In 2009, 43 patients with limbic encephalitis of unknown origin were investigated in order to identify the antigen. Among these, antibodies of ten patients showed a similar pattern of reactivity to neuropil of brain and cerebellum. Further tests showed that the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) was the target antigen in these patients. Until now, this series of 10 patients is the major source of information concerning anti-AMPA receptor encephalitis (Lai et al. 2009).

The AMPAR is an ionotropic glutamate receptor concentrated at synapses, mediating most of the fast excitatory neurotransmission in the brain (Shepherd and Huganir 2007). The receptors are composed of various combinations of four subunit proteins, GluR1-4 (Lai et al. 2009; Shepherd and Huganir 2007; Granger et al. 2011). Expression of subunits is developmentally regulated and is brain region specific. Most receptors consist of two different subunits, mainly GluR1-2 or GluR2-3 (Shepherd and Huganir 2007). In anti-AMPAR encephalitis, antibodies are shown to react to cell surface GluR1 and GluR2 subunits. Antibody reaction leads to a decrease in the number of receptors at synapses and a decrease of receptors along dendrites. Removal of antibodies from neuronal cultures has shown to restore receptor number and localization of AMPAR clusters (Lai et al. 2009).

The patients investigated to isolate the antigen were nine women and one man. Age ranged from 38 to 87 years; median age was 60 years. A series of four female patients age 51–71 has been reported as well (Graus et al. 2010). These data suggest that the disease predominantly occurs in older women. Seven out of ten patients had a tumor. In four patients tumor was diagnosed concurrent with the first episode of encephalitis, in two patients concurrent with a relapse, and in one patient 6 months after limbic encephalitis. Tumors were thymic carcinoma, thymoma, non-SCLC, SCLC, and breast cancer. Four tumors were tested on GluR1 and GluR2 proteins and all showed to be positive (Lai et al. 2009).

Eleven of fourteen reported patients had presented with classical limbic encephalitis, showing confusion, disorientation, and memory loss evolving within 8 weeks. One patient presented with a 4-month history of symptoms suggesting a rapidly progressive dementia and two patients had atypical acute psychosis. Four out of ten patients had seizures (Lai et al. 2009; Graus et al. 2010).

Most patients have CSF lymphocytic pleocytosis (Lai et al. 2009). Antibodies to the AMPAR can be detected with both immunohistochemical staining of neuropil or cell-based assay, using either serum or CSF. However, one patient was tested negative in serum although CSF was positive (Graus et al. 2010). Brain MRI shows increased FLAIR signal in medial temporal lobes (Lai et al. 2009). Rapid brain atrophy evolving within 5 days was seen on serial MRIs in a pregnant 30-year-old woman with anti-AMPAR encephalitis (Wei et al. 2013; Hutchinson et al. 2008).

In the series of ten patients, nine were treated with immunotherapy and tumor treatment when appropriate. Immunotherapy usually consisted of combinations of plasma exchange, corticosteroids, and/or intravenous immunoglobulin. Azathioprine and cyclophosphamide were both used in one patient (Lai et al. 2009).

All nine treated patients showed major improvement after first episode of limbic encephalitis. Three patients had fully recovered without relapse. One patient had mild persistent depression, apathy, and reduced verbal fluency at 3 months. Five patients had one to three relapses. Relapses occurred up to 101 months after the initial disease episode. Recovery from relapses can be incomplete, with persistent memory loss and behavior problems. One patient died shortly after prolonged status epilepticus at second relapse (Lai et al. 2009).

The NMDA receptor and AMPA receptor are ionotropic glutamate receptors, as mentioned earlier. In contrast, there are several metabotropic subtypes of glutamate receptors (mGluR). These receptors indirectly activate ion channels through a signaling cascade involving G proteins. Antibodies directed to the mGluR5 and mGluR1 subtype have been reported in patients with encephalitis. The mGluR5 and mGluR1 receptors are very similar in their amino acid sequences, but antibodies to these receptors do not cross react. The brain distributions differ between the receptors, just as the clinical picture described in disease (van Coevorden-Hameete et al. 2014; Lancaster et al. 2011a).

In 1982, Ian Carr wrote a moving personal paper about the subacute loss of memory and psychosis in his 15-year-old daughter Jane, who subsequently appeared to have Hodgkin’s lymphoma (Carr 1982). He called this paraneoplastic disease the Ophelia syndrome, after the character in Shakespeare’s Hamlet. More recently three patients with a comparable clinical picture in Hodgkin lymphoma were reported. In these patients mGluR5 was detected as the target antigen of the antibodies (Lancaster et al. 2011a; Mat et al. 2013; Sillevis et al. 2000). Antibodies were detected in serum in two patients (CSF not available) and in CSF in the third patient (serum not available). All three patients received tumor treatment, either chemotherapy or radiotherapy. Only one patient had immunotherapy with intravenous methylprednisolone. All patients had a favorable outcome of both oncological and neurological disease, similar to Carr’s daughter.

Subacute cerebellar ataxia has been associated with antibodies to mGluR1 in serum and CSF in five patients. The first two patients had had Hodgkin’s disease 2 and 9 years earlier and were in remission (Sillevis et al. 2000). A third had an adenocarcinoma of the prostate and two patients had no tumor (Lancaster et al. 2011a; Iorio et al. 2013; Marignier et al. 2010). The pathogenic role of mGluR1 antibodies has been demonstrated by the induction of cerebellar symptoms in mice after passive transfer of patient’s antibodies (Sillevis et al. 2000). Biopsy specimen of lymph node of one patient with mGluR1 antibodies and Hodgkin’s disease failed to show mGluR1 RNA. In contrast, immunohistochemistry analysis showed mGluR1 expression in the adenocarcinoma of the patient with prostate cancer (Sillevis et al. 2000; Iorio et al. 2013). Neurological outcome in the five reported patients is variable, ranging from complete recovery to no improvement, despite immunotherapy.

Antibodies to the glycine receptor (GlyR) were initially detected in a patient with progressive encephalomyelitis with rigidity and myoclonus (PERM) in 2008 (Hutchinson et al. 2008). In the following years, antibodies were seen in several patients with the clinical spectrum of both stiff person syndrome (SPS) and PERM (Alexopoulos et al. 2013; McKeon et al. 2013).

GlyRs are chloride channels facilitating inhibitory neurotransmission in the brain and spinal cord. GlyRs are expressed on the cell surface membrane and are composed of two GlyRα subunits, with four variants, GlyRα1–4, and three GlyRβ subunits. GlyR antibodies are thought to be directed to the GlyRα1 subunit (van Coevorden-Hameete et al. 2014). Receptor dysfunction results in loss of inhibitory neurotransmission in the brainstem and spinal cord, leading to abnormal discharges of motor neurons and widespread muscular rigidity (Bourke et al. 2013; De Blauwe et al. 2013).

SPS and PERM are part of a clinical spectrum of antibody-mediated disease in both children and adults (Clardy et al. 2013). Several antibodies have been associated with these syndromes. Antibodies directed to the intracellular protein glutamic acid decarboxylase (GAD65) are detected in 60–80 % of the patients with SPS, but the pathogenic role of these antibodies remains controversial (De Blauwe et al. 2013). Anti-amphiphysin antibodies have been detected in some patients, associated with breast cancer. Passive transfer of patients’ immunoglobulin to rats has been reported to induce stiffness and spasms, suggesting some pathogenic role of these antibodies (Geis et al. 2010; Sommer et al. 2005), even though these antibodies act to intracellular antigens. Anti-GlyR antibodies are detected in only 9–15 % of the patients with SPS but are probably much more common in PERM. Several arguments support the pathogenic role of anti-GlyR antibodies in these disorders. Mutations in the GlyRα gene are identified in hereditary hyperekplexia. Pharmacological disturbance of the GlyR causes muscle cramps. The favorable effect of immunotherapy is also supporting the pathogenicity of antibodies, especially antibodies directed to extracellular proteins such as GlyR (Hutchinson et al. 2008; van Coevorden-Hameete et al. 2014).

Whether anti-GlyR antibody production is a paraneoplastic phenomenon is unknown. SPS and PERM with anti-GlyR antibodies have been reported in patients with thymoma, small cell lung cancer, breast cancer, and chronic lymphocytic leukemia but in a considerable amount of patients without a tumor as well (Kyskan et al. 2013; Derksen et al. 2013).

Anti-GlyR antibodies have been detected in 3–6 % of the patients with epilepsy of unknown cause (Brenner et al. 2013; Ekizoglu et al. 2014).

Patients with SPS show lower extremity and lumbar stiffness and spasms. PERM has been described as a severe variant of SPS with whole-body stiffness, myoclonic jerks, autonomic features, and brainstem signs, usually ocular motility disorders. Patients may experience hyperekplexia and bulbar signs such as trismus and laryngospasm (Hutchinson et al. 2008; Alexopoulos et al. 2013; Iizuka et al. 2012; Vincent et al. 2011).

Standard CSF examination in PERM is usually normal or shows mild lymphocytosis.

Anti-GlyR antibodies can be detected in both serum and CSF using cell-based assay. Immunohistochemistry of the brain shows no staining.

MRI of the brain and spinal cord is unremarkable. Electromyography findings can be characteristic for hyperekplexia, showing involuntary continuous motor unit activity.

Successful treatment of SPS and PERM has been described in case reports. Immunotherapy with either steroids, intravenous immunoglobulin, or plasmapheresis might be beneficial. Azathioprine and rituximab have been used with success as well (Bourke et al. 2013; Clardy et al. 2013; Kyskan et al. 2013; Damasio et al. 2013). Stiffness and spasms can be symptomatically treated with clonazepam, diazepam, baclofen, phenytoin, or gabapentin (Bourke et al. 2013; Kyskan et al. 2013; Mas et al. 2011).

After initial improvement on immunotherapy, PERM patients tend to relapse. Three reported patients responded well on immunotherapy during relapse, although one of them had sustained disabilities (Hutchinson et al. 2008; Kyskan et al. 2013; Damasio et al. 2013).

Antibodies to the voltage-gated potassium channel (VGKC) were initially detected in patients with acquired neuromyotonia, a peripheral nerve disorder characterized by muscle cramps, impaired relaxation, and stiffness (Shillito et al. 1995). A pathogenic role of anti-VGKC antibodies was subsequently suspected in Morvan’s syndrome, showing neuromyotonia accompanied by autonomic and cognitive symptoms and insomnia (Barber et al. 2000). The similarity of the central nervous system symptoms of Morvan’s syndrome with symptoms seen in limbic encephalitis has led to the analysis and identification of anti-VGKC antibodies in two patients with limbic encephalitis in 2001 (Buckley et al. 2001). Antibodies were eventually thought to be directed to the Kv1.1, 1.2, and 1.6 subunits of the VGKC receptor (Kleopa et al. 2006). However, the exact role of VGKC antibodies remained controversial as no laboratory succeeded in showing staining with serum in VGKC-transfected cells. In the year 2010, this reconsideration led two laboratories to identify simultaneously that these antibodies are not directed to the subunits of the VGKC itself but to the VGKC-associated proteins leucine-rich glioma-inactivated protein 1 (LGI1) and contactin-associated protein 2 (Caspr2) (Irani et al. 2010b; Lai et al. 2010). Anti-LGI1 antibodies are mainly associated with CNS disorders such as limbic encephalitis and epilepsy. Caspr2 antibodies are predominantly associated with peripheral nerve hyperexcitability and the combination of CNS and PNS symptoms in Morvan’s syndrome but have been described with (limbic) encephalitis as well.

A significant part of the patients testing positive in the VGKC-radioimmunoassay (VGKC-RIA) do not have anti-LGI1 or anti-Caspr2 antibodies. This is an emerging heterogeneous group of patients. The clinical significance of a positive VGKC-RIA in these patients is currently unknown, which poses a threat for both under- and overdiagnosis as well as under- and overtreatment.

The use of several cutoff values for positive VGKC-RIA titers and the tendency to perform grouped analysis of all VGKC-RIA-positive patients, even after the discovery of the LGI1 and Caspr2 subtypes, are a significant limitation in reviewing clinical symptoms and response to treatment.

LGI1 is a secreted glycoprotein, in contrast to the previously described membrane-bound proteins. LGI1 binds to the cell membrane via a disintegrin and metalloprotease domain-containing protein (ADAM). LGI1 connects presynaptic ADAM23 to postsynaptic ADAM22, thereby influences synaptic transmission (van Coevorden-Hameete et al. 2014; Fukata et al. 2010). This LGI1 transsynaptic fine-tuning is thought to have an antiepileptic effect (Fukata et al. 2010). Antibodies are thought to react to LGI1 co-expressed with ADAM22 or ADAM23 (Fukata et al. 2010; Shin et al. 2013; Ohkawa et al. 2013). No in vivo studies have been performed to establish a pathogenic role of LGI1 antibodies in limbic encephalitis. However, genetic disruption of LGI1 is known to cause an epileptic syndrome and, as seizures are a hallmark symptom in anti-LGI encephalitis, makes its pathogenic role more likely.

Caspr2 is a membrane protein in the juxtaparanodes in myelinated axons in both the peripheral and central nervous system (Irani et al. 2010b; Lancaster et al. 2011b). Caspr2 stabilizes Kv1.1 and 1.2 channels. The role of Caspr2 antibodies has not been studied in vivo. Caspr2 antibodies reactivity might disrupt this colocalization of Kv1.1 and 1.2, diminishing repolarization and thereby causing hyperexcitability. Mutation in the Caspr2 coding gene, CNTNAP2, is known to cause childhood-onset refractory epilepsy with mental retardation (van Coevorden-Hameete et al. 2014).

The exact incidence of VGKC-complex encephalitis and its subtypes is unknown. Pooling of study results is impeded by the differences between countries in test technics and its interpretation.

The VGKC-RIA is requested for several indications. High titers are most commonly seen in limbic encephalitis. In a cohort of 125 patients with antibody-mediated encephalitis, 11.2 % had anti-LGI antibodies and only 0.8 % showed anti-Caspr2 antibodies (Shin et al. 2013). In two cohorts of VGKC-RIA-positive patients with limbic encephalitis, LGI antibodies were detected in respectively 49/64 (77 %) and 9/10 (90 %) patients (Irani et al. 2010b; Butler et al. 2014). Median age at onset of anti-LGI antibody-mediated limbic encephalitis is 60 years (range 30–80) and there is a male predominance (Lai et al. 2010; Shin et al. 2013). Tumors are uncommon (Irani et al. 2010b).

Anti-Caspr2 antibodies are rarely seen in limbic encephalitis and are more often associated with peripheral nervous system disorders, such as neuromyotonia, or in Morvan’s syndrome, showing both PNS and CNS features. Tumors, most frequently thymoma, are seen in the minority of anti-Caspr2 antibody-positive patients, mainly in those with Morvan’s syndrome (Irani et al. 2010b).

Positive VGKC-RIA is seen in unexplained adult-onset epilepsy in 4–6 % of the patients (Lilleker et al. 2013; Majoie et al. 2006). In one cohort of six VGKC-RIA-positive patients with unexplained epilepsy, only one patient showed anti-LGI1 and one other patient had anti-Caspr2 antibodies (Lilleker et al. 2013). Another cohort of 18 epilepsy patients with positive VGKC-RIA showed positive CBA for LGI1 in 14 patients and Caspr2 in 1 patient, but the clinical picture of this cohort is different. Most patients had cognitive symptoms as well, suggesting limbic encephalitis (Quek et al. 2012).

Patients with a positive test result by VGKC-RIA who do not show anti-LGI1 or anti-Caspr2 antibodies might react to a VGKC-complex protein yet to be identified. This mainly applies to patients with a clear clinical syndrome such as limbic encephalitis or Morvan’s syndrome. The clinical relevance of antibodies is highly controversial in patients with an atypical or peripheral nervous system syndrome showing positive VGKC-RIA without LGI1 or Caspr2 antibodies.

A positive VGKC-RIA is unfrequently seen in children, although several cases of movement disorders, insomnia, peripheral nervous system disorders, seizures, and limbic encephalitis have been reported. Unfortunately, reactivity to LGI1 and Caspr2 in these patients is unknown (Dhamija et al. 2011).

Patients with anti-LGI1 encephalitis show the typical features of limbic encephalitis such as seizures, memory deficit, confusion, and behavioral problems. A minority of the patients show autonomic dysfunction (Irani et al. 2010b; Shin et al. 2013). Hyponatremia is common (~60 %). Typical for anti-LGI1 encephalitis is the occurrence of faciobrachial dystonic seizures (FBDS). These brief involuntary abnormal movements are unilateral, involving the arm, usually ipsilateral face, and less commonly the trunk or a leg. A minority of the patients produce vocalizations at the start of the FBDS, and loss of consciousness is uncommon. FBDS occur very frequent, up to 100 times per day (Andrade et al. 2011; Irani et al. 2011). The etiology of FBDS was analyzed in three patients. Longer events showed generalized flattened EEG pattern just before the onset of movement, confirming that these events are tonic seizures (Andrade et al. 2011; Irani et al. 2011). In the majority of the patients, FBDS precede the onset of limbic encephalitis. Patients usually have other seizure types and cognitive symptoms within a few weeks. Early recognition of FBDS as a prodromal feature of anti-LGI encephalitis might enable early treatment.

Patients with anti-Caspr2 antibodies may have limbic encephalitis. More common, these antibodies are seen in Morvan’s syndrome, a combination of central and peripheral nervous system symptoms, showing muscle weakness, cramps, autonomic features, and insomnia. Anti-Caspr2 antibodies are also associated with neuromyotonia, a peripheral nerve hyperexcitability syndrome.

Standard CSF examination is usually unremarkable, although mild pleocytosis or mildly elevated protein can be seen. Serum hyponatremia is common in anti-LGI1 encephalitis (Lai et al. 2010; Shin et al. 2013).