Stiff-person syndrome (SPS) is a disease of the CNS, which manifests with progressive muscle stiffness, typically in the trunk and extremities, as well as spontaneous or triggered spasms. Up to 80% of patients show a high serum titer and intrathecal synthesis of anti-glutamic acid decarboxylase (GAD) antibodies. Around 5 % of all SPS cases are paraneoplastic and usually associated with antibodies against amphiphysin.
These diseases are characterised by progressive destruction of the myelin sheath. The demyelinating foci are predominantly in the brain and spinal cord. The loss of myelin impairs neuronal signal transduction, leading to motor, visual and sensory disorders. These encompass neuromyelitis optica spectrum disorders (NMOSD), which affect in particular the optical nerve and the spinal cord. NMOSD is associated with pathogenic antibodies against the CNS water-channel protein aquaporin-4 (AQP-4). Antibodies against myelin oligodendrocyte glycoprotein (MOG) occur in around 20 % of AQP-4-negative NMOSD patients. Antibodies against MOG also occur in other demyelinating diseases of the CNS, e.g. acute demyelinating encephalomyelitis (ADEM). The determination of anti-AQP-4 and anti-MOG antibodies enables early delimitation from multiple sclerosis, the most important differential diagnosis.
The peripheral nervous system can also be the target of autoaggression, affecting nerves, ganglia or myelin sheaths. Manifestations encompass motor paralysis, sensitivity disorders or dysautonomy. Autoantibodies against cell-membrane glycolipids or glycoproteins of neurons or glial cells are diagnostically definitive for many forms of peripheral neuropathy. Antibodies against gangliosides are characteristic markers for Guillain-Barré syndrome and its variants, for example, acute motor axonal neuropathy (GM1/ GM1b/ GD1a/ GalNac-GD1a IgG), Miller-Fisher syndrome (GQ1b/GT1a IgG), and multifocal motor neuropathy (GM1/GD1b/ asialo-GM1 IgM). Further, IgM antibodies against myelin-associated glycoprotein (MAG) typically occur in demyelinating polyneuropathy with monoclonal IgM gammopathy.
In myasthenia gravis (MG) and Lambert-Eaton myasthenic syndrome (LEMS) the dominant symptom is muscle weakness, which is mainly due to antibody-mediated transmission disorders of the neuromuscular synapses. Antibodies against nicotinic acetylcholine receptors (AChR) are detected in 85 to 90% of patients with generalised MG. Additional reactivities against antigens of the striated muscle (e.g. titin) often occur in connection with neoplasia (thymoma in 15% of all myasthenia cases) and a severe disease course.