Isaacs syndrome with CASPR2 antibody: A series of three cases

Systematic Review of the Clinical Characteristics and Management of Isaac Syndrome

OBJECTIVES

Isaac syndrome (IS) is a condition characterized by peripheral nerve hyperexcitability caused by voltage-gated potassium channel (VGKC)-complex antibodies. Muscle twitching, stiffness, hypertrophy, and dysautonomic characteristics, such as hyperhidrosis, are common manifestations. The syndrome can be autoimmune or paraneoplastic, with thymoma being a common cause of paraneoplastic IS. Furthermore, this condition could be handed down from one generation to another. However, there is limited information regarding outcomes, relapses, associated syndromes, associated malignancies (other than thymoma), and treatment options. Despite its rarity, there remains a need for effective management strategies for patients with IS. To address this gap, we conducted a systematic review to summarize the most common and effective treatments of IS in immunomodulatory agents and symptomatic medications, as well as to describe outcomes, relapses, and associated malignancies. Altogether, this review serves to guide clinical practice recommendations for IS and highlight areas for further research.

METHODS

We used the Preferred Reporting Items for Systematic Reviews and Meta-Analyses protocol to conduct a systematic review of cases reposted through the PubMed and Google Scholar databases. The terms "Isaac Syndrome" and "Acquired Neuromyotonia" were used. The Joanna Briggs Institute's critical appraisal tool was used to evaluate the quality of the included studies.

RESULTS

We identified 61 case reports and 4 case series, comprising a total of 70 patients with IS (mean age at onset: 42.5 ± 18 years, and 69% were males). Fourteen cases reported relapses. Thymoma was the most common malignancy associated with IS, followed by lymphoma. Among various serum antibodies, voltage-gated potassium channel-complex antibodies were the most reported antibodies elevated in IS (reported in 38 patients and elevated in 21 patients [55.2%]), followed by acetylcholine ganglionic receptor antibodies, which were reported in 30% of patients (n = 21) and were elevated in 5 cases. The most common electromyography findings were myokymic discharges (n = 22), followed by fasciculations (n = 21) and neuromyotonia (n = 19). For treatment, combining anticonvulsants such as carbamazepine with immunotherapy therapy showed the best results in controlling the symptoms. Among immunotherapy therapies, the combination of plasma exchange plus intravenous high-dose steroids achieved the best results in the acute treatment of IS ([n = 6], with improvement noted in 83.3% [n = 5] of cases). Among the symptomatic treatments with anticonvulsants, carbamazepine was the most efficacious anticonvulsant in treatment of IS, with an average effective dosing of 480 mg/day (carbamazepine was used in 32.3% of acute treatment strategies [n = 23], with improvement noted in 73.9% [n = 17] of cases).

CONCLUSIONS

IS a rare neuromuscular syndrome that tends to affect middle-aged men. These patients should be screened for thymoma and other malignancies such as lymphomas. The management of IS symptoms can be challenging, but based on our review, the combination of multiple immunosuppressives such as IV steroids and plasmapheresis with anticonvulsants such as carbamazepine seems to achieve the best results.

Epidemic Japanese B encephalitis combined with contactin-associated protein-like 2 antibody-positive autoimmune encephalitis: A case report

BACKGROUND

It is not uncommon to develop viral encephalitis. Epidemic Japanese B encephalitis infection combined with contactin-associated protein-like 2 (CASPR-2) antibody-positive autoimmune encephalitis has not been reported at present. In clinical work, we need to consider more options.

CASE SUMMARY

A 32-year-old male worker presented with headache, fever and call-unresponsive presentation. Complete cranial magnetic resonance image showed symmetrical abnormal signals in bilateral medial temporal lobe, bilateral thalamus and basal ganglia. Improved lumbar puncture showed that cerebrospinal fluid protein and cell count increased significantly. Viral encephalitis was considered, and the patient's consciousness still increased rapidly after antiviral treatment. Further detection of Cerebrospinal fluid Japanese B encephalitis virus Polymerase Chain Reaction positive, serum autoimmune encephalitis antibody showed CASPR-2 antibody positive (1:320), the patient's condition gradually improved after plasma exchange treatment. 3 mo later, the serum CASPR-2 antibody was negative and the patient's condition was stable.

CONCLUSION

This article reports the world’s first case of Epidemic Japanese B encephalitis infection combined with CASPR-2 antibody-positive autoimmune encephalitis, with a view to raising awareness.

Rituximab Was Effective in Relieving Symptoms of Isaacs Syndrome: A Case Report

We presented a 23-year-old patient who had experienced neuromyotonia in his left leg. Although he tested negative for anti-LGI1 and anti-CASPR2 antibodies, we diagnosed him with Isaacs syndrome due to myokymic discharges on electromyography and symptoms being relieved by intravenous methylprednisolone (IVMP) and intravenous immunoglobulin (IVIg). IVMP, IVIg, plasma exchange, or cyclosporine treatment did not provide a long-term response; however, rituximab showed long-term improvement. Rituximab should be considered early in the treatment of patients with antibody-negative Isaacs syndrome who are responsive to immunotherapy, including IVMP, IVIg, and plasma exchange, and have long-term symptoms that are hard to control.

Autoimmune neuromyotonia

PURPOSE OF REVIEW

Autoimmune neuromyotonia encompasses a group of rare immune-mediated neurological disorders frequently associated with anti-contactin-associated protein-like 2 (CASPR2) antibodies and featuring clinical and electrical signs of peripheral nerve hyperexcitability (PNH). We aim to summarize the current knowledge on immune-mediated neuromyotonia, focusing on clinical presentations, pathophysiology, and management.

RECENT FINDINGS

Neuromyotonia is a major feature of several autoimmune neurological syndromes characterized by PNH with or without central neurological system involvement. Experimental and clinical evidence suggest that anti-CASPR2 antibodies are directly pathogenic in autoimmune neuromyotonia patients.

SUMMARY

Neuromyotonia, a form of PNH, is a major feature in several syndromes associated with anti-CASPR2 antibodies, including cramp-fasciculation syndrome, Isaacs syndrome, Morvan syndrome, and autoimmune limbic encephalitis. Diagnosis relies on the identification of motor, sensory, and autonomic signs of PNH along with other neurological symptoms, anti-CASPR2 antibody-positivity, and of characteristic electroneuromyographic abnormalities. Paraneoplastic associations with thymoma are possible, especially in Morvan syndrome. Patients usually respond to immune-active treatments, including steroids, intravenous immunoglobulins, plasma exchanges, and rituximab.

Effect of low-dose rituximab treatment on autoimmune nodopathy with anti-contactin 1 antibody

Background

Autoimmune nodopathy with anti-contactin-1 (CNTN1) responds well to rituximab instead of traditional therapies. Although a low-dose rituximab regimen was administered to patients with other autoimmune diseases, such as myasthenia gravis and neuromyelitis optica spectrum disorders, and satisfactory outcomes were obtained, this low-dose rituximab regimen has not been trialed in anti-CNTN1-positive patients.

Methods

Anti–CNTN1 nodopathy patients were enrolled in this prospective, open-label, self-controlled pilot study. A cell-based assay was used to detect anti-CNTN1 antibodies and their subclasses in both serum and cerebrospinal fluid. Clinical features were evaluated at baseline, 2 days, 14 days, and 6 months after single low-dose rituximab treatment (600 mg). The titers of the subclasses of anti-CNTN1 antibody and peripheral B cells were also evaluated at baseline, 2 days, and 6 months after the rituximab regimen.

Results

Two patients with anti–CNTN1 antibodies were enrolled. Both patients had neurological symptoms including muscle weakness, tremor, sensory ataxia, numbness and mild nephrotic symptoms. In the field of neurological symptoms, sensory ataxia markedly improved, and the titer of anti-CNTN1 antibody as well as CD19+ B cells decreased only two days following low-dose rituximab treatment. Other neurological symptoms improved within two weeks of rituximab treatment. At the 6-month follow-up, all neurological symptoms steadily improved with steroid reduction, and both the anti-CNTN1 antibody titer and CD19+ B cells steadily decreased. No adverse events were observed after this single low-dose rituximab treatment.

Conclusions

We confirmed the clinical efficacy of low-dose rituximab by B cell depletion in autoimmune nodopathy with anti-CNTN1 antibody. This rapid and long-lasting response suggests that low-dose rituximab is a promising option for anti-CNTN1 nodopathy.

Isaacs' syndrome as the initial presentation of malignant thymoma and associated with double-positive voltage-gated potassium channel complex antibodies, a case report

BACKGROUND

Isaacs' syndrome is a peripheral nerve hyperexcitability (PNH) syndrome due to peripheral motor nerve instability. Acquired Isaacs' syndrome is recognized as a paraneoplastic autoimmune disease with possible pathogenic voltage-gated potassium channel (VGKC) complex antibodies. However, the longitudinal correlation between clinical symptoms, VGKC antibodies level, and drug response is still unclear.

CASE PRESENTATION

A 45-year-old man had progressive four limbs soreness, muscle twitching, cramps, and pain 4 months before admission. Electromyography (EMG) studies showed myokymic discharges, neuromyotonia, and an incremental response in the high-rate (50 Hz) repetitive nerve stimulation (RNS) test. Isaacs' syndrome was diagnosed based on clinical presentations and EMG reports. Serum studies showed positive VGKC complex antibodies, including leucine-rich glioma-inactivated 1 (LGI1) and contactin-associated protein-like 2 (CASPR2) antibodies. The acetylcholine receptor antibody was negative. Whole-body computed tomography (CT) and positron emission tomography revealed a mediastinal tumor with the great vessels encasement, right pleura, and diaphragm seeding. Biopsy confirmed a World Health Organization type B2 thymoma, with Masaoka stage IVa. His symptoms gradually improved and both LGI1 and CASPR2 antibodies titer became undetectable after concurrent chemoradiotherapy (CCRT) and high dose steroid treatment. However, his Isaacs' syndrome recurred after the steroid was reduced 5 months later. Follow-up chest CT showed probable thymoma progression. LGI1 antibody turned positive again while CASPR2 antibody remained undetectable.

CONCLUSIONS

Our patient demonstrates that Isaacs' syndrome could be the initial and only neuromuscular manifestation of malignant thymoma. His Isaacs' syndrome is correlated well with the LGI1 antibody level. With an unresectable thymoma, long-term immunosuppressant therapy may be necessary for the management of Isaacs' syndrome in addition to CCRT for thymoma.

Tacrolimus as a therapeutic option in patients with acquired neuromyotonia

OBJECTIVE

To analyze the clinical characteristics and outcomes of patients diagnosed with acquired neuromyotonia and who were treated with tacrolimus.

METHODS

A single center, retrospective study was performed on patients with acquired meuromyotonia whose treatment included tacrolimus. The clinical information, antibody tests, and electromyography results were reviewed. The Numeric Rating Scale for pain and modified Rankin scale were used to quantify outcomes.

RESULTS

This study included four patients who presented with fasciculation or myokymia in their limbs. Electromyography suggested peripheral nerve hyperexcitability. Autoantibodies including contactin-associated protein 2 (CASPR2), leucine-rich glioma inactivated protein 1 (LGl1) or IgLON5 antibody were detected in three patients, and another patient had Sjogren's syndrome. Initial treatment included membrane-stabilizing drugs and/or corticosteroids. Tacrolimus was administered at a dose of 3 mg once daily to all patients. All patients showed clinical improvement after the treatment. No recurrence was observed after gradual tapering or discontinuation of therapy during follow-up.

CONCLUSIONS

Tacrolimus may be a therapeutic option for acquired neuromyotonia. Further studies on tacrolimus in larger patient cohort should be performed.

Case report: Isolated muscle neuromyotonia, as presenting feature of Isaacs' syndrome

An autoimmune form of Isaacs' syndrome is commonly associated with VGKC complex antibodies and characterized by continuous muscle activity of extremity muscles. Here, we describe a CASPR2 and LGI1 positive patient with neuromyotonia clinically and electrophysiologically isolated to gastrocnemius muscles only. IVIG course and plasma exchange were ineffective, but symptoms significantly improved after a course of high-dose steroids. This case demonstrates that focal hyperexcitability should raise suspicion for autoimmunity. LGI1 antibody can be positive in patients with only peripheral nerve system involvement and if one treatment fails, other should be tried.

Systematic review of the clinical spectrum of CASPR2 antibody syndrome

BACKGROUND

Contactin-associated protein-like 2 (CASPR2) autoantibody disease has a variable clinical phenotype. We present a case report and performed a systematic review of the literature to summarize: (1) the clinical phenotype of patients with CASPR2 antibodies, (2) the findings in neurological investigations, and (3) the associated neuroimaging findings.

METHODS

A chart review was performed for the case report. A systematic review of the medical literature was performed from first available to June 13, 2018. Abstracts were screened, and full-text peer-reviewed publications for novel patients with CASPR2 positivity in serum or cerebrospinal fluid (CSF) were included. Selected publications were reviewed, and relevant information was collated. Data were analyzed to determine overall frequency for demographic information, clinical presentations, and investigation findings.

RESULTS

Our patient was a previously healthy 61-year-old male with both serum and CSF CASPR2 antibodies who presented with limbic encephalitis and refractory epilepsy. He was successfully treated with immunosuppression. For our systematic review, we identified 667 patients from 106 studies. Sixty-nine percent were male. Median age was 54 years (IQR 39-65.5). Median disease duration was 12 months (IQR 5.6-20). Reported overall clinical syndromes were: autoimmune encephalitis [69/134 (51.5%)], limbic encephalitis [106/274 (38.7%)], peripheral nerve hyperexcitability [72/191 (37.7%)], Morvan syndrome [57/251 (22.7%)], and cerebellar syndrome [24/163 (14.7%)]. Patients had positive serum [642/642 (100%)] and CSF [87/173 (50.3%)] CASPR2 antibodies. MRI was reported as abnormal in 159/299 patients (53.1%), and the most common abnormalities were encephalitis or T2 hyperintensities in the medial temporal lobes, or hippocampal atrophy, mesial temporal sclerosis, or hippocampal sclerosis. FDG-PET was abnormal in 30/35 patients (85.7%), and the most common abnormality was temporomesial hypometabolism. The most commonly associated condition was myasthenia gravis (38 cases). Thymoma occurred in 76/348 patients (21.8%). Non-thymoma malignancies were uncommon [42/397 (10.6%)].

CONCLUSIONS

Most patients have autoimmune or limbic encephalitis and corresponding abnormalities on neuroimaging. Other presentations include peripheral nerve hyperexcitability or Morvan syndromes, cerebellar syndromes, behavioral and cognitive changes, and more rarely movement disorders. The most commonly associated malignancy was thymoma and suggests a role for thymoma screening in CASPR2-related diseases.

Autoimmune Channelopathies at Neuromuscular Junction

The neuromuscular junction, also called myoneural junction, is a site of chemical communication between a nerve fiber and a muscle cell. There are many types of channels at neuromuscular junction that play indispensable roles in neuromuscular signal transmission, such as voltage-gated calcium channels and voltage-gated potassium channels on presynaptic membrane, and acetylcholine receptors on post-synaptic membrane. Over the last two decades, our understanding of the role that autoantibodies play in neuromuscular junction disorders has been greatly improved. Antibodies against these channels cause a heterogeneous group of diseases, such as Lambert-Eaton syndrome, Isaacs' syndrome and myasthenia gravis. Lambert-Eaton syndrome is characterized by late onset of fatigue, skeletal muscle weakness, and autonomic symptoms. Patients with Isaacs' syndrome demonstrate muscle cramps and fasciculation. Myasthenia gravis is the most common autoimmune neuromuscular junction channelopathy characterized by fluctuation of muscle weakness. All these disorders have a high risk of tumor. Although these channelopathies share some common features, they differ for clinical features, antibodies profile, neurophysiological features, and treatments. The purpose of this review is to give a comprehensive insight on recent advances in autoimmune channelopathies at the neuromuscular junction.

Structural mapping of hot spots within human CASPR2 discoidin domain for autoantibody recognition

Accumulating evidence has showed that anti-CASPR2 autoantibodies occur in a long list of neurological immune disorders including limbic encephalitis (LE). Belonging to the well-known neurexin superfamily, CASPR2 has been suggested to be a central node in the molecular networks controlling neurodevelopment. Distinct from other subfamilies in the neurexin superfamily, the CASPR subfamily features a unique discoidin (Disc) domain. As revealed by our and others' recent studies, CASPR2 Disc domain bears a major epitope for autoantibodies. However, structural information on CASPR2 recognition by autoantibodies has been lacking. Here, we report the crystal structure of human CASPR2 Disc domain at a high resolution of 1.31 Å, which is the first atomic-resolution structure of the CASPR subfamily members. The Disc domain adopts a total β structure and folds into a distorted jellyroll-like barrel with a conserved disulfide-bond interlocking its N- and C-termini. Defined by four loops and located in one end of the barrel, the "loop-tip surface" is totally polar and easily available for protein docking. Based on structure-guided epitope prediction, we generated nine mutants and evaluated their binding to autoantibodies of cerebrospinal fluid from twelve patients with limbic encephalitis. The quadruple mutant G69N/A71S/S77N/D78R impaired CASPR2 binding to autoantibodies from eleven LE patients, which indicates that the loop L1 in the Disc domain bears hot spots for autoantibody interaction. Structural mapping of autoepitopes within human CASPR2 Disc domain sheds light on how autoantibodies could sequester CASPR2 ectodomain and antagonize its functionalities in the pathogenic processes.

Contactin-associated protein-like 2, a protein of the neurexin family involved in several human diseases

Contactin-associated protein-like 2 (CASPR2) is a cell adhesion protein of the neurexin family. Proteins of this family have been shown to play a role in the development of the nervous system, in synaptic functions, and in neurological diseases. Over recent years, CASPR2 function has gained an increasing interest as demonstrated by the growing number of publications. Here, we gather published data to comprehensively review CASPR2 functions within the nervous system in relation to CASPR2-related diseases in humans. On the one hand, studies on Cntnap2 (coding for CASPR2) knockout mice revealed its role during development, especially, in setting-up the inhibitory network. Consistent with this result, mutations in the CNTNAP2 gene coding for CASPR2 in human have been identified in neurodevelopmental disorders such as autism, intellectual disability, and epilepsy. On the other hand, CASPR2 was shown to play a role beyond development, in the localization of voltage-gated potassium channel (VGKC) complex that is composed of TAG-1, Kv1.1, and Kv1.2. This complex was found in several subcellular compartments essential for action potential propagation: the node of Ranvier, the axon initial segment, and the synapse. In line with a role of CASPR2 in the mature nervous system, neurological autoimmune diseases have been described in patients without neurodevelopmental disorders but with antibodies directed against CASPR2. These autoimmune diseases were of two types: central with memory disorders and temporal lobe seizures, or peripheral with muscular hyperactivity. Overall, we review the up-to-date knowledge on CASPR2 function and pinpoint confused or lacking information that will need further investigation.

Current Treatment Options for Peripheral Nerve Hyperexcitability Syndromes

PURPOSE OF REVIEW

Peripheral nerve hyperexcitability (PNH) syndromes are divided into primary and secondary groups based on the presence or absence of demonstrable peripheral nerve disease. In this review, we systematically evaluate the evidence for current therapies and supportive managements based on autoimmune, paraneoplastic, and genetic components in pathophysiology reported in the literature.

RECENT FINDINGS

Current therapy options are based on symptomatic management as well as focusing the underlying immune/genetic/paraneoplastic pathology by immunosuppressants, chemotherapy, and surgery. Further research is desired to provide treatment options geared specifically towards addressing PNH. Supportive care can also be an area for future research.