Physiologic Myopathy Due to Aging
Introduction
Aging is associated with a gradual decline in muscle mass, strength, and function, a condition termed sarcopenia. While sarcopenia may be considered part of the normal aging process, it represents a physiologic form of myopathy—characterized by reduced muscle performance not caused by a primary neuromuscular disease but by complex age-related biological changes. This age-related myopathy can lead to decreased mobility, increased risk of falls, and loss of independence in elderly individuals. Differentiating physiologic myopathy from pathological myopathies is important for diagnosis and management.
Physiology/Pathophysiology
- Loss of muscle mass (atrophy): There is a progressive decline in the number and size of muscle fibers, particularly type II (fast-twitch) fibers, which are responsible for power and rapid movements. This leads to reduced muscle strength and endurance.
- Motor neuron loss: Aging is associated with a loss of alpha motor neurons in the spinal cord, leading to muscle fiber denervation. Although reinnervation by surviving neurons may occur, it is often incomplete or inefficient.
- Mitochondrial dysfunction: Mitochondria in aging muscle cells show decreased oxidative capacity, increased production of reactive oxygen species (ROS), and impaired energy production, contributing to muscle fatigue.
- Altered protein homeostasis: There is an imbalance between protein synthesis and degradation, with a relative increase in proteolysis. Anabolic resistance (reduced response to protein intake or exercise) further contributes to muscle wasting.
- Hormonal changes: Declines in anabolic hormones, such as testosterone, growth hormone, and insulin-like growth factor-1 (IGF-1), contribute to muscle catabolism.
- Increased inflammation: Chronic low-grade systemic inflammation (“inflammaging”) with elevated levels of cytokines like IL-6 and TNF-α promotes muscle protein breakdown and impairs regeneration.
Manifestations
Physiologic myopathy due to aging develops gradually and presents with several characteristic features. Muscle weakness is common, typically symmetric and proximal, affecting large muscle groups such as the thighs and shoulders. Visible muscle atrophy may be observed, particularly in the limbs, often accompanied by reduced endurance and increased fatigue during physical activity. Impaired balance and coordination resulting from muscle weakness can increase the risk of falls and fractures. Functional decline is frequently observed, with difficulty performing everyday tasks like climbing stairs or rising from a chair.
In some individuals, sarcopenic obesity may occur, where muscle loss is masked by increased fat mass, and while not a disease itself, aging-related myopathy significantly contributes to frailty and may resemble or coexist with other neuromuscular disorders.
