206 Diseases, Disorders, and Injuries of the Muscular System

Muscular Dystrophy

Muscular dystrophy (MD) is a group of muscle diseases characterized by non-functional muscle proteins that impair proper function.

Muscular dystrophy (MD) is a group of muscle diseases characterized by the creation of non-functional muscle proteins that weakens muscles and impairs proper function. Specifically the proteins dystrophin and the dystrophin protein complexes are altered and nonfunctional, unable to join with actin filaments. This leads to disruptions of mechanical stabilization and calcium concentration regulation within the sarcomeres, altering the ability of filaments to bind and cause contraction. Progression of the disease includes death of muscle tissue.

Muscular Dystrophy: In affected muscle the tissue becomes disorganized and the concentration of dystrophin (green) is greatly reduced.

Types of Muscular Dystrophy

Several types of MD exist, including Duchenne MD, the first description of a MD disease, noted by Guillame Duchenne, a French neurologist, after study of 13 young boys presenting symptoms. Other forms include Becker, limb-girdle, congenital, facioscapulohumeral, myotonic, oculopharyngeal, distal, and Emery-Dreifuss MD. Most of these diseases affect multiple organ systems including the cardiovascular, gastrointestinal, nervous, and endocrine systems. Although not typically affecting intellectual abilities, a subset of MD patients do exhibit cognitive impairment, behavioral, vision, and speech problems likely linked to MD, and this is seen particularly in DMD. MD-affected individuals with susceptible intellectual impairment are diagnosed through molecular characteristics but not through problems associated with disability.

Causes of Muscular Dystrophy

Muscular dystrophy is generally an inherited disease, with men affected at much higher rates than women. Women are often carriers of the disease gene variant, but not affected themselves, although other patterns of inheritance exist for the different types of MD. However, mutations of the dystrophin gene and nutritional defects (with no genetics history) at the prenatal stage are also possible in about 33% of people affected by DMD.

Exercise-Induced Muscle Damage

Exercise damages muscles due to eccentric and concentric muscle loading and often results in delayed onset muscle soreness (DOMS).

Exercise damages muscles due to eccentric and concentric muscle loading. Resistance training, and particularly high loading during eccentric contractions, results in delayed onset muscle soreness (DOMS). Previously attributed to the accumulation of lactic acid during exercise, it is now understood that DOMS is due to structural damage in sarcomeres, particularly to the z-disks and contractile filaments. Z-disks provide the structural support for the contractile filaments of the sliding filament mechanism. Overloading of muscles damages these connections and the orientation of the contractile filaments.

The Sarcomere: Delayed onset muscle soreness is caused by structural damage to the Z disk and myosin and actin filaments.

Causes of Exercise-Induced Muscle Damage

Acute inflammation of the muscle cells, as understood in exercise physiology, can result after induced eccentric and concentric muscle training. Participation in eccentric training and conditioning, including resistance training and activities that emphasize eccentric lengthening of the muscle such as downhill running on a moderate to high incline, can result in considerable soreness within 24 to 48 hours, even though blood lactate levels, previously thought to cause muscle soreness, were much higher with level running. This has been noted especially in marathon runners whose muscle fibers revealed remarkable damage after both training and marathon competition. The onset and timing of this gradient damage to the muscle parallels the degree of muscle soreness experienced by the runners.

This disruption of the muscle fibers triggers white blood cells to increase following the induced muscle soreness, leading to the inflammatory response observation from the induced muscle soreness. Elevations in plasma enzymes, myoglobinemia, and abnormal muscle histology and ultrastructure are concluded to be associated with the inflammatory response. High tension in the contractile-elastic system of muscle results in structural damage to the muscle fiber and plasmalemma and its epimysium, perimysium, and endomysium. The mysium damage disrupts calcium homeostasis in the injured fiber and fiber bundles, resulting in necrosis that peaks about 48 hours after exercise. The products of the macrophage activity and intracellular contents (such as histamines, kinins, and K+) accumulate outside the cells. These substances then stimulate the free nerve endings in the muscle; a process that appears accentuated by eccentric exercise, in which large forces are distributed over a relatively small cross-sectional area of the muscle.

The acute, and delayed nature of muscle soreness is indicative of an inflammatory immune response. Damage to the sarcomeres causes aninflux of white blood cells, leading to inflammation, which is itself associated with increased plasma enzyme concentration, myoglobinemia, and abnormal muscle structure and histology. A further response to sarcomere damage is necrosis following damage to the mysium, which peaks about 48 hours following exercise.

The muscle adapts rapidly to the structural damage caused by exercise, and further soreness and damage in later exercise events is mitigated.

Carpal Tunnel Syndrome

Carpal tunnel syndrome is an entrapment median neuropathy of the median nerve due to its compression at the wrist in the carpal tunnel.

Carpal tunnel syndrome (CTS) is an entrapment median neuropathy, causing paresthesia, pain, numbness, and other symptoms in the distribution of the median nerve due to its compression at the wrist in the carpal tunnel. The pathophysiology is not completely understood but can be attributed to the compression of the median nerve traveling through the carpal tunnel.

Untreated Carpal Tunnel Syndrome: Patients with CTS experience numbness, tingling, or burning sensations in the thumb and fingers, in particular the index, middle fingers, and radial half of the ring fingers, which are innervated by the median nerve. Less-specific symptoms may include pain in the wrists or hands and loss of grip strength.

Symptoms of Carpal Tunnel Syndrome

Patients with CTS experience numbness, tingling, or burning sensations in the thumb and fingers, in particular the index, middle fingers, and radial half of the ring fingers, which are innervated by the median nerve. Less-specific symptoms may include pain in the wrists or hands and loss of grip strength. The numbness often occurs at night, with the hypothesis that the wrists are held flexed during sleep. It can be relieved by wearing a wrist splint that prevents flexion. Long-standing CTS leads to permanent nerve damage with constant numbness, atrophy of some of the muscles of the thenar eminence, and weakness of palmar abduction. Pain in carpal tunnel syndrome is primarily numbness that is so intense that it wakes one from sleep. Pain in electrophysiologically verified CTS is associated with misinterpretation of nociception and depression. Palliative treatments for CTS include use of night splints and corticosteroid injection. The only scientifically established disease modifying treatment is surgery to cut the transverse carpal ligament.

Causes of Carpal Tunnel Syndrome

Most cases of CTS are of unknown causes, or idiopathic. Carpal tunnel syndrome can be associated with any condition that causes pressure on the median nerve at the wrist. A variety of patient factors can lead to CTS, including heredity, size of the carpal tunnel, associated local and systematic diseases, and certain habits. Some common conditions that can lead to CTS include obesity, oral contraceptives, hypothyroidism, arthritis, diabetes, and trauma. Carpal tunnel syndrome is also associated with repetitive activities of the hand and wrist, in particular with a combination of forceful and repetitive activities. Non-traumatic causes generally happen over a period of time, and are not triggered by one certain event. Many of these factors are manifestations of physiologic aging.

Treatments of Carpal Tunnel Syndrome

Treatments of CTS include immobilization of the wrist, steroid injection, activity modification, physical therapy, massage, medication, or surgery to release the transverse carpal ligament.