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Stiff person syndrome (SPS) is a rare, progressive syndrome that affects the nervous system, specifically the brain and spinal cord. Symptoms may include extreme muscle stiffness, rigidity and painful spasms in the trunk and limbs, severely impairing mobility. Spasms can generate enough force to fracture bone. People with SPS often have heightened sensitivity to noise, sudden movements, and emotional distress, which can set off muscle spasms. Persistent symptoms can lead to abnormal posturing of the spine, such as being hunched over. The syndrome affects twice as many women as men.^[1][2]

SPS is caused by increased muscle activity due to decreased inhibition of the central nervous system. It is thought to have an autoimmune component and is often associated with diabetes, as well as other autoimmune diseases such as thyroiditis, vitiligo, and pernicious anemia.^[1][2] It may be diagnosed after having various tests including blood tests (such as for glutamic acid decarboxylase (GAD) antibodies which is elevated in about 2 in 3 people with SPS), a lumbar puncture, and electromyography. Treatment aims to control symptoms and improve mobility. Examples of treatments that have been used for SPS, include benzodiazepines, muscle relaxants, intravenous immune globulin (IVIG) therapy, plasmapheresis (also called plasma exchange), and rituximab.^[1][2] While some people with SPS may maintain reasonable levels of activity with treatment, the majority become disabled over time.^[2]

Stiff person syndrome (SPS) is a progressive syndrome characterized by recurrent episodes of severe muscle stiffness, rigidity, and painful spasms in the trunk and limbs. The age that symptoms begin can vary, but most people start experiencing symptoms between ages 30 and 60. Spasms can be prolonged and extremely forceful, with the ability to generate enough force to fracture bone. They may cause a person to fall when walking or standing. Spasms are especially likely or may worsen during times of emotional distress, when being touched, when there is sudden movement, or with noise.^[2][3]

Over time, persistent symptoms can lead to abnormal posturing of the spine, such as being stiffened and hunched over.^[1] Daily activities such as getting into or out of bed, getting up from a chair, or dressing may become increasingly difficult.^[2] People with SPS also may become fearful and anxious about navigating daily life, which in turn may trigger additional spasms. Many people with SPS develop depression as the syndrome progresses and quality of life becomes severely impaired.^[2][3]

Scientists don't yet understand the complete picture of what causes stiff person syndrome, but research indicates that it is the result of an abnormal autoimmune response in the brain and spinal cord.^[1] Autoimmune responses occur when the immune system mistakenly attacks the body.

Most people with stiff person syndrome have antibodies that are made to attack glutamic acid decarboxylase (GAD). GAD is a protein in some neurons that are involved in making a substance called gamma-aminobutyric acid (GABA), which is responsible for controlling muscle movement. The symptoms of stiff person syndrome may develop when the immune system mistakenly attacks the neurons that produce GAD. When GAD is not working properly, there is not enough GABA to help control muscle movement. The exact role that deficiency of GAD plays in the development of stiff person syndrome is not fully understood.^[3]

Some individuals with stiff person syndrome will have antibodies to amphiphysin, a protein involved in the transmission of signals from one neuron to another. Individuals with these antibodies have a higher risk for developing breast, lung, or colon cancer.^[3][4][5][6]

As is the case with most autoimmune diseases, genetic factors involved in causing stiff person syndrome have not been established. While most cases appear to occur in an isolated manner, there have been reported cases of multiple people in the same family being affected by SPS.^[7] Although one specific genetic change (mutation) is not known to cause stiff person syndrome, it is thought that genetics in combination with other factors may play a role in causing SPS.^[3]

A diagnosis of stiff person syndrome (SPS) is typically made based on symptoms, a detailed medical history, and various tests used to support the diagnosis or rule out other diseases with overlapping symptoms.^[4][8] One commonly used test is a blood test to detect the presence of glutamic acid decarboxylase (GAD) antibodies.About 60-80% of people with SPS have antibodies against GAD that can be detected on a blood test.^[8] The absence of GAD antibodies does not rule out SPS, but the presence of high levels of GAD antibodies strongly supports the diagnosis.^[5] GAD antibodies may also be measured in the cerebral spinal fluid from a lumbar puncture.^[9]

Additionally, a doctor may recommend electromyography (EMG), which records electrical activity in skeletal muscles.^[4][8] The EMG of a person with SPS typically shows continuous motor activity in the skeletal muscles.^[4][8] 

Other testing that may be used to confirm or rule out the diagnosis includes:

• A hemoglobin A1Ctest to rule out diabetes mellitus.
• A complete blood countto rule out pernicious anemia.
• A thyroid-stimulating hormone (TSH)test to rule out thyroiditis.
• A lumbar puncture to look for oligoclonal bandswhich indicate that the central nervous system is inflamed (these bands can be seen in about two thirds of people who have GAD antibodies).^[3]

Genetic testing currently is not available because the underlying genetic cause of stiff person syndrome has not been established.^[5]

Treatment aims to control symptoms and improve mobility and function. While some people on treatment for SPS may maintain reasonable levels of activity, the majority become increasingly disabled over time. Treatment options depend on the symptoms and severity in each person and may include:^[2][3]

• Benzodiazepines - these are drugs that slow down the nervous system and may relieve muscle spasms and anxiety. They are generally considered the best initial therapy for SPS. Examples include diazepamand clonazepam.
• Baclofen- this is a muscle relaxant that may be used for people in whom benzodiazepines are not effective or not well-tolerated. Some people benefit from using baclofen in addition to benzodiazepines.
• Immune modulating therapies - these may be considered in people with severe symptoms who do not experience relief with benzodiazepines and baclofen. Options may include intravenous immune globulin (IVIG) therapy, plasmapheresis(also called plasma exchange), and  rituximab. However studies supporting the effectiveness and safety of these therapies for SPS are limited.

Source: https://rarediseases.info.nih.gov/diseases/5023/stiff-person-syndrome