patients these fibers projected into the red nuclei and thalami without decussating. This was demonstrated by absence of the transverse fibers at the level of the inferior colliculi of the midbrain, as absence of the characteristic “focal red dot” deep within the interpeduncular fissure on color coded FA texture and FA tensor maps (Fig 6). Red dot sign was seen in normal control subjects.Failure of the superior cerebellar peduncles (SCP) to decussate was also demonstrated by fibre tractography (Fig 7 and Fig 8). The SCP in both patients showed a obvious connection from the cerebellum to both the sensory and motor cortices when compared to normal subjects (Fig 9).
In control subjects FA texture maps, FA tensor maps and fibre tratography demonstrate decussation of the SCP at the level of midbrain (Fig 10, Fig 11 and Fig 12).
In the two JS patients the deep cerebellar nuclei (dentate nuclei) appeared more lateralized displaced by the widened fourth ventricle(Fig 13) when compared to normal control subjects (Fig 14).
JS is a rare autosomal recessive disorder representing developmental defects of the cerebellar vermis. The neuroradiologic hallmark of the molar tooth sign has traditionally aided the diagnosis. Only recently case reports and small case series have used DTI data sets to unravel the complex neuro-architecture of this rare posterior fossa malformation. The largest DTI case series by Poretti et al demonstrated that the fibers the superior cerebellar peduncles (SCP) did not decussate in the mesencephalon regardless of phenyotype-genotype subgroups .6 Widjaja et and Merlini et al have also described such findings.7,8 In our study, we confirmed and extended these findings to two more JS patients. In addition to fiber tractography, we found that FA texture map was useful in demonstrating absence of the transverse fibers at the level of the midbrain, with the absence of the characteristic "red dot sign"deep within the interpeduncular fossa.
To date, DTI studies of JS patients, as well as in our cases, have all demonstrated complete absence of decussating fibers of the SCP irrespective of genetic types of JSRD.6 Parisi et al, using functional magnetic resonance imaging, suggested at least some degree of decussation of the superior cerebellar peduncles and of the corticospinal tract.2 The difference maybe attributed to the heterogeneity of JS; however, it is difficult to determine if the current resolution of DTI and FT is sensitive enough to differentiate between complete absence versus decreased volume of the decussating fibers.
Lee et al pointed out an interesting observation in a JS patient—fiber tractography of the SCP showed a strong connection from the cerebellum to both the sensory and motor cortices.9 In both of our JS patients with fiber tractography performed, we were able to demonstrate this finding. This suggests that there is some modified connection from the cerebellum to the cerebral cortex in Joubert syndrome. Poretti et al showed that the corticospinal tract failed to cross in the caudal medulla in all the patients. Unfortunately, we could not validate this neuropathologic feature as our scanning parameter did not extend down to the caudal medulla. Lastly, in both of our patients, we demonstrated the known finding of abnormally shaped deep cerebellar nuclei namely dentate nuclei which were more laterally located and had a straight configuration.
Although we have confirmed key features of white matter tract abnormalities in two JS patients using DTI, we are only beginning to unravel the complexity of the aberrant fiber tracts in JS. The neuropathologic findings by Yachnis and Rorke suggested that there are other extensive malformations of the brain stem and fiber tract yet to be described by neuroimaging techniques.10 Pathologic findings suggested a more extensive malformations of the medulla included hypoplasia of the inferior olivary nuclei, solitary nuclei and tracts, and the nucleus and spinal tracts of trigeminal nerve (cranial nerve V). Even more striking was dysplasia of the caudal medulla at the cervicomedullary junction, which was characterized by the absence of a posterior median sulcus, neuronal swelling and axonal spheroids in the region of malformed nuclei gracilis and cuneatus, and absence of pyramidal decussation.
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