Choroid plexus volume is enlarged in clinically isolated syndrome patients with optic neuritis

Radiomics-based analysis of choroid plexus abnormalities in neuromyelitis optica spectrum disorders and multiple sclerosis and their clinical implications

BACKGROUND

The choroid plexus (CP) is closely linked to inflammation in multiple sclerosis (MS). While the CP volume is enlarged in MS compared with healthy controls (HC), no such changes are observed in neuromyelitis optica spectrum disorder (NMOSD), a disease with similar clinical and imaging features to MS. It remains unclear whether the CP plays a similar role in NMOSD as in MS.

PURPOSE

To investigate the abnormal CP radiomics in NMOSD and MS and explore their clinical implications.

METHODS

This retrospective study included 111 MS, 69 Aquaporin-4 (AQP4)-IgG positive NMOSD, and 82 HC, with age and sex matching. Radiomics features of the CP were extracted from T1-weighted images after automated segmentation, including shape, first order statistics (intensity), and texture features (N=1051). Analysis of covariance was used to assess group differences in these features, and 11 classic machine learning algorithms were employed to construct disease classification models. Moreover, partial correlation analysis was performed to further explore the relationships between differential radiomics features and clinical measures, such as Expanded Disability Status Scale (EDSS) and Symbol Digit Modalities Test (SDMT).

RESULTS

Compared with HC, MS exhibited significant differences in 453 features, including shape, intensity, and texture, while NMOSD displayed differences in 102 intensity and texture features, with no differences in shape. NMOSD and MS differed in 178 features, primarily texture (P < 0.05, Bonferroni correction). In the classification models based on CP radiomics features, the best AUC for MS vs HC was 0.935 (95% CI: 0.830 - 0.997) with the Partial Least Squares Regression Generalized Linear Model (plsRglm), while for NMOSD vs HC, it was 0.822 (95% CI: 0.629 - 0.962) with the Light Gradient Boosting Machine (LightGBM). Meanwhile, the best AUC for NMOSD vs MS was 0.832 (95% CI: 0.667 - 0.960) with the Quadratic Discriminant Analysis (QDA). Furthermore, of the 453 abnormal radiomics features of MS patients, 120 were significantly correlated with EDSS and 234 with SDMT scores (P < 0.05, FDR correction), while no radiomics features in NMOSD were significantly correlated with clinical scores (P > 0.05, FDR correction).

CONCLUSION

Radiomics can detect varying degrees of CP abnormalities in NMOSD and MS, suggesting CP involvement in the pathophysiology of NMOSD, albeit to a lesser extent than in MS. It may help understand the potential pathophysiological differences between the two diseases and their impact on clinical monitoring.

Dynamics of choroid plexus volume is associated with the presence and development of fatigue in multiple sclerosis

BACKGROUND

Immune-mediated processes are implicated in the pathogenesis of fatigue, a common symptom in multiple sclerosis (MS). The choroid plexus (CP) regulates central nervous system (CNS) immune homeostasis and undergoes volumetric modifications possibly contributing to MS-related fatigue. We explored the association between MS-related CP volume changes and fatigue dynamics.

METHOD

Eighty-five patients with MS and 68 healthy controls (HC) underwent brain 3T MRI, neurological evaluation and Modified Fatigue Impact Scale (MFIS) at two timepoints (median follow-up=1.4 years). Normalised brain and regional grey matter (GM) volumes were obtained using FSL-SIENAx, FIRST, SIENA and tensor-based morphometry. CP volumes were quantified with in-house methods, and longitudinal changes were analysed using linear mixed models.

RESULTS

At baseline, 25 (29%) patients with MS had fatigue (f-MS) (MFIS ≥38). Compared with HC, patients with MS had significantly higher brain T2-lesion volume, lower brain, deep GM, cortical volumes and higher CP volume (false discovery rate (FDR)-p ≤0.024). Compared with non-fatigued (nf-MS) patients, f-MS were older, more disabled (FDR-p ≤0.002) and showed numerically higher CP volume (FDR-p=0.076). At follow-up, 41 (68%) nf-MS remained non-fatigued (nf-FU-MS) and 19 (32%) developed fatigue (f-FU-MS). Patients with MS showed higher brain and deep GM atrophy rates versus HC (FDR-p ≤0.048), whereas clinical, lesional and brain volumetric changes were not significantly different among MS groups (FDR-p ≥0.287). CP volume significantly increased in all MS groups compared with HC (FDR-p ≤0.043), with greater enlargement in f-FU-MS versus nf-FU-MS (FDR-p=0.048).

CONCLUSIONS

Larger CP and greater enlargement are associated with the presence and development of fatigue in MS, likely reflecting dynamic inflammatory states within the CNS, supporting the immunological contribution to MS-related fatigue.

Advances in neuroimaging of multiple sclerosis

PURPOSE OF REVIEW

To summarize recent advancements in understanding multiple sclerosis (MS) pathophysiology, predicting disease course, and monitoring treatment responses using MRI.

RECENT FINDINGS

Paramagnetic rim lesions (PRLs) are highly specific to MS and clinically relevant. Detected from the earliest disease phases, PRLs aid in distinguishing MS from other conditions, improving diagnostic accuracy. Moreover, PRLs are associated with more severe disability and measures of brain damage and may predict disease progression. Similarly, slowly expanding lesions (SELs) are associated with more severe disability and predict a more severe disease course. Disease-modifying therapies have limited effectiveness in reducing PRLs or SELs. Choroid plexus (CP) enlargement is associated with structural brain damage and clinical disability and predicts disease evolution. Enlarged perivascular spaces (ePVS) suggest microangiopathic changes rather than direct MS-related inflammation. Glymphatic dysfunction, evaluated using diffusion tensor image analysis along the perivascular space, emerges early in MS and correlates with disability, cognitive impairment, and structural brain damage. Aging and comorbidities exacerbate MS-related damage, complicating diagnosis and treatment. Emerging technologies, such as brain-age paradigms, aim to disentangle aging from MS-specific neurodegeneration.

SUMMARY

Advances in MRI have highlighted the clinical significance of chronic inflammation and glymphatic dysfunction as early contributors to MS progression as well as the interplay between aging, comorbidities and MS.

Retinal Vascular Morphology Reflects and Predicts Cerebral Small Vessel Disease: Evidences from Eye-Brain Imaging Analysis

Cerebral small vessel disease (SVD) involves ischemic white matter damage and choroid plexus (CP) dysfunction for cerebrospinal fluid (CSF) production. Given the vascular and CSF links between the eye and brain, this study explored whether retinal vascular morphology can indicate cerebrovascular injury and CP dysfunction in SVD. We assessed SVD burden using imaging phenotypes like white matter hyperintensities (WMH), perivascular spaces, lacunes, and microbleeds. Cerebrovascular injury was quantified by WMH volume and peak width of skeletonized mean diffusivity (PSMD), while CP volume measured its dysfunction. Retinal vascular markers were derived from fundus images, with associations analyzed using generalized linear models and Pearson correlations. Path analysis quantified contributions of cerebrovascular injury and CP volume to retinal changes. Support vector machine models were developed to predict SVD severity using retinal and demographic data. Among 815 participants, 578 underwent ocular imaging. Increased SVD burden markedly correlated with both cerebral and retinal biomarkers, with retinal alterations equally influenced by cerebrovascular damage and CP enlargement. Machine learning models showed robust predictive power for severe SVD burden (AUC was 0.82), PSMD (0.81), WMH volume (0.77), and CP volume (0.80). These findings suggest that retinal imaging could serve as a cost-effective, noninvasive tool for SVD screening based on vascular and CSF connections.

Volumetric Changes of the Choroid Plexus before and after Spinal CSF Leak Repair

BACKGROUND AND PURPOSE

Patients with intracranial hypotension from spinal CSF leaks have increased choroid plexus volumes in response to CSF leakage. The purpose of this study was to assess changes in choroid plexus volumes in patients before and after spinal CSF leak repair.

MATERIALS AND METHODS

This was a retrospective, institutional review board-approved study on patients with spinal CSF leak who had pre- and post-CSF leak repair MRI examinations. Brain MRIs with contrast were performed on a 1.5/3T scanner with acquisition of 3D T1 postcontrast (eg, Bravo, MPRAGE, and so forth). Choroid plexus volumes at the level of the trigonum ventriculi were calculated for the left and right sides on all pre- and posttreatment MRIs using Visage-7 segmentation tools. Basic demographic data, type of CSF leak, and choroid plexus volumes were recorded for all patients. Basic 2-tailed t tests were used to compare choroid plexus volumes between the pre- and posttreatment groups.

RESULTS

Twenty patients with spontaneous intracranial hypotension from spinal CSF leaks were included. Eleven patients (55%) had a type 1a (ventral tear) spinal CSF leak, 5 patients (25%) had type 1b (lateral tear), and 4 patients (20%) had a type 3 spinal CSF leak. The mean age was 47.6 years (SD, 13.8 years). The mean choroid plexus volumes pretreatment were 0.82 cm3 (SD, 0.29 cm3) compared with 0.38 cm3 (SD, 0.19 cm3) posttreatment (P value 0.01).

CONCLUSIONS

Significantly decreased choroid plexus volumes were seen in patients with spontaneous intracranial hypotension following spinal CSF leak repair. This finding highlights the modulation and dynamic role of the choroid plexus in states of low CSF volumes.

The impact of white matter brain lesions on processing speed deficits in systemic lupus erythematosus patients

BackgroundThe mechanism of cognitive dysfunction in systemic lupus erythematosus (SLE) is still not fully understood. Even though many SLE patients present some neurological dysfunction, including various cognitive deficits, neither a specific pattern of cognitive dysfunction nor specific structural changes associated with cognitive impairment in SLE patients have been established. Moreover, although prevalent and bothersome, cognitive deficits have not been included in the most recent SLE diagnostic criteria.PurposeThe aim of this study was to determine the relationship between the presence of white matter lesions (WMLs) and cognitive dysfunction in patients diagnosed with SLE.Research Design33 SLE patients underwent 3 T brain magnetic resonance imagining (MRI) and an extensive battery of psychological tests, including Automated Neuropsychological Assessment Metrics (ANAM) and the standard pen and paper neuropsychological tests. Patients were stratified into two groups based on the presence (N = 15) or absence (N = 18) of WMLs. Psychometric scores were compared between the two groups.Results and conclusionsSignificant deficits in cognitive functions were observed. Patients with WMLs showed deficits in attention and executive functions, as well as memory deficits in comparison to the group without WMLs. As measured with: STROOP Test (executive function), Color Trail Test (CTT) (attention), Californian Verbal Learning Test (CVLT) (memory), and from ANAM tests with: Procedural Reaction Time (PRT) (attention), Code Substitution Delayed (CS_D) (memory), Spatial Processing (SP) (visuospatial functions), Tower Puzzle (TP) (executive functions), 2 Choice Reaction Time (2CHRT) (attention), Running Memory CPT (RM CPT) (memory), Matching Grids (Mat GR) (visuospatial functions), Go/No _Go inhibition (Go/No_Go Go) (executive functions). Additionally, we analyzed structural volumetric measures derived from a comprehensive segmentation pipeline recon-all using Freesurfer 5.3. Significant differences were identified for the following structures' volumes: right choroid plexus, left choroid plexus, right lateral ventricle. All these structures had a greater volume in patients with WMLs.

Imaging the Choroid Plexus

The choroid plexus (CP), a highly vascularized structure within the ventricles of the brain, has come under increased scrutiny over the last several years as potentially having a role in the pathophysiology of multiple sclerosis (MS). Originally consider as being only responsible for the production of cerebrospinal fluid, it is now widely recognized that the CP is also involved in immunosurveillance and immune cell trafficking. Histopathology studies have found several immunological changes in donor tissue, including the accumulation of inflammatory cells. These findings have been corroborated by animal studies combining immunohistopathology and magnetic resonance imaging (MRI), showing dynamic changes in CP volume that track immune cell infiltration into the CP itself. Subsequent in vivo studies in persons with MS using MRI have suggested that while CP volume increases very early in the disease, CP inflammation continues to have a role throughout later stages as well. Together with recent advances in image processing methods, the analysis of prospective studies as well as existing datasets will help shed further light on the underlying pathophysiological changes within the CP. Such studies are needed to better understand if the CP may represent a novel therapeutic target to ultimately impact the evolution of the disease.

Choroid Plexus Volume in Pediatric-Onset Multiple Sclerosis

BACKGROUND AND OBJECTIVES

Recent studies suggest that the choroid plexus (CP) may function as a site of access of inflammatory cells into the CNS in multiple sclerosis (MS). Pediatric-onset MS (POMS) is characterized by a high inflammatory burden, as evidenced by a high relapse rate and volume of T2 lesions, making patients with POMS an informative population to evaluate choroid plexus volume (CPV). The objectives of the study were (1) to evaluate CPV at symptom onset in participants with POMS compared with healthy controls (HCs); (2) to evaluate changes in CPV in the first year of disease in participants with POMS; and (3) to evaluate associations between CPV, brain volumes, relapse activity, and disability in participants with POMS.

METHODS

Baseline 1.5T MRI scans were acquired from 23 participants with POMS and 23 age-matched and sex-matched HCs; 18 participants with POMS also had 12-month follow-up MRI scans. The CP of the lateral ventricles was segmented manually. CP and brain structure volumes were normalized for total intracranial volume. The number of relapses, T2 and gadolinium-enhancing T1 lesion counts, and Expanded Disability Status Scale (EDSS) scores at 12 months were also analyzed. Baseline CPVs were compared between groups using the Wilcoxon exact test, and CPV change from baseline to 12 months in participants with POMS was compared using the Wilcoxon signed-rank test. The relationship between CPV and brain volumetric measures, T2 lesion volumes, lesion count, number of relapses, and EDSS scores was assessed through Spearman correlation.

RESULTS

The median normalized CPV was 1.51 × 10-3 (interquartile range [IQR]: 1.32-1.76) in POMS baseline scans and 1.21 × 10-3 (IQR: 1.1-1.47) in HC scans (p = 0.001). CPV did not significantly change at 12 months in the 18 participants with POMS with follow-up scans (p = 0.352). CPV in participants with POMS and HCs correlated with lateral ventricular volume (p = 0.012 for both groups) but did not correlate with brain and T2 lesion volumes or lesion count at baseline, nor with relapse activity or EDSS scores at 12 months in participants with POMS.

DISCUSSION

CPV measured at baseline is greater in participants with POMS than in HCs. Baseline CPV did not predict higher disease activity or worse neurologic outcomes over 1 year. While higher CPV may be an early feature of inflammation in MS, its strong correlation with ventricular volumes could also reflect enlargement secondary to the mechanical attachment of CP to the ventricular wall.

Choroid plexus volume changes in multiple sclerosis: insights from a systematic review and meta-analysis of magnetic resonance imaging studies

PURPOSE

Multiple sclerosis (MS) is a chronic autoimmune disease characterized by the destruction of the myelin sheath within the central nervous system. The etiology of MS involves a complex interplay of genetic, environmental, and immunological factors. Recent studies indicated the potential role of the choroid plexus (CP) in the pathogenesis and progression of MS. This systematic review aims to assess existing research on the volume alterations of the CP in MS patients compared to the normal population.

METHODS

A comprehensive search was conducted across databases including PubMed, Embase, Scopus, and Web of Science up to June 2024. Data from the included studies were synthesized using a meta-analytical approach with a random-effects model, assessing heterogeneity with the I2 and Tau-squared indices.

RESULTS

We included 17 studies in this systematic review. The meta-analysis, which included data from eight studies reporting CP volume relative to TIV, found a statistically significant increase in CP volume in MS patients compared to healthy controls (HCs). The SMD was 0.77 (95% CI: 0.61 to 0.93), indicating a large effect size. This analysis showed no heterogeneity (I² = 0%). A separate meta-analysis was conducted using five studies that reported CP volume as normalized volume, resulting in an SMD of 0.63 (95% CI: 0.2-1.06).

CONCLUSION

This study demonstrates an increase in CP volume among MS patients compared to HCs, implying the potential involvement of CP in MS pathogenesis and/or progression. These results show that CP might serve as a radiological indicator in the diagnosis and prognosis of MS.

Volumetric Changes in the Choroid Plexus Associated with Spontaneous Intracranial Hypotension in Patients with Spinal CSF Leak

BACKGROUND AND PURPOSE

The choroid plexus contains specialized ependymal cells responsible for CSF production. Recent studies have demonstrated volumetric and perfusion changes in the choroid plexus with age and neurodegenerative disorders, however, volumetric changes in the choroid plexus in low pressure states is not known. The purpose of this study is to evaluate volumetric differences in choroid plexus size in patients with spontaneous intracranial hypotension (SIH) resultant from spinal CSF leaks compared with healthy controls.

MATERIALS AND METHODS

This was a retrospective, institutional review board-approved study. Patients with MRI evidence of SIH and a spinal CSF leak diagnosed on myelography and subsequently confirmed at surgery were included in this study. All patients included in this study including age-matched healthy controls had a brain MRI performed on a either a 1.5 or 3T scanner with acquisition of 3D T1 postcontrast (eg, BRAVO, MPRAGE, etc). In all patients, the trigonum ventriculi volume, in the atria of the lateral ventricles, was contoured by using Visage-7 segmentation tools on the volumetric postcontrast T1 sequence. A basic 2-tailed t test was used to compare choroid plexus volumes between the 2 groups.

RESULTS

Thirty-four patients were included with 17 patients with SIH with spinal CSF leak and 17 healthy control patients who were age- and sex-matched. The mean age of patients was 45 years, standard deviation 14 years. The mean volume of the choroid plexus for patients with SIH with spinal CSF leak was 1.2 cm3 (standard deviation = 0.26) compared with 0.63 cm3 (standard deviation = 0.31) in the control group (P < .0001).

CONCLUSIONS

Results of this study demonstrate a higher choroid plexus volume in patients with SIH with spinal CSF leak compared with age- and sex-matched healthy controls. This likely reflects compensatory mechanisms to counteract intracranial hypotension by increasing CSF production as well as increased vascularity of the choroid plexus through expansion of the intracranial blood pool.

Choroid plexus enlargement is associated with future periventricular neurodegeneration in multiple sclerosis

BACKGROUND

The choroid plexus (CP), located within the ventricles of the brain and the primary producer of cerebrospinal fluid, has been shown to be enlarged in patients with multiple sclerosis (MS) and linked to periventricular remyelination failure. Atrophied T2-lesion volume (aT2-LV), a promising neurodegenerative imaging marker in progressive MS (PMS), reflects the volume of periventricular lesions subsumed into cerebrospinal fluid over the follow-up.

METHODS

In a cohort of 143 people with relapsing-remitting MS (RRMS) and 53 with PMS, we used 3T magnetic resonance imaging (MRI) to quantify CP volume (CPV) at baseline and aT2-LV over an average of 5.4 years of follow-up. Partial correlations, adjusting for age and sex, and linear regression analyses were used to assess the relationships between imaging measures.

RESULTS

In both cohorts, CPV was associated with aT2-LV in both the RRMS group (r = 0.329, p < 0.001) as well as the PMS group (r = 0.522, p < 0.001). In regression analyses predicting aT2-LV, ventricular volume (final adjusted R2 = 0.407, p < 0.001) explained additional variance beyond age, sex, and T2-lesion volume in the RRMS group while CPV (final adjusted R2 = 0.446, p = 0.009) was retained in the PMS group.

CONCLUSION

Findings from this study suggest that the CP enlargement is associated with future neurodegeneration, with a particularly relevant role in PMS.

Enlarged choroid plexus in relapsing-remitting multiple sclerosis may lead to brain structural changes through the glymphatic impairment

OBJECTIVES

To investigate the potential link among choroid plexus (CP) volume, glymphatic clearance and brain structural change in relapsing-remitting multiple sclerosis (RRMS) patients.

MATERIALS AND METHODS

Sixty-five RRMS patients and 48 healthy controls (HC) underwent MRI examination. The diffusion tensor image analysis along the perivascular space (DTI-ALPS) was calculated to reflect glymphatic system function. The brain structure volume and DTI-ALPS index were compared between RRMS and HC. The mediating effect of the DTI-ALPS index between CP volume and brain structural changes was further investigated. The longitudinal changes of brain structure and DTI-ALPS index were compared in 20 RRMS patients.

RESULTS

Compared to HC, CP volume in RRMS was significantly increased (P < 0.001), and DTI-ALPS index was significantly decreased (P = 0.001). The volumes of white matter, thalamus, putamen and pallidum were significantly decreased in RRMS, and the volumes of lateral ventricle and third ventricle were increased. Mediation analysis showed DTI-ALPS index partially mediated the association between CP enlargement and deep gray matter (DGM) atrophy in RRMS, and between CP enlargement and ventricle enlargement. CP volume and DTI-ALPS index were also significantly correlated with Expanded Disability Status Scale (EDSS) (P = 0.006, P = 0.043). Notably, the variation of DTI_ALPS index during the follow-up period were significantly and negatively correlated with the variation of EDSS (P = 0.045).

CONCLUSION

Enlarged CP volume and decreased DTI_ALPS index may be closely related to DGM atrophy and ventricular enlargement in RRMS, and may be potential imaging markers of clinical disability.

The Time Trajectory of Choroid Plexus Enlargement in Multiple Sclerosis

Choroid plexus (CP) can be seen as a watchtower of the central nervous system (CNS) that actively regulates CNS homeostasis. A growing body of literature suggests that CP alterations are involved in the pathogenesis of multiple sclerosis (MS) but the underlying mechanisms remain elusive. CPs are enlarged and inflamed in relapsing-remitting (RRMS) but also in clinically isolated syndrome (CIS) and radiologically isolated syndrome (RIS) stages, far beyond MS diagnosis. Increases in the choroid plexus/total intracranial volume (CP/TIV) ratio have been robustly associated with increased lesion load, higher translocator protein (TSPO) uptake in normal-appearing white matter (NAWM) and thalami, as well as with higher annual relapse rate and disability progression in highly active RRMS individuals, but not in progressive MS. The CP/TIV ratio has only slightly been correlated with magnetic resonance imaging (MRI) findings (cortical or whole brain atrophy) and clinical outcomes (EDSS score) in progressive MS. Therefore, we suggest that plexus volumetric assessments should be mainly applied to the early disease stages of MS, whereas it should be taken into consideration with caution in progressive MS. In this review, we attempt to clarify the pathological significance of the temporal CP volume (CPV) changes in MS and highlight the pitfalls and limitations of CP volumetric analysis.

Longitudinal enlargement of choroid plexus is associated with chronic lesion expansion and neurodegeneration in RRMS patients

BACKGROUND AND OBJECTIVE

We explored dynamic changes in the choroid plexus (CP) in patients with relapsing-remitting multiple sclerosis (RRMS) and assessed its relationship with chronic lesion expansion and atrophy in various brain compartments.

METHODS

Fifty-seven RRMS patients were annually assessed for a minimum of 48 months with 3D FLAIR, pre- and post-contrast 3D T1 and diffusion-weighted magnetic resonance imaging (MRI). The CP was manually segmented at baseline and last follow-up.

RESULTS

The volume of CP significantly increased by 1.4% annually. However, the extent of CP enlargement varied considerably among individuals (ranging from -3.6 to 150.8 mm3 or -0.2% to 6.3%). The magnitude of CP enlargement significantly correlated with central (r = 0.70, p < 0.001) and total brain atrophy (r = -0.57, p < 0.001), white (r = -0.61, p < 0.001) and deep grey matter atrophy (r = -0.60, p < 0.001). Progressive CP enlargement was significantly associated with the volume and extent of chronic lesion expansion (r = 0.60, p < 0.001), but not with the number or volume of new lesions.

CONCLUSION

This study provides evidence of progressive CP enlargement in patients with RRMS. Our findings also demonstrate that enlargement of the CP volume is linked to the expansion of chronic lesions and neurodegeneration of periventricular white and grey matter in RRMS patients.

Understanding the Pathophysiology and Magnetic Resonance Imaging of Multiple Sclerosis and Neuromyelitis Optica Spectrum Disorders

Magnetic resonance imaging (MRI) has been extensively applied in the study of multiple sclerosis (MS), substantially contributing to diagnosis, differential diagnosis, and disease monitoring. MRI studies have significantly contributed to the understanding of MS through the characterization of typical radiological features and their clinical or prognostic implications using conventional MRI pulse sequences and further with the application of advanced imaging techniques sensitive to microstructural damage. Interpretation of results has often been validated by MRI-pathology studies. However, the application of MRI techniques in the study of neuromyelitis optica spectrum disorders (NMOSD) remains an emerging field, and MRI studies have focused on radiological correlates of NMOSD and its pathophysiology to aid in diagnosis, improve monitoring, and identify relevant prognostic factors. In this review, we discuss the main contributions of MRI to the understanding of MS and NMOSD, focusing on the most novel discoveries to clarify differences in the pathophysiology of focal inflammation initiation and perpetuation, involvement of normal-appearing tissue, potential entry routes of pathogenic elements into the CNS, and existence of primary or secondary mechanisms of neurodegeneration.