Acute motor axonal neuropathy

Overview

Acute motor axonal neuropathy (AMAN) is a rare and severe subtype of Guillain–Barré syndrome (GBS), a group of acute autoimmune neuropathies that affect the peripheral nervous system. Unlike the classic form of GBS (acute inflammatory demyelinating polyradiculoneuropathy, or AIDP), AMAN specifically targets motor nerves and causes rapid-onset weakness without sensory loss. It is most commonly seen in children and young adults and is especially prevalent in parts of Asia, Central and South America, and China.

Causes

AMAN is caused by an autoimmune response in which the body's immune system mistakenly attacks the axons of motor nerves. This autoimmune attack is often triggered by a preceding infection, particularly:

• Campylobacter jejuni (a common cause of bacterial gastroenteritis)

• Other viral or bacterial infections

The immune response may generate antibodies that cross-react with components of the motor nerve axon membrane—especially gangliosides like GM1 and GD1a—leading to inflammation, damage, and loss of nerve conduction.

Symptoms

The hallmark symptom of AMAN is rapidly progressive weakness, which often begins in the legs and ascends to involve the arms and respiratory muscles. Key features include:

• Acute flaccid paralysis

• Absent or reduced deep tendon reflexes

• Symmetric limb weakness

• Respiratory failure (in severe cases)

• No significant sensory deficits (unlike other GBS variants)

Some patients may also experience facial or bulbar weakness, but sensory function (touch, pain, temperature) typically remains intact.

Diagnosis

Diagnosing AMAN involves a combination of clinical evaluation, laboratory studies, and neurophysiological tests. Key steps include:

• Clinical examination: Rapid-onset, symmetric motor weakness without sensory loss

• Nerve conduction studies (NCS): Show reduced or absent motor nerve action potentials with preserved sensory conduction, consistent with axonal degeneration

• Electromyography (EMG): Supports axonal motor nerve involvement

• Cerebrospinal fluid (CSF) analysis: May show elevated protein with normal cell count (albuminocytologic dissociation), though this may be absent early in the illness

• Serology: Antibodies to GM1 or GD1a gangliosides may be present

• History of recent infection: Especially gastrointestinal illness due to Campylobacter jejuni

Treatment

Treatment for AMAN is aimed at halting the immune attack and supporting nerve recovery. The mainstays of treatment include:

• Intravenous immunoglobulin (IVIG): A standard therapy that modulates the immune system and reduces nerve damage

• Plasmapheresis (plasma exchange): An alternative to IVIG that removes pathogenic antibodies from the blood

• Supportive care: Including respiratory support, nutrition, and physical therapy

• Monitoring for complications: Such as autonomic dysfunction, infections, or deep vein thrombosis

Early initiation of IVIG or plasmapheresis is crucial for improving outcomes and preventing progression to respiratory failure.

Prognosis

The prognosis of AMAN varies. Many patients experience significant recovery over weeks to months, especially with early treatment. However, because AMAN involves direct axonal injury (as opposed to demyelination), recovery can be slower and more incomplete compared to other GBS variants.

Key prognostic factors include:

• Severity of initial weakness

• Time to treatment initiation

• Requirement for mechanical ventilation

While some individuals recover fully, others may have lasting weakness or require long-term rehabilitation. Mortality is low in modern intensive care settings but can occur in severe or untreated cases.