Etiology and pathogenesis
Pathology. There are no distinctive or universally present neuropathologic abnormalities in paraneoplastic opsoclonus, nor does the severity of histologic changes at autopsy correlate with the degree of patients' neurologic impairment. Diffuse dropout of Purkinje cells was present in approximately one half of reported autopsied cases, ranging from mild to nearly complete (51; 06). Some cases with Purkinje cell loss also had perivascular mononuclear cell infiltrates in the cerebellum. Neuronal loss or perivascular mononuclear cell infiltrates in the brainstem, especially in the inferior olivary nuclei, may be present in addition to or instead of Purkinje cell loss (51; 01; 57; 06). In several reported autopsies of adults with paraneoplastic opsoclonus, there were no histopathologic abnormalities in either the cerebellum or brainstem (51).
It was previously believed that opsoclonus was primarily caused by dysfunction or loss of the "omnipause" neurons in the pontine reticular formation, but autopsy of the pons has not demonstrated these histologic changes (51). A separate hypothesis postulates that opsoclonus is the result of injury to Purkinje cells in the dorsal cerebellar vermis, leading to disinhibition of the fastigial cerebellar nuclei (59; 50). This is supported by cerebellar pathology in some (but not all) patients and by the report of two adults with opsoclonus in whom functional MR imaging showed abnormally high activation of deep cerebellar nuclei (27). The lack of consistent or universal pathologic lesions in patients with opsoclonus may imply that some patients have immune-mediated cerebellar neuronal dysfunction sufficient to cause clinical disease but short of causing neuronal cell death.
The proposed hypothesis underlying the development of paraneoplastic opsoclonus myoclonus syndrome is thought to be due to an aberrant autoantibody mediated humoral and cell-mediated immune response. There is thought to be cross-reactivity of immune cells targeting tumor cell antigens with neuronal tissue, resulting in an inflammatory response directed at neurons within the cerebellum and brainstem (11). It is probable that the exact nature of the immune dysfunction differs between subsets of patients. Inflammatory infiltrates in the brain or antineuronal antibodies in the serum of some patients with paraneoplastic opsoclonus indirectly support an autoimmune etiology, but the actual pathophysiologic mechanisms remain unknown.
The strongest evidence for an autoimmune pathogenesis of paraneoplastic opsoclonus is the presence in some patients of antineuronal autoantibodies in serum and CSF. Table 1 summarizes the autoantibodies associated with paraneoplastic opsoclonus in adults.
Table 1. Antineuronal Antibodies in Adults with Opsoclonus
Autoantibody |
Tumor(s) |
Cellular Location |
Amphiphysin |
Breast, small cell lung carcinoma |
Intracellular (with transient expression on cell surface) |
Anti-neuronal nuclear antibody type 1(ANNA1; anti-Hu) |
Small cell lung carcinoma |
Intracellular |
Anti-neuronal nuclear antibody type 2 (ANNA2; anti-Ri) |
Breast, ovarian, small cell lung carcinoma |
Intracellular |
Contactin-Associated Protein-like 2(CaspR2) IgG |
Non-small cell lung carcinoma | |
Collapsin response-mediator protein 5 (CRMP5; CV2) |
Small cell lung carcinoma, thymoma, others |
Intracellular |
Voltage gated calcium channel (VGCC) |
Small cell lung carcinoma |
Intracellular |
Ma1/ Ma2 (Ta) |
Germ cell tumors, others |
Intracellular |
Neuronal intermediate filament IgG |
None: Anaplasma infection prior | |
N-methyl-D-aspartate receptor (NMDA-R) |
Ovarian teratoma |
Cell surface |
Purkinje cell cytoplasmic antibody type 1 (PCA1; anti-Yo) |
Breast, ovarian |
Intracellular |
Glutamic acid decarboxylase (GAD) 65 |
Thymoma (rarely) |
Intracellular |
GABA-B |
Small cell lung cancer |
Cell Surface |
GABA-A |
Thymoma |
Cell Surface |
Kelch-like (KLHL) protein 11 (KLHL11) |
Ovarian cancer (reported in half of published papers) |
Intracellular |
Anti-SOX-1 |
Small cell lung cancer | |
Zic4 |
Small cell lung carcinoma |
Unknown |
ANNA2 was initially described in association with paraneoplastic opsoclonus myoclonus syndrome in women with breast or ovarian carcinoma (39). Reports show that the clinical syndromes and the tumors associated with ANNA2 are heterogeneous (47). ANNA2 antibodies can also be associated with small cell lung cancers but have also been found in non-small cell lung adenocarcinoma, and less commonly with other tumors (47). Most patients with paraneoplastic opsoclonus myoclonus syndrome and ANNA2 antibodies have limb and gait ataxia and brainstem dysfunction as well. However, ANNA2 antibodies are not specific to opsoclonus myoclonus syndrome. Some patients with ANNA2 antibodies have features of multifocal encephalomyelitis or brainstem encephalitis, but not opsoclonus (20). A retrospective review demonstrated opsoclonus with or without myoclonus in 28% of patients, and opsoclonus rarely appeared in isolation (55).
ANNA2-IgG stains the nuclei of neurons, with some tissue immunofluorescence cytoplasmic staining (39; 47). In contrast to ANNA1, ANNA2 does not react with the neurons of the dorsal root ganglia, sympathetic ganglia, or myenteric plexus (23). ANNA2 antibodies are thought to be directed against two 55 and 80kDa onconeural antigens, NOVA-1 and NOVA-2 (07; 60). The Nova proteins share sequence homology with a group of nuclear RNA-binding proteins believed to be involved in the regulation of mRNA splicing (30). This family of proteins is distinct from the RNA-binding proteins that react with ANNA1 antibodies. The Nova-1 and Nova-2 genes are differentially expressed by various subgroups of CNS neurons and are also expressed by gynecologic and lung tumors (07; 60).
As with other antineuronal antibodies associated with neurologic paraneoplastic syndromes, the role of intracellular onconeural antibodies remains unknown. Antibodies like ANNA2 likely represent an epiphenomenon or marker of primarily T-cell-mediated inflammation (47). A large proportion of patients with paraneoplastic opsoclonus myoclonus syndrome have no identifiable antineuronal antibodies (04; 52; 03).
Patients with paraneoplastic opsoclonus myoclonus syndrome may have antineuronal antibodies other than ANNA2 antibodies (Table 1). Several reported patients with opsoclonus, breast cancer, and diffuse cerebellar degeneration have PCA1 (anti-Yo) antibodies (45). Patients with small cell lung carcinoma or neuroblastoma, in whom opsoclonus is a component of multifocal paraneoplastic encephalomyelitis, may have ANNA1 antibodies that react with a group of 35 to 40 kd neuronal RNA-binding proteins. There are individual reports of patients with paraneoplastic opsoclonus and other antineuronal antibodies such as anti-Ma1 or anti-Ma2 antibodies and underlying tumors, particularly testicular germ cell (28), amphiphysin antibodies associated with breast or small cell lung carcinoma (48; 53; 57), CRMP5 antibodies associated with small cell lung cancer or thymoma (61), voltage-gated calcium channel antibodies associated with small cell lung cancer (32), anti-HNK1 antibodies associated with small cell lung cancer (02), and unclassified patterns of reactivity associated with small cell lung cancer or other neoplasms (09; 05). Opsoclonus myoclonus syndrome has been reported in patients with antibodies in which an association with malignancy is less frequent or where no underlying malignancy is identified, including against glutamic acid decarboxylase (GAD)65 (54). There are several reports of opsoclonus myoclonus syndrome associated with antibodies to NMDA-R with and without ovarian teratoma, GABAA receptors (46), or GABAB receptors (12). An initial paper describing a novel autoantibody KLHL11 did not describe opsoclonus myoclonus as an associated phenotype, but opsoclonus myoclonus syndrome in the presence of KLHL11 antibodies and ovarian teratoma has subsequently been reported (41; 42); further investigations are necessary to clarify this discrepancy.
An important note must be made regarding the recent use of immune-checkpoint inhibitors therapies for the management of several malignancies. A range of autoimmune disorders have occurred in the context of these medications, many of which with autoantibodies. Several cases have been reported, including one with ipilimumab/nivolumab opsoclonus myoclonus syndrome in a patient with malignant pleural mesothelioma that started 10 weeks after oncologic therapy was initiated (40).
As with children, nonparaneoplastic opsoclonus in adults frequently occurs after a respiratory or gastrointestinal illness. Patients develop acute to subacute onset opsoclonus, with myoclonus and ataxia. The syndrome is monophasic, and most patients make a good recovery within several weeks. Patients tend to be younger than their paraneoplastic counterparts (04). Parainfectious opsoclonus myoclonus syndrome is associated with several infections, including St. Louis encephalitis, Japanese encephalitis, herpes simplex, West Nile virus encephalitis (35), Epstein-Barr virus encephalitis, cytomegalovirus encephalitis, human herpesvirus 6 encephalitis, Powassen infection, and CNS Lyme disease. Persons with HIV infection may develop opsoclonus myoclonus syndrome in the seroconversion phase or during immune reconstitution after starting antiretroviral therapy or as a manifestation of CNS escape (24; 08). Opsoclonus myoclonus syndrome has been associated with diseases such as malaria, dengue, scrub-typhus, and chikungunya (14; 49; 38). A single case of post vaccination opsoclonus myoclonus syndrome has been reported after antirubella vaccination (37).
Opsoclonus myoclonus, among other movement disorders, has been reported post Sars-CoV-2 infection (19).
Opsoclonus may rarely occur in patients with structural lesions of the thalamus or upper midbrain, hemorrhage (34), neoplasms, and even sarcoidosis. Neurovascular compression in the brainstem has also been reported (44). Opsoclonus has been reported in patients with toxic or metabolic encephalopathies, including hyperosmolar nonketotic coma, organophosphate toxicity, amitriptyline overdose, cyclosporine toxicity, intravenous phenytoin, diazepam, as well as severe uremia (10; 25).