Stiff-person syndrome (SPS), also known as stiff-man syndrome (SMS), is a rare neurologic disorder of unclear cause characterized by progressive rigidity and stiffness. The stiffness primarily affects the truncal muscles and is superimposed by spasms, resulting in postural deformities. Chronic pain, impaired mobility, and lumbar hyperlordosis are common symptoms. SPS occurs in about one in a million people and is most commonly found in middle-aged people. A small minority of patients have the paraneoplastic variety of the condition. Variants of the condition, such as stiff-limb syndrome which primarily affects a specific limb, are often seen. SPS was first described in 1956. Diagnostic criteria were proposed in the 1960s and refined two decades later. In the 1990s and 2000s the roles of antibodies in the condition became clearer. SPS patients generally have glutamic acid decarboxylase (GAD) antibodies, which seldom occur in the general population. In addition to blood tests for GAD, electromyography tests can help confirm the condition's presence. Benzodiazepine-class drugs are the most common treatment; they are used for symptom relief from stiffness. Other common treatments include baclofen, intravenous immunoglobin and rituximab. There is a limited but encouraging therapeutic experience of haematopoietic stem cell transplantation for SPS. Signs and symptoms Stiff-person syndrome (SPS) is often separated into several subtypes, based on the cause and progression of the disease. There are three clinical classifications of SPS Classic SPS, associated with other autoimmune conditions and usually GAD-positive; Partial SPS variants Progressive encephalomyelitis with rigidity and myoclonus (PERM). Around 70% of those with SPS have the "classic" form of the disease. People with classic SPS typically first experience intermittent tightness or aching in the muscles of the trunk. These muscles repeatedly and involuntarily contract, causing them to grow and rigidify. Eventually, rigidified muscles reduce the affected person's range of motion, slow their voluntary movements, and may cause them to have abnormal posture, particularly lumbar hyperlordosis (a distinctive curve in the lower back). Rigid trunk muscles can also prevent the chest and abdomen from expanding, causing shortness of breath and early satiety. In many people with SPS, muscle rigidity eventually progresses from the trunk to the limbs – first affecting muscles closest to the trunk, then further. Stiffened limbs can affect a person's balance and gait, causing awkward 'statue-like' falls where the affected person cannot put out their arms to soften the impact. Alongside growing stiffness, many with SPS develop bouts of muscle spasms