Anti-VGKC-complex encephalitis

Anti-VGKC-complex encephalitis are caused by antibodies against the voltage gated potassium channel-complex (VGKC-complex) and are implicated in several autoimmune conditions including limbic encephalitis, epilepsy and neuromyotonia (i.e. Isaacs' Syndrome).

Research into autoimmune encephalitis have advanced a lot recently. Both tumour associated (paraneoplastic) and non-tumour associated conditions are recognized. Different antibodies are more or less selective for different parts of the brain. Antibodies are directed towards intracellular antigens in the classic paraneoplastic syndromes or synaptic proteins in paraneoplastic or non-tumour associated conditions. The VGKC-complex associated autoimmune encephalitis is an example of the latter form.

Antibodies directed against VGKC were first reported in neuromyotonia.[1] Further studies led to VGKC antibodies being recognized in Morvan’s syndrome and limbic encephalitis as well.[2] It was for many years erroneously presumed that antibodies detected in a VGKC assay were targeted against the channel itself. But the heterogeneous presentation of patients was difficult to explain. The original assays for the detection of VGKC antibodies used Iodine-125 labelled dendrotoxin and the relatively mild detergent 2% digitonin on mammalian brain homogenate, and VGKC with complexed proteins was extracted.[1] Co-precipitated proteins would thus also be detected in this assay. It’s now acknowledged that most VGKC antibodies are instead directed towards associated/complexed proteins. In a particular study of 96 patients with VGKC antibodies detected with the radioimmunoprecipitation assay, only 3 (3%) had antibodies towards the Kv1 subunit of the VGKC channel, 55 (57%) had antibodies against Leucine-rich, glioma Inactivated 1 (LGI1), 19 had antibodies reacting with Contactin-associated protein 2 (CASPR 2), 5 had antibodies against Contactin-2 and 18 (19%) had antibodies with unknown specificity.[3] Of the patients with Contactin-2 antibodies, 4/5 had antibodies against other antigens as well.

Signs and symptoms

Signs and symptoms depend on the targeted antigen, but the features in patients with different antibodies often overlap. The most characteristic feature found in a case series was cognitive impairment and seizures in anti-LGI-1 positive patients, and peripheral motor hyperexcitability in anti-CASPR2 positive patients.[4] Some patients have other coexisting autoimmune diseases.

anti-LGI-1 encephalitis: Patients with anti-LGI1 encephalitis have limbic encephalitis with amnesia and/or confusion (100%) and seizures (84-92 %) [3][5] Other reported features include hyponatremia (in 60%), movement disorders (myoclonus/dyskinesia), sleep disorders (hypersomnia, insomnia, REM sleep behavior disorder, sleep reversal) and ataxia. Tonic seizures, with movements of the leg, arm or face, refractory to treatment with anti-epileptic drugs may precede the disorders, and should lead to testing for anti-LGI1 antibodies.[6]

Anti-CASPR2 nervous system manifestations: Patients with anti-CASPIR2 antibodies develop symptoms from the CNS and/or the peripheral nervous system.[7] The classic presentation is with Morvan’s syndrome, a disease with the features of neuromyotonia (i.e. peripheral hyperexcetability) and limbic encephalitis. Other patients present with isolated neuromyotonia or limbic encephalitis.

Patients with anti-DPPX encephalitis present with symptoms of hyperexcetability such as agitation, tremor, muscle rigidity and gastrointestinal symptoms.[7]

Causes

The causes are in general unknown. Some of the patients have tumours. The number of patients with anti-LGI1 antibodies and tumours have been reported to be between 0 and 11% in anti-LGI-1 antibody positive patients, and between 0-30 % in anti-CASPR2 antibody positive patients.[3][5] Tumours have not been reported in patients with anti-DPPX antibodies.[7] The associated tumour is often a thymoma.[7]

Pathophysiology

Patients with autoimmune encephalitis have antibodies towards synaptic proteins with neuronal functions. The antibodies are presumed to be disease causing, just like the acetylcholine receptor antibodies in myasthenia gravis.

The voltage-gated potassium channel (VGKC), like other ion channels, belong to a multiprotein complex. Some of the proteins which associates to the channel directly/indirectly include, but are not limited to, LGI1, CASPR2, Contactin2, DPPX, ADAM22 and ADAM23. LGI1 is a secreted neuronal protein which binds to ADAM22 and ADAM23 and thus crosslinks pre-synaptic VGKC with post-synaptic AMPA receptor. An inherited form of human epilepsy known as autosomal dominant partial epilepsy with auditory features (ADPEAF) has been found to be caused by mutations of the LGI1 gene. LGI1 knockout mice cause lethal epilepsy and the heterozygous mice have a lowered seizure threshold.[8] Mutations of CNTNAP2 (the gene encoding CASPR2) have been reported to be associated with intellectual disability, motor impairment and epilepsy.[9]

Diagnosis

The diagnosis of autoimmune encephalitis is based on the exclusion of differential diagnosis. The first step is a clinical evaluation Biochemical tests may reveal hyponatremia and the other features of SIADH.[10] Relevant supplementary tests are listed in table 1.[11] The diagnosis of VGKC-complex associated limbic encephalitis should be suspected in both men and women presenting with subacute debut of disorientation, confusion and amnesia especially when associated with seizures and signal change of the medial temporal lobe on MRI.

Investigation methods Comments
MRI Exclusion of differential diagnosis (e.g. cerebrovascular disease). Hyperintensity of affected regions on T2-images is characteristic. Sensitivity 60-84 %.[3][5]
EEG Exclusion of differential diagnosis. Non-characteristic findings may be found in autoimmune encephalitis.
Lumbar puncture Exclusion of differential diagnosis (e.g. infectious diseases). Abnormalities may be found in autoimmune encephalitis.
Immunofluorescence/immunohistochemistry On mammalian brain.
Rodent hippocampal culture
Cell based assays Using cell lines (e.g. HEK 293) transfected with recombinant proteins. Commercial products exists.
Radioimmunoassay Non specific. Have the ability to detect antibodies towards undescribed VGKC-complex associated antigens. The complex probably contains unidentified antigens since about 20% of patients have antibodies which can’t be detected with the more specific assays.

Treatment

The treatment is largely based on the treatment of anti-NMDAR encephalitis which is the most common autoimmune encephalitis. Treatment of an associated tumour is implicated in all paraneoplastic neurologic syndromes. Case series have been published where treatment has consisted of combinations of IVIG, plasmapheresis, glucocorticoids/other immunosuppressant drugs and rituximab.

Prognosis

The prognosis with treatment is generally good and much better than in the classic paraneoplastic syndromes. Patients, in one of the two biggest case series, with anti-CASPR2 antibodies w/o tumours improved in average from a Modified Rankin Score (MRS) of 4 (moderate severe disability) to a MRS of 1 (no significant disability despite symptoms).[3] The patients with anti-CASPR2 antibodies and tumours often deteriorated despite of treatment. The improvements seen in patients with anti-LGI1 antibodies were slightly better.

Epidemiology

The mean incidence of anti-LGI1 in Denmark is 1,1 per million compared to 3,3 per million for anti-NMDAR antibodies (NR1).[12] The disorders may be underdiagnosed due to the lack of knowledge about the conditions and the limited availability of diagnostic tests. The group of patients with anti-LGI1 antibody associated limbic encephalitis have a male predominance and a mean age of about 60 years (variation 30–80 years).[5] The other disorders with associated antibodies also seem to affect mostly older men.

See also

Anti-NMDAR encephalitis
Limbic encephalitis

References

  1. 1 2 Hart IK, Waters C, Vincent A, Newland C, Beeson D, Pongs O, et al. (1997). "Autoantibodies detected to expressed K+ channels are implicated in neuromyotonia.". Ann Neurol. 41 (2): 238–46. PMID 9029073. doi:10.1002/ana.410410215.
  2. ↑ Vincent A (2008). "Autoimmune channelopathies: John Newsom-Davis's work and legacy. A summary of the Newsom-Davis Memorial Lecture 2008.". J Neuroimmunol. 201–202: 245–9. PMID 18722023. doi:10.1016/j.jneuroim.2008.07.007.
  3. 1 2 3 4 5 Irani SR, Alexander S, Waters P, Kleopa KA, Pettingill P, Zuliani L, et al. (2010). "Antibodies to Kv1 potassium channel-complex proteins leucine-rich, glioma inactivated 1 protein and contactin-associated protein-2 in limbic encephalitis, Morvan's syndrome and acquired neuromyotonia.". Brain. 133 (9): 2734–48. PMC 2929337 Freely accessible. PMID 20663977. doi:10.1093/brain/awq213.
  4. ↑ Klein CJ, Lennon VA, Aston PA, McKeon A, O'Toole O, Quek A, et al. (2013). "Insights from LGI1 and CASPR2 potassium channel complex autoantibody subtyping.". JAMA Neurol. 70 (2): 229–34. PMC 3895328 Freely accessible. PMID 23407760. doi:10.1001/jamaneurol.2013.592.
  5. 1 2 3 4 Lai M, Huijbers MG, Lancaster E, Graus F, Bataller L, Balice-Gordon R, et al. (2010). "Investigation of LGI1 as the antigen in limbic encephalitis previously attributed to potassium channels: a case series.". Lancet Neurol. 9 (8): 776–85. PMC 3086669 Freely accessible. PMID 20580615. doi:10.1016/S1474-4422(10)70137-X.
  6. ↑ Irani SR, Michell AW, Lang B, Pettingill P, Waters P, Johnson MR, et al. (2011). "Faciobrachial dystonic seizures precede Lgi1 antibody limbic encephalitis.". Ann Neurol. 69 (5): 892–900. PMID 21416487. doi:10.1002/ana.22307.
  7. 1 2 3 4 Dalmau J, Rosenfeld MR (2014). "Autoimmune encephalitis update.". Neuro Oncol. 16 (6): 771–8. PMC 4022229 Freely accessible. PMID 24637228. doi:10.1093/neuonc/nou030.
  8. ↑ Fukata Y, Lovero KL, Iwanaga T, Watanabe A, Yokoi N, Tabuchi K, et al. (2010). "Disruption of LGI1-linked synaptic complex causes abnormal synaptic transmission and epilepsy.". Proc Natl Acad Sci U S A. 107 (8): 3799–804. PMC 2840530 Freely accessible. PMID 20133599. doi:10.1073/pnas.0914537107.
  9. ↑ Gregor A, Albrecht B, Bader I, Bijlsma EK, Ekici AB, Engels H, et al. (2011). "Expanding the clinical spectrum associated with defects in CNTNAP2 and NRXN1.". BMC Med Genet. 12: 106. PMC 3162517 Freely accessible. PMID 21827697. doi:10.1186/1471-2350-12-106.
  10. ↑ Vincent A, Buckley C, Schott JM, Baker I, Dewar BK, Detert N, et al. (2004). "Potassium channel antibody-associated encephalopathy: a potentially immunotherapy-responsive form of limbic encephalitis.". Brain. 127 (Pt 3): 701–12. PMID 14960497. doi:10.1093/brain/awh077.
  11. ↑ Dalmau, J Rosenfeld, MR. Paraneoplastic and autoimmune encephalitis. In: UpToDate, Post TW (Ed), UpToDate, Waltham, MA. (Accessed on October 11, 2015.)
  12. ↑ Somnier, F. E. (Apr 2015). Autoimmune encephalitis History & current knowledge (PDF). Copenhagen, Denmark: Statens Serum Institut. p. 6. Retrieved 2015-10-12.
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