Acute Inflammatory Demyelinating Polyneuropathy (AIDP) is the most common form of Guillain-Barré Syndrome (GBS), representing a serious, acute disorder affecting the peripheral nervous system. This condition involves the immune system mistakenly attacking the body’s own nerve cells, leading to rapidly progressive muscle weakness. AIDP is considered a medical urgency because the weakness can progress quickly to involve muscles necessary for breathing. Prompt identification and treatment are paramount to improving patient outcomes.
Understanding the Autoimmune Attack
AIDP is classified as an autoimmune response where the body’s defenses are misdirected against the peripheral nerves, which are the nerves outside the brain and spinal cord. This subtype of GBS accounts for approximately 90% of cases in North America and Europe. The attack targets the myelin sheath, a fatty layer that insulates the nerve fibers (demyelination).
The immune system’s error is often triggered by a preceding illness, usually a respiratory or gastrointestinal infection occurring one to six weeks before neurological symptoms begin. The bacterium Campylobacter jejuni is the most frequently identified trigger worldwide, though viruses like Cytomegalovirus and Epstein-Barr virus are also implicated. The mechanism involves molecular mimicry, where antibodies created to fight the infection mistakenly recognize and attack components of the peripheral nerve myelin due to structural similarities.
The destruction of the myelin sheath impairs the ability of the nerves to transmit electrical signals efficiently. When this sheath is damaged, the signals slow down significantly or stop completely, directly causing the muscle weakness and sensory changes experienced by patients. This inflammatory process can also attract immune cells, like macrophages, which contribute to the stripping away of the myelin from the nerve axons.
Recognizing the Physical Manifestations
The hallmark of AIDP is rapidly progressive and generally symmetric muscle weakness that typically begins in the lower extremities. Patients often first notice difficulty walking, climbing stairs, or a general sense of heaviness in their legs. This weakness characteristically ascends, moving upward to involve the arms and truncal muscles.
Sensory symptoms, such as tingling, numbness, or a “pins and needles” feeling (paresthesias), frequently accompany or precede muscle weakness. Pain, especially in the back and limbs, is also a common complaint. A defining clinical sign is areflexia, the absence of deep tendon reflexes, which is caused by the demyelination of the nerve roots.
The progression of physical symptoms is acute and reaches its maximum severity quickly. In most cases, the greatest level of weakness, known as the nadir, is reached within two weeks of the first symptom. Over 90% of patients have reached their peak disability by four weeks. In severe cases, the weakness can involve the cranial nerves, causing difficulty with facial movement, swallowing, and speaking. About 20% to 30% of patients develop respiratory muscle weakness severe enough to require mechanical ventilation.
How Doctors Confirm the Condition
The diagnostic process begins with a detailed neurological examination to assess muscle strength, sensation, and reflexes, looking for the characteristic pattern of ascending weakness and absent reflexes. To confirm the diagnosis and characterize the nerve damage, physicians rely on specialized electrodiagnostic studies.
Nerve Conduction Studies (NCS) and Electromyography (EMG) measure the speed and strength of electrical signals passing through the nerves and muscles. In AIDP, NCS typically reveals features of demyelination, such as a significant reduction in nerve conduction velocity, prolonged distal latency, and the presence of conduction block, where the signal stops completely along a segment of the nerve. These findings distinguish AIDP from other forms of GBS that primarily involve axonal damage.
Another procedure central to the diagnosis is a lumbar puncture, often called a spinal tap, which collects a sample of cerebrospinal fluid (CSF). The classic finding in AIDP is called albuminocytologic dissociation, characterized by a high concentration of protein in the CSF with a near-normal white blood cell count. While highly suggestive, this finding is not always present in the first week of symptoms, as protein levels increase over the course of the first few weeks.
Medical Interventions and Recovery
Treatment for AIDP is time-sensitive and focuses on reducing the severity of the autoimmune attack and providing supportive care. The two primary immune-modulating therapies are Intravenous Immunoglobulin (IVIg) and Plasma Exchange (Plasmapheresis). These treatments are equally effective and should be started within two weeks of symptom onset for maximum benefit.
IVIg involves administering concentrated antibodies sourced from healthy donors directly into the bloodstream. These antibodies neutralize the harmful autoantibodies and block the damaging effects of the immune response on the nerves. Plasma Exchange (PLEX) removes the patient’s blood, separates the plasma containing the damaging antibodies, and replaces it with a substitute solution or donor plasma before returning the blood cells to the body.
Supportive care is important, often requiring admission to an intensive care unit for close monitoring. This includes surveillance of respiratory function and blood pressure, as autonomic nervous system involvement can cause dangerous fluctuations. For those who develop respiratory failure, mechanical ventilation is provided until their strength begins to return.
The recovery phase is variable and can be lengthy, often starting weeks after the condition plateaus. Most patients experience a good recovery, with approximately 77% able to walk independently six months after symptom onset. However, full recovery can range from a few months to over a year, and a small percentage of individuals may be left with some residual weakness or sensory deficit.