Relevance of Antibody Testing in Movement Disorders

Not too long ago, genetic movement disorders were considered a niche area. With new technologies came the discovery of novel disorders and a better understanding of long‐known disorders, leading to new treatments and propelling research with treatment trials on the horizon (eg, gene therapy in aromatic L‐amino acid decarboxylase [AADC] deficiency with a viral vector expressing AADC). ¹ , ² Most importantly, these advances have changed practice where genetic testing, including those hitherto new technologies like whole exome and whole genome sequencing, have become widely available leading to earlier and easier diagnosis of familial neurological disease.

Similar developments are ongoing in neuroimmunology, with obvious repercussions on the area of autoimmune movement disorders. We recognize an ever‐expanding spectrum of new antibodies and new syndromes. Antibody testing is widely available, and it is hoped that akin to genetic testing, it will become more affordable overtime.

In contrast to most of the neurogenetic movement disorders, the diagnosis of an autoimmune etiology immediately guides treatment and management of the patient. For example, immunotherapy, screening for underlying tumors or associated autoimmunity and disease monitoring (depending on the syndrome: tumors, associated autoimmunity, relapses). This article outlines the relevance of antibody testing to movement disorders, that is, the role of antibodies in making a diagnosis, influencing treatment, and monitoring decisions (see Fig. 1).

FIG. 1.

Graphic summary of the role of autoantibodies in movement disorders.

Neuronal Antibody‐Related Movement Disorders

Neuronal antibody‐related movement disorders (NARMD) encompass antibody‐mediated movement disorders (where the antibody is pathogenic) and antibody‐associated movement disorders (where the antibodies are not pathogenic, but are useful biomarkers). For the diagnosis of NARMD, clinical acumen and specific antibody testing are of paramount importance to avoid missing potentially treatable disease.

Although the classical presentation of autoimmune encephalitides involves a rapid onset, some NARMD have an insidious onset and a slowly progressive course that mimic neurodegenerative disease (in particular NARMD with antibodies against LGI1, DPPX, CASPR2, and IgLON5). ³ , ⁴ , ⁵ , ⁶ Patients with “atypical” movement disorder presentations (eg, related to ant‐iNMDAR, GABA_AR, IgLON5, or CASPR2) have a high risk of misdiagnosis. ⁷ , ⁸ , ⁹ , ¹⁰ , ¹¹ Another pitfall to be avoided is clinical presentations that closely resemble classic neurodegenerative disease (eg, SEZ6L2 antibodies mimicking progressive supranuclear palsy.) ¹² Knowledge of the characteristic phenotypes and red flags, as well as a high index of clinical suspicion are, therefore, crucial so that the physician pre‐selects those patients who should undergo antibody testing, as often the diagnostic surrogate markers (imaging, cerebrospinal fluid [CSF]), although of use, are not sensitive enough.

Surrogate Markers Versus Antibody Testing

CSF analysis and brain magnetic resonance imaging (MRI) are helpful in excluding relevant differential diagnoses of autoimmune encephalitis (eg, infectious or neurodegenerative diseases, cerebral vasculitis, or cerebral neoplasms.) ¹³ , ¹⁴ Moreover, the new criteria for autoimmune encephalitis allow a diagnosis based on suggestive clinical features, CSF, and MRI findings. ¹⁵

However, MRI or the basic CSF analysis have too low sensitivity as to be relied on as screening tools. Brain MRI in autoimmune encephalitis is often normal or unspecific. ¹⁶ CSF profiles differ across the different antibody‐associated disease. Even in NARMD where pleocytosis, intrathecal immunoglobulin G (IgG) synthesis, or raised CSF protein are more common, ~40% will have a normal cell count, 50% will not have intrathecal IgG synthesis, and 30% will have normal protein levels. ¹⁷ Studies suggest that by relying only on these criteria, ~13% to 15% of patients will go undiagnosed. ³ , ¹¹ Therefore, it should be emphasized that normal brain imaging and CSF analysis do not exclude an autoimmune encephalitis or NARMD. Rather, antibody testing is critical to make the diagnosis, and this “molecular” diagnostic approach has important implications.

Syndromic Versus Exact “Molecular” Diagnosis

The difference between a syndromic and a “molecular” diagnosis based on a specific antibody result impacts treatment, prognosis, and monitoring. As in genetics, where one phenotype can be associated with multiple genes (genetic heterogeneity), one movement disorder phenotype can be associated with multiple antibodies (antibody heterogeneity), each however with different implications. ¹⁸ , ¹⁹ For example, stiff person spectrum disorder (SPSD) can be associated with anti‐glutamic acid decarboxylase (GAD), GlyR, amphiphysin, or dipeptidyl‐peptidase–like protein‐6 (DPPX), with differing implications. ²⁰ With GAD‐antibodies, the most common subtype, we expect to see a chronic disease course even when treated with immunotherapy, and the physician needs to look out for associated autoimmunity (neurological: cerebellar ataxia, focal epilepsy; non‐neurological: type 1 diabetes, autoimmune thyroid disease, pernicious anemia). With GlyR‐antibodies a monophasic course with a very good recovery with immunotherapy can be seen and searching for a thymoma (and if present, its removal) is mandatory. Amphyiphysin‐antibodies suggest a paraneoplastic cause associated with breast or lung cancer, and the cancer treatment defines the prognosis. DPPX‐antibodies are rarely associated with neoplasms (B cell neoplasms in ~7%), but require constant and sometimes aggressive immunotherapy. ⁴ Table S1 summary the response to immunotherapy and prognosis differences between other surface antibodies relevant to clinical practice. ²¹ , ²² , ²³ Further examples of antibody heterogeneity (one phenotype associated with multiple antibodies) are manifold and outlined in a previous article. ²⁴ Last, but not least, it is worth highlighting that onconeuronal antibodies not only predict cancer, but also help narrow down which cancer to look for (important for the choice of appropriate investigations). ²⁵ For example, KHLH‐11 antibodies are associated with germ cell tumors that might not be detected with a total body fluorodeoxyglucose‐positron emission tomography. Ri and Hu antibodies combined suggest that lung cancer is more likely than breast cancer to be present, whereas Ri antibodies alone indicate a higher probability of an underlying breast cancer. In summary, knowledge of the exact antibody helps to identify relevant related illness, guides treatment decisions, and informs prognosis.

Antibody Testing in Clinical Practice

Availability and affordability of antibody testing still varies greatly across different countries, but it is hoped that this will improve akin to the evolution in genetic testing.

Just as with genetic testing, antibody testing has some limitations and pitfalls, and the physician needs to be aware of the relevance of the appropriate test and specimen. ²⁶

Antibody tests are not designed as screening tools in an unselected population, but they need to be applied using clinical acumen. Importantly, using paired serum and CSF samples increases the specificity and sensitivity of antibody testing. ¹⁵ , ²⁷ , ²⁸ , ²⁹ It also allows detection of intrathecal autoantibody synthesis (as opposed to diffusion across the blood‐CSF barrier) by calculating the antibody index (CSF/serum difference of antibody amounts per weight unit IgG; normal <1.5). ³⁰ A positive antibody index is a strong predictor of central nervous system autoimmunity; this is especially helpful when assessing the neurological relevance or otherwise of antibodies occasionally found in healthy subjects or associated with non‐neurological disease, such as anti‐GAD. ⁴ , ³⁰ , ³¹ , ³² , ³³

Another pitfall can be the testing for IgM or IgA antibodies in some laboratories. In the context of NARMD, IgG antibodies, but not IgM or IgA antibodies, are of established diagnostic or pathogenic relevance. ³⁴ , ³⁵ , ³⁶ Moreover, when faced with ambiguous test results testing with a different laboratory method (eg, using tissue‐based immunohistochemistry to confirm the staining pattern is in accordance with the result from the cell‐based assay) is recommended. Sometimes it is worthwhile for the physician to contact the neuroimmunology research laboratory to discuss an individual patient with an unexpected negative result. Given all these caveats, it is really important to avoid over relying on an unexpected antibody test result that overrides clinical acumen. Similarly, it should be kept in mind that we treat patients, not laboratory results. Management decisions should primarily take into account the clinical history, neurological examination, and disease course.

Lessons From Genetics

Similar to genetics where gene panels rather than single gene tests have become routine, antibody testing is best done in panels. Panels account for the fact that one phenotype can be associated with different underlying antibodies (antibody heterogeneity), and one antibody can be associated with different phenotypes (phenotypic heterogeneity).

Like in genetics, where we recognize variants of unknown significance (VUS), antibodies of unknown significance (AUS) pose a difficult problem in clinical practice. ²⁶ The term AUS was introduced, analogous to VUS, to highlight that sometimes positive antibody test results may not indicate disease and hence, the clinician must avoid misdiagnosis. There are certain conceptual differences between VUS and AUS. In genetics, there is ultimately a certain dichotomy where gene variants are benign or pathogenic. In contrast, an antibody may be pathogenic, but also non‐pathogenic antibodies may be relevant biomarkers. Moreover, methodological issues leading to “false positive” antibodies can be an issue. Although the American College of Medical Genetics and Genomics has proposed certain criteria to interpret the significance of genetic findings, more research is needed in neuroimmunology to guide the physician in dealing with AUS. ²⁶

Conclusion

Antibody testing is becoming more and more relevant in clinical practice. It is more sensitive and specific than MRI and basic CSF parameters in making a diagnosis of NARMD. An exact “molecular” diagnosis, rather than a syndromic diagnosis, has various important implications for treatment, monitoring, and prognosis. Clinicians need to be also aware of the limitations and pitfalls of antibody testing and apply it only in patients selected with clinical acumen, using paired serum, and CSF samples and appropriate panels.

Author Roles

(1) Research project: A. Conception, B. Organization, C. Execution; (2) Statistical Analysis: A. Design, B. Execution, C. Review and Critique; (3) Manuscript Preparation: A. Writing of the first draft, B. Review and Critique;

B.B.: 3A, 3B.

Disclosures

Ethical Compliance Statement: The author confirms that the approval of an institutional review board was not required for this work. I confirm that I have read the Journal's position on issues involved in ethical publication and affirm that this work is consistent with those guidelines.

Funding Sources and Conflicts of Interest: No specific funding was received for this work. There are no conflicts of interest to report.

Financial Disclosures for Previous 12 Months: B.B. is supported by the Hurka Foundation and has received a research grant from the Koetser Foundation. B.B. has received roaylties from Oxford University Press.

Supporting information

Table S1. Differences in prognosis and treatment response for NMDAR, Caspr2, and LGI1 antibodies. Data extrapolated from ²¹ , ²² , ²³