Thalamic projection fiber integrity in de novo Parkinson’s disease

Introduction

Parkinson’s disease (PD) is a progressive neurodegenerative disorder with a slow, predictable course through vulnerable regions of subcortical and cortical gray matter (GM)¹. The pathological hallmark of PD is the selective and substantial loss of dopaminergic neurons in the substantia nigra (SN) pars compacta^{1, 2}. It is generally accepted that a reduction of dopamine in the SN pars compacta underlies dysfunction within the direct basal ganglia (BG) pathway³, which results in reduced excitation of cortical neurons by the thalamus⁴. This increased inhibition of cortical-subcortical circuits in PD results in bradykinesia and varying degrees of cognitive impairment^5–7. Thus, while dysfunction of subcortical structures such as the BG and thalamus have historically been conceptualized as a consequence of SN pars compacta degeneration, there is also evidence to suggest that the thalamus may also undergo structural changes⁸, display functional abnormalities⁹, and be a site of direct disease pathology in PD^{1, 10, 11}. Given that symptom presentation in PD only occurs after significant dopamine loss, it is important to examine the integrity of structures further into the direct BG pathway, such as the thalamus, particularly early in the disease.

While conventional magnetic resonance imaging (MRI) has limited utility as an investigative and diagnostic tool in PD, recent advances in neuroimaging techniques have allowed for the non-invasive, in vivo assessment of PD-related pathology^{12, 13}. For example, diffusion tensor imaging (DTI) measures within the SN have shown excellent sensitivity and specificity in differentiating de novo PD patients from healthy controls¹⁴. Recent studies support the reduction in fractional anisotropy (FA) values within the SN of PD patients^15–17 and also suggest that microstructural changes occur in areas beyond the SN, including the thalamus¹⁶. As a technique, DTI is based upon the diffusivity of water molecules, which is variably restricted by different tissues. FA values, which are calculated from the diffusion tensor images, range from 0 to 1, with 0 representing isotropic diffusion and 1 anisotropic diffusion. In white matter (WM), where the tissue is highly organized, the free diffusion of water is limited, resulting in FA values near 1. FA values calculated in WM reflect a combination of several factors, including axonal density, degree of myelination, and orientational coherence^{18, 19}. The direction of diffusion is less restricted in GM, resulting in FA values near 0. It is unclear exactly what FA values represent in GM structures, but one hypothesis is that they reflect neuronal density, with lower FA values representing a decrease in density²⁰.

Post-mortem studies demonstrate that PD differentially affects specific thalamic nuclei^{10, 11, 21–23}, indicating that there could be degeneration or disruption of the microstructural integrity of the thalamus. To date, most studies utilizing DTI to investigate the thalamus have focused on the thalamus as a single entity, and have not evaluated the integrity of specific nuclei or the critical cortical-subcortical fibers that project from the thalamus^{16, 24–26}. Further, these studies examined relatively advanced PD patients who had been taking antiparkinsonian medication^{16, 24–26}, thus making it unclear whether differences between the patients and controls were caused by the disease, medication, or a combination of both.

In the current study, we conducted a detailed examination of the projection fibers from specific thalamic nuclei in de novo PD patients, allowing investigation of the disease without concern for the potential effects of antiparkinsonian medication. Using a high-resolution DTI sequence at 3 Tesla (T), we placed small regions of interest (ROIs) in six thalamic regions and used the voxels within the ROIs as seeds for fiber tracking. Similar methods have been applied previously with high inter-rater reliability in a study of traumatic brain injury²⁷. Our hypothesis was that there would be degeneration of thalamic projection fibers in early-stage, untreated PD, and that fibers from specific nuclei may be preferentially affected.

Materials and Methods

Results

The two-way interaction between group and hemisphere was not significant for any of the six thalamic regions (all Ps > .4). The main effect of hemisphere was only significant for the VL (P = .012), with higher FA values on the left than right side. There was a significant main effect of group for three of the six thalamic regions. As presented in Figure 2, FA values were reduced in the PD compared to the control group for fibers projecting from the AN (P = .016), VA (P = .009), and DM (P = .006). In addition, the reduction in FA values from the VL approached significance for the PD compared to the control group (P = .065). The FA values in the VPL/VPM and PU projection fibers did not differ significantly between the groups (both Ps > .4).

For the three thalamic regions that differed significantly between the groups, we performed a correlation analysis to investigate the relationship between UPDRS Part III scores and FA values in PD. There was a significant correlation between UPDRS Part III scores and FA values in the AN (r = −.487, P = .029), indicating a moderate negative association; as UPDRS Part III scores (i.e., motor impairment) increased, FA values decreased. The correlations between UPDRS Part III scores and FA values in the DM, VA, and VL were not significant (all Ps > .2).

Discussion

The thalamus plays an important role in the output pathway of the BG circuitry. Not only does it transmit nearly all cortical inputs, including cognitive, sensory, and motor information, but it also modulates and integrates this information. Further, cerebellar output influences the cortical areas involved in motor processing via the thalamus. The thalamus is organized into distinct nuclear regions, from which diffuse and specific efferent projections are sent to cortical, cerebellar, and subcortical regions³.

The present study was the first to examine the integrity of thalamic nuclei projections in de novo PD using in vivo DTI tractography. We showed that there were reduced FA values in the fibers projecting from the AN, VA, and DM, but not the VPL/VPM or PU, in PD patients compared to controls. Further, the reduction in FA values approached significance in the VL of the PD patients. Although the pathological cause of these reduced FA values is not known, it may reflect demyelination, neuronal loss, or gliosis.^{20, 33–36} We also showed that there was a significant negative correlation between FA values in the AN and UPDRS Part III scores. This relationship should be investigated further in future work. Collectively, our results provide preliminary evidence of thalamic fiber degeneration in early-stage PD and shed light on the extent of disrupted thalamic circuitry as a result of the disease.

Our DTI results are largely consistent with post-mortem studies of the thalamus in PD. Rub et al¹¹ reported evidence of mild inclusion pathology in the AN, VA, VL, and DM of patients with mild to severe PD. Further, there was no evidence of pathology in the VPM and VPL, and relatively little pathology in the PU of these patients. However, other studies have reported that the volume and number of neurons in the AN and DM are not reduced significantly in PD^21–23.

The AN, VA, and DM are considered specific nuclei, in that they relay information to functionally distinct cortical regions. The AN is an associative nucleus that is part of the major limbic pathway, and primarily projects information related to emotion and memory to the cingulate gyrus³⁷. Similarly, the DM is involved in emotion³⁸ and cognition and is connected to the amygdala³⁹ and the anterior cingulate and prefrontal cortices^{37, 40, 41}. The VA, which in combination with the VL is known as the “motor thalamus,” primarily relays somatomotor information to the supplementary motor area from the BG. In functional imaging studies of PD, the supplementary motor area is often hypoactive during movement tasks compared to controls^{42, 43}. Taken together, damage to these different thalamic projection fibers may result in widespread impairments^{39, 44} and may contribute to symptom presentation in PD.

Although ex vivo studies remain the gold standard for pathological confirmation of PD, it is important to study the brain in vivo to gain insight into the structures affected early in the disease and to evaluate differences across disease stages. As a technique, DTI has proven useful in identifying structural abnormalities in the SN of PD patients in vivo^{14, 17, 24, 26, 45}. However, DTI studies comparing the thalamus between PD patients and controls have been less conclusive. In a large cohort, Chan et al ⁴⁶ did not observe differences in FA values at 1.5 T between mild to severe PD patients on antiparkinsonian medication and controls in an ROI that covered a single slice of the thalamus. Similarly, there were no differences in MD values (FA values were not reported) at 1.5 T in a circular ROI placed on one slice of the thalamus between early-stage, medicated PD patients and controls⁴⁷. However, in another large cohort, Péran et al ²⁶ reported reduced FA and MD values at 3 T in the whole thalamus of early-stage, but medicated, PD patients and controls. Most recently, a DTI study that examined the thalamus using both voxel-wise and ROI-based approaches reported reduced FA values at 4 T in mild to severe, medicated PD patients versus controls¹⁶. Collectively, these studies suggest that magnet field strength and/or ROI size and placement may affect our ability to detect microstructural changes in the thalamus.

As discussed earlier, the nuclei of the thalamus have specialized connectivity and functions, and thus may be differentially affected in disease states, as supported by post-mortem research. By studying the thalamus as a single entity, it is unclear whether significant effects in large ROIs or the whole thalamus are driven by degeneration in a few select nuclei and non-significant effects are the result of placing small ROIs on relatively spared nuclei. The present study addressed these concerns by using high-resolution fiber tracking at 3 T, allowing for specific thalamic nuclei to be studied with great sensitivity. Indeed, we showed that FA values were reduced in the AN, VA, and DM in the PD patients compared to controls. There was also a marginally significant reduction in FA values from the VL projections of the PD patients.

Although DTI has proven useful in studying sub-regions of the SN in PD¹⁴, relatively few studies have used this technique to examine individual thalamic nuclei. In fact, to date, only the DM and VL have been examined in PD. Consistent with the role of the DM in affect and cognition, Li et al ²⁵ reported that PD patients with depression had reduced FA values compared to non-depressed PD patients, and that these FA values correlated negatively with depression severity. Given that depression is prevalent in PD⁴⁸, it is possible that the patients in our study had greater affective disturbances than the controls. The VL segment of the motor thalamus receives input from the cerebellum⁴⁹ and BG and projects primarily to the pre- and primary motor cortices, retaining its topographic organization throughout. Two previous MRI studies at 1.5 T have examined the VL in PD, neither of which showed differences in DTI measures between the PD and control groups^{45, 50}. In the present study, the difference in FA values between the two groups approached significance in the VL. Using a high field strength scanner to test larger samples may increase the ability to detect microstructural changes in the VL.

There were two thalamic regions whose FA values did not differ significantly between the groups, namely the VPL/VPM and PU, both of which are sensory nuclei. Human DTI tractography has shown that the PU is connected ipsilaterally to several cortical and subcortical areas, particularly those in the visual pathways⁵¹. Although its function is not completely understood, the PU is thought to play an important role in higher-order visual processing and integration, including visual attention and oculomotor behavior⁵². The VPL and VPM relay touch and proprioceptive information from the contralateral body and head, respectively, to the primary and secondary somatosensory cortices⁴¹. The fact that we did not show a significant group difference in the PU or VPL/VPM suggests that visual and somatosensory processing should be relatively intact in early-stage PD.

There are a few limitations to the present study. First, a detailed neuropsychological examination was not performed on the PD patients. Given that cognitive and affective problems occur frequently in PD⁴⁸ and we found differences in thalamic nuclei known to be involved in emotion and cognitive processing, it is important to collect these data in the future to better understand the relationship between thalamic degeneration and these non-motor functions in PD. Second, the reduction of FA values in the VL of PD patients did not quite reach significance, suggesting that larger sample sizes or more impaired patients may be necessary to detect a difference in this nucleus. Third, intralaminar and midline nuclei (e.g., centromedian-parafasicular complex) were not examined because their boundaries could not be distinguished on the MRI scans with acceptable inter-rater reliability. These nuclei, which are involved in arousal and awareness⁵³, have shown severe pathology in post-mortem studies of PD^{11, 21–23}. Fourth, data in superior cortical areas were not collected due to limitations in the number of slices that can be collected within a reasonable acquisition time. This limited quantification of the tract projections.

Conclusion

The present study provides the first in vivo evidence that there is a disruption of the projection fibers in certain thalamic nuclei, namely the AN, VA, and DM, in de novo PD patients. These results suggest that structural degeneration occurs beyond the SN in early PD and thus help further our understanding of the neural structures affected by the disease. Specifically, we showed that nuclei involved in motor, cognitive, and affective processes are disrupted, whereas those involved in sensory processes are relatively spared. In addition, this study showed that the current methodology not only has utility in examining the microstructural integrity of thalamic nuclei in traumatic brain injury, ²⁷ but also in PD. Future research should use these methods longitudinally to determine whether thalamic fiber integrity changes with disease progression and whether or not it is affected by antiparkinsonian medication. Further, a detailed neuropsychological examination should be conducted along with DTI of thalamic regions to better understand the impact of fiber degeneration on cognitive, affective, and behavioral functioning in PD.