Multi-parametric neuroimaging evaluation of cerebrotendinous xanthomatosis and its correlation with neuropsychological presentations

Cerebrotendinous xanthomatosis (CTX) is a rare, autosomal recessive, lipid storage disease. Leukoencephalopathy, signal abnormalities of dentate nuclei of the cerebellum, is frequently reported in conventional brain magnetic resonance imaging (MRI) [1]. Recent studies have shown that brain damage in CTX patients extends well beyond the abnormalities observed on conventional neuroimaging studies [1], and that this maybe responsible for the neurologic manifestations.

Voxel-based morphometry (VBM) is an unbiased, whole brain quantitative method that can detect atrophies of gray matter (GM) and white matter (WM) [2]. Diffusion tensor imaging (DTI) can be used to investigate WM tract pathology since it is sensitive to water diffusion [2]. Fractional anisotropy (FA) and mean diffusivity (MD) [3] can be derived from DTI to provide information on micro-structural changes. FA is a reflection of the degree of directionality of cellular structures within the fiber tracts [4, 5], while MD represents the magnitude of diffusion and reveals information on tissue integrity [6]. Analyses of axial and radial diffusivities provide potential measures of the mechanisms that underlie diffusion tensor changes. Tract-based spatial statistics (TBSS) focuses on spatial localization of diffusion-related changes using voxel-wise comparison between patients and normal controls [7]. Diffusion MRI provides quantitative measurements compared with conventional MRI in detecting WM pathology and is better suited for a more comprehensive evaluation of diffusion changes in WM [3].

In this study, we chose 3-dimensional T1 MRI, DTI and 99 mTc-ethyl cysteinate dimer (ECD) single photon emission computed tomography (SPECT) to explore the structural and functional changes in five CTX patients. The multi-parametric modalities provided quantitative measurements which could be compared with age-matched controls and provided a better understanding of the regions being affected in CTX as compared with conventional imaging. Correlations between the neuroimaging modalities and with neuropsychological presentations were also analyzed.

With the advent of molecular genetic analysis, the role of conventional MRI in diagnosing CTX has been considerably reduced [1]. Despite this, quantitative assessment of WM integrity is still important clinically since neuronal and mitochondrial dysfunction are known to occur in tissues that appear normal [15]. The cross modality comparison in this study indicates that CTX features in the brain are complex and structural imaging analysis such as diffusion measures and VBM may provide complimentary information to conventional MRI.

Combined with VBM and SPECT results, several patterns are observed in our study. First, the cerebellum, especially the peri-dentate region, is the major target of involvement in CTX both functionally and structurally. Second, despite the extensive GM involvement, hypo-perfusion in SPECT is consistent with the results of GM atrophy with a higher t value, which suggests that GM atrophy plays a key role in decreased perfusion. Third, decreased FA and increased MD in the left cerebral peduncle, bilateral peri-ventricular and peri-dentate regions correspond with the WM atrophy shown by VBM, suggesting irreversible processes in these selective regions. Changes in diffusion preceded VBM in the anterior/posterior corpus callosum and other WM regions suggesting improved sensitivity of DTI in detecting WM changes. Lastly, although treatment with CDCA is reported to delay or even reverse disease progression [16–18], progressive peri-dentate WM changes observed in our longitudinal study may argue otherwise.

The current study reports increased diffusivity in normal appearing WM, beyond the affected WM regions where both decreased FA and increased MD were noticeable. In theory, changes in diffusion can be attributed to an increase in extra-cellular space, more myelin permeability with axonopathy or the presence of elements other than myelin sheaths within these structures, which have a greater diffusion capacity than water [19, 20]. The VBM results here show regions of WM atrophy that may explain the effect of tissue atrophy to increase extra-cellular space and change water diffusion. In most case reports on CTX, WM lesions are referred to as demyelinating [21–24]. However, this term is considered relatively loose because there is also extensive axonal loss in the cerebellar and midbrain lesions. Pathology reports by both Soffer et al [25] and Pop et al [26] delicately describe the pathology of WM tract and emphasize the examination of myelin and axon integrity. Their reports suggested that demyelination did not result from abnormal myelin composition in CTX. Instead, myelin loss is secondary to axonal loss. They also indicated that the occurrence of axonal spheroids in the brain stem suggested the pathology of WM as axonal degeneration.

The role played by axonal injury may also partly explain the irreversibility and progression of cognitive decline in the study patients despite CDCA treatment. Extensive astrocytosis accompanied by sheets of macrophages and crystalline clefts, that are extensively reported in the literature on CTX pathology, may cause a change in water diffusion [21–24]. However, based on the fact that the diffusion coefficients of these elements are smaller than water, FA and MD changes in our study were less likely to be from the contribution of these substances. Combining the pathology features previously reported, demyelination, axonopathy, and tissue atrophy all occur in CTX.

Because diffusion indices such as FA and MD are derivatives of three tensor eigenvalues, the correct interpretation of diffusion changes should consider the contribution of each eigenvalue. The study results show that increased MD from DTI data are contributions by the increments of axial and radial eigenvalues. Decreased FA can be attributed to the fact that the increment in radial diffusivity is larger than the corresponding ones. Increased radial diffusivity is likely to reflect myelin damage [19]. Increased axial diffusivity is related to axonopathy, which has previously been observed in neurologic disorders with axonal lesions [20, 21]. Therefore, analysis of axial and radial diffusivity also suggests the presence of both demyelinating and with axonopathic lesions in CTX.

Only one case report [27] in the literature mentions SPECT findings in CTX, which showed cerebellar and bi-parietal hypo-perfusion. The current study results not only reiterate previous findings but also show that the perfusion patterns reflected the pattern of GM atrophy. The radioactivity of ECD SEPCT is dependent on regional blood perfusion [28] and is reasonable to show the relationship with the atrophic pattern. However, from CTX pathology literature, the cyto-architecture of the cerebral cortex is preserved, showing no apparent neuronal loss or any significant neuronal changes [15]. It is presumed that the "dying back" phenomenon is a mechanism of GM atrophy because these neurons are unable to maintain metabolic integrity. The major involvement of the pyramidal tract in our FA results with greater peri-Sylvian atrophy in VBM support this notion. The toxic accumulation of cholestanol, crystalline clefts, and the presence of gliosis and macrophages may cause axonal damage and further neuronal degeneration. The greater atrophy of WM in the posterior peri-ventricular region is also evidence of this hypothesis.

There is increasing evidence that WM abnormalities are associated with dementia [29]. Our study reveals that in addition to cortical atrophy, there is also damage in the major associative fiber tracts in CTX. These tracts are unique to humans and cannot be identified as discrete fiber bundles in rodents, primates, or human fetal brains [30]. The net effect of major associative fiber injury in CTX suggests the impairment of higher cognitive functions, which are also well demonstrated in the current correlation results. As regards the neuropsychological presentation, the five patients already had mild to moderate mental retardation upon diagnosis. Although the imaging resolution of this DTI study does not reveal cellular-level connectivity information for the brain cortex, it is still of great interest to investigate deficits associated with these bundles. Therefore, in addition to GM involvement, the cognitive presentations with the correlation study of the CTX patients can be partly explained by the poor integrity of these fiber tracts. The involvement of the associative tracts supports the theory that CTX is a developmental disorder [30].

There are several limitations to this study. First, this study enrolled a small number of patients. Because of the rarity of this disorder, the interpretation presented here might represent the results related to the two families. Whether it can be applied to the general population of CTX families requires a larger population study. Second, we cannot provide histology proof in this study because this is a cross sectional study on human subjects. Therefore, the interpretation of the pathological changes through neuroimaging findings (as reflected in changes in axial and radial eigenvalues) cannot be proved. Nonetheless, the results are consistent with previous pathology studies where myelin damage is constant with or without axonal destruction. It is worth pointing out that a direct correlation between axial and radial diffusivity parameters and the microscopic pathology of white matter injury has not been conclusive. As such, interpretation of changes in axial/radial eigenvalues as demyelination or axonopathy [31, 32] should be treated with great care, especially in regions where neuron fibers cross [33].

This study shows that DTI and VBM can provide complementary information regarding the GM or WM involvement in CTX, and that they are more sensitive than conventional neuroimaging studies. From imaging study results, WM involvement including demyelination, axonal changes and tissue loss suggest irreversibility, especially in the peridentate areas. Major associative fiber involvement as well as cortical atrophy account for the broad spectrum of cognitive deficits in these patients. The cognitive decline and image changes in conventional MRI indicate neuro-toxicity and possible irreversibility of this disease despite using CDCA.