Guillain–Barré syndrome (GBS) is a rapid-onset muscle weakness as a result of damage to the peripheral nervous system. Many experience changes in sensation or develop pain, followed by muscle weakness beginning in the feet and hands. The symptoms develop over half a day to four weeks. During the acute phase, the disorder can be life-threatening with about a quarter developing weakness of the breathing muscles and requiring mechanical ventilation. Some are affected by changes in the function of the autonomic nervous system, which can lead to dangerous abnormalities in heart rate and blood pressure.

This autoimmune disease is caused by the body's immune system mistakenly attacking the peripheral nerves and damaging their myelin insulation. Sometimes this immune dysfunction is triggered by an infection. The diagnosis is usually made based on the signs and symptoms, through the exclusion of alternative causes, and supported by tests such as nerve conduction studies and examination of the cerebrospinal fluid. Various classifications exist, depending on the areas of weakness, results of nerve conduction studies, and the presence of antiganglioside antibodies. It is classified as an acute polyneuropathy.

In those with severe weakness, prompt treatment with intravenous immunoglobulins or plasmapheresis, together with supportive care, will lead to good recovery in the majority. Some may experience ongoing difficulty with walking, painful symptoms, and some require long-term breathing support. Guillain–Barré syndrome is rare, at one to two cases per 100,000 people every year. The syndrome is named after the French neurologists Georges Guillain and Jean Alexandre Barré, who described it with André Strohl in 1916.

Symptoms
The first symptoms of Guillain–Barré syndrome are numbness, tingling, and pain, alone or in combination. This is followed by weakness of the legs and arms that affects both sides equally and worsens over time. The weakness can take half a day to over four weeks to reach maximum severity, and then becomes steady. In one in five people, the weakness continues to progress for as long as four weeks. The muscles of the neck may also be affected, and about half experience involvement of the cranial nerves which supply the head and face; this may lead to weakness of the muscles of the face, swallowing difficulties and sometimes weakness of the eye muscles. In 8%, the weakness affects only the legs (paraplegia or paraparesis). Involvement of the muscles that control the bladder and anus is unusual. In total, about a third of people with Guillain–Barré syndrome continue to be able to walk. Once the weakness has stopped progressing, it persists at a stable level ("plateau phase") before improvement occurs. The plateau phase can take between two days and six months, but the most common duration is a week. Pain-related symptoms affect more than half, and include back pain, painful tingling, muscle pain and pain in the head and neck relating to irritation of the lining of the brain.

Causes
Many people with Guillain–Barré syndrome have experienced the signs and symptoms of an infection in the 3–6 weeks prior to the onset of the neurological symptoms. These may consist of upper respiratory tract infection (rhinitis, sore throat) or diarrhea. In children, particularly those younger than six years old, the diagnosis can be difficult and the condition is often initially mistaken (sometimes for up to two weeks) for other causes of pains and difficulty walking, such as viral infections, or bone and joint problems.

Two thirds of people with Guillain–Barré syndrome have experienced an infection before the onset of the condition. Most commonly these are episodes of gastroenteritis or a respiratory tract infection. The Epstein–Barr virus/HHV-4 and varicella zoster virus/HHV-3 and the bacterium Mycoplasma pneumoniae have been associated with GBS. The tropical viral infection dengue fever has been associated with episodes of GBS. Previous hepatitis E virus infection has been found to be more common in people with Guillain–Barré syndrome. Zika virus has been linked to Guillain–Barré syndrome.

Treatment
Plasmapheresis and intravenous immunoglobulins (IVIg) are the two main immunotherapy treatments for GBS. Plasmapheresis attempts to reduce the body's attack on the nervous system by filtering antibodies out of the bloodstream. Similarly, administration of IVIg neutralizes harmful antibodies and inflammation. These two treatments are equally effective and a combination of the two is not significantly better than either alone. Plasmapheresis speeds recovery when used within four weeks of the onset of symptoms. IVIg works as well as plasmapheresis when started within two weeks of the onset of symptoms, and has fewer complications. IVIg is usually used first because of its ease of administration and safety. Its use is not without risk; occasionally it causes liver inflammation, or in rare cases, kidney failure. Glucocorticoids alone have not been found to be effective in speeding recovery and could potentially delay recovery.

Prognosis
Guillain–Barré syndrome can lead to death as a result of a number of complications: severe infections, blood clots, and cardiac arrest likely due to autonomic neuropathy. Despite optimum care this occurs in about 5% of cases. There is a variation in the rate and extent of recovery. The prognosis of Guillain–Barré syndrome is determined mainly by age (those over 40 may have a poorer outcome), and by the severity of symptoms after two weeks. Furthermore, those who experienced diarrhea before the onset of disease have a worse prognosis. On the nerve conduction study, the presence of conduction block predicts poorer outcome at 6 months. In those who have received intravenous immunoglobulins, a smaller increase in IgG in the blood two weeks after administration is associated with poorer mobility outcomes at six months than those whose IgG level increased substantially. If the disease continues to progress beyond four weeks, or there are multiple fluctuations in the severity (more than two in eight weeks), the diagnosis may be chronic inflammatory demyelinating polyneuropathy which is treated differently. The health-related quality of life (HRQL) after an attack of Guillain–Barré syndrome can be significantly impaired. About a fifth are unable to walk unaided after six months, and many experience chronic pain, fatigue and difficulty with work, education, hobbies and social activities.