What is an intrinsic property of the skeletal muscle group affected in a patient with myasthenia gravis (MG), characterized by generalized weakness, bilateral ptosis, and double vision, which improves with edrophonium (Tensilon) administration and ice pack placement?

From the Guidelines

The skeletal muscle group affected in this patient with myasthenia gravis is characterized by a small mass per motor unit. This patient presents with classic symptoms of myasthenia gravis, including fatigable weakness, ptosis, diplopia, and positive response to both ice pack testing and edrophonium (Tensilon test) 1. Myasthenia gravis affects the neuromuscular junction, particularly impacting muscles with small motor units such as the extraocular muscles, facial muscles, and bulbar muscles. These muscles have fewer muscle fibers per motor neuron compared to larger muscle groups, making them more susceptible to the effects of acetylcholine receptor antibodies 1. This small mass per motor unit allows for fine, precise movements but also makes these muscles particularly vulnerable to neuromuscular transmission disorders. The other options (high myoglobin content, high density of mitochondria, increased ATP generation, and high ATPase activity) are characteristics of muscles designed for sustained contractions or rapid, powerful movements, not the fine motor control muscles primarily affected in myasthenia gravis. Some key points to consider in the management of myasthenia gravis include:

• Recognition of the disease and prevention of morbidity and mortality from generalized myasthenia gravis 1
• Reduction of diplopia (if present)
• Restoration of normal ocular alignment, appearance, and improved self-image
• Restoration of binocular function
• Reduction of compensatory head position (if present) The most recent and highest quality study on this topic is from 2024, which highlights the importance of early recognition and treatment of myasthenia gravis to prevent morbidity and mortality 1.

From the Research

Clinical Presentation and Diagnosis

The patient's symptoms, including generalized weakness, drooping of eyelids, and double vision, are characteristic of myasthenia gravis (MG) 2, 3, 4. The improvement in symptoms after the administration of edrophonium and the use of ice packs over the eyes also support this diagnosis.

Pathophysiology of Myasthenia Gravis

Myasthenia gravis is an autoimmune disease caused by antibodies against the acetylcholine receptor (AChR) or other related proteins in the postsynaptic muscle membrane 3, 4. This leads to defective transmission at the neuromuscular junction and results in muscle weakness and fatigue.

Intrinsic Properties of Skeletal Muscle

The skeletal muscle group affected in MG has several intrinsic properties, including:

• A small mass per motor unit, which is a characteristic of extraocular and other muscles affected in MG
• High density of mitochondria, which is not directly related to the pathophysiology of MG
• Increased amount of ATP generated, which is not a characteristic of the skeletal muscle group affected in MG
• High ATPase activity, which is not directly related to the pathophysiology of MG
• High myoglobin content, which is not a characteristic of the skeletal muscle group affected in MG

Based on the available evidence, the correct answer is:

• A small mass per motor unit, as extraocular muscles have a small mass per motor unit and are often affected in MG 3, 4.