Myasthenia Gravis: Understanding Limb Weakness

Physical strength originates from muscle contractions. Strong contractions generate powerful movements, while weak contractions result in diminished strength. Under normal conditions, when the body receives an external stimulus, the signal travels through sensory nerves to the central nervous system. The sensory cortex processes this information and sends it to the motor cortex, which then transmits commands to the muscles, triggering contraction. A strong neural signal enhances muscle force, while a weaker signal results in reduced strength. However, nerves and muscles are distinct tissues and cannot directly communicate. Instead, they rely on neuromuscular junctions to relay signals. Any disruption along this neural pathway can lead to muscle weakness or even paralysis.

What Is Myasthenia Gravis?

Myasthenia Gravis (MG) is an autoimmune disorder mediated by antibodies against acetylcholine receptors. It involves cell-mediated immunity and complement activation, primarily affecting the postsynaptic membrane at the neuromuscular junction. This disruption impairs the transmission of nerve signals to muscles, leading to skeletal muscle weakness. As an autoimmune disease, MG causes the immune system to mistakenly attack the body's own neuromuscular connections, resulting in fatigue and muscle dysfunction.

Classification of Myasthenia Gravis in Adults

Adult-onset Myasthenia Gravis is clinically categorized into six types:

Ocular Type (15–20%): Limited to the extraocular muscles, this type presents with symptoms like ptosis (drooping eyelids) and diplopia (double vision).
Mild Generalized Type (30%): Affects eye, facial, and limb muscles but spares the respiratory muscles. Patients can maintain independence in daily activities without experiencing a myasthenic crisis.
Moderate Generalized Type (25%): Involves multiple limb muscle groups and may lead to difficulties in chewing, swallowing, and speech, although respiratory function remains unaffected and no crisis occurs.
Acute Fulminant Type (15%): Develops rapidly, often within weeks or months, progressing to respiratory failure and requiring intubation or tracheostomy. This type carries a high mortality rate.
Late-onset Severe Type (10%): Typically evolves from Type I, IIA, or IIB within about two years and is associated with a high risk of mortality.
Atrophic Form: Muscle atrophy may appear within six months of onset.

Why Do Myasthenia Gravis Patients Experience Weakness in Arms and Legs?

Myasthenia Gravis specifically affects the communication between nerves and muscles at the neuromuscular junction. As a result, muscle signals become weakened or even completely blocked, leading to varying degrees of muscle fatigue and weakness. When this signal failure becomes severe, patients may experience paralysis—known as a myasthenic crisis. Since the majority of MG types involve limb muscles, it is common for patients to report weakness in their hands and feet. This symptom significantly impacts mobility and daily functioning, making early diagnosis and treatment essential for improving quality of life.