Muscular dystrophy (MD) refers to a heterogeneous group of inherited disorders, characterized clinically by progressive degeneration of skeletal muscle fibers leading to muscle weakness and wasting of varying severity. Some forms of muscular dystrophy are seen in infancy or childhood while others may not appear until middle age or later. Most of the forms of MD are multi-system disorders with manifestations in body systems including the heart, gastrointestinal and nervous systems, endocrine glands, skin, eyes and other organs including brain.
There are 9 major forms of Muscular Dystrophies:
Duchenne Muscular Dystrophy (DMD):
It is the most common and severe form of muscular dystrophy in children, affecting males only. It is a X-linked hereditary disease caused by absence of a structural protein termed dystrophin.
Pathogenesis & Genetics:
- X-linked inheritance (this disorder affects primarily males, and females who are carriers have milder symptoms)
- X chromosome (Xp21) contains a gene that encodes Dystrophin (427-kDa)
- Dystrophin is part of a complex structure involving several other protein components. The “dystrophin-glycoprotein complex” helps anchor the structural skeleton within the muscle cells, through the outer membrane of each cell, to the tissue framework that surrounds each cell.
- Mutation (mostly deletion) results in a virtual absemce of dystrophin protein causing impaired contractile activity of the muscle fibers.
- Normal at birth with onset of symptoms by age 5
- Progressive muscular weakness
- Usually wheelchair-bound by age 10-12
- Weakness begins in the pelvic girdle muscles and then extends to the shoulder girlde
- Difficulty in rising and climbing
- Calf pseudo-hypertrophy (muscle atrophy due to increase in fat and connective tissue)
- May develop heart failures and arrhythmias
- Mental retardation may occur
- Respiratory insufficiency and pulmonary infections
- Death results from respiratory failure and cardiac decompensation by the 2nd or 3rd decade
– Elevated serum creatine kinase level
– Histologic abnormalities (Morphology):
- variation in size of muscle fiber
- increased numbers of internalized nuclei
- degeneration, necrosis, phagocytosis of muscle fibers followed by regeneration
- fatty infiltration (replacement of muscles by fat)
– Muscle biopsy:
- Immunostain shows decreased dystrophin protein
- DNA analysis by PCR
Becker’s Muscular Dystrophy (BMD):
It is similar to DMD (regarded as milder form of DMD) in most of the aspects except:
- It is less common and not as severe as DMD.
- Mutation produces an altered dystrophin protein (truncated and partially functional).
- Symptoms appear later (usually appears between the ages of 2 and 16 but can appear as late as age 25) and progresses slowly.
- Spine deformity and restrictive lung disease are rare, while cardiac involvement is common.
- Survival is usually into old age.
Myotonic Musuclar Dystrophy (MMD):
- The most common form of muscular dystrophy in adults, myotonic muscular dystrophy affects both men and women, and it usually appears any time from early childhood to adulthood. In rare cases, it appears in newborns (congenital MMD).
- Myotonia the sustained involuntary contraction of a group of muscles, is the cardinal neuromuscular symptom in this disease. Patients often complain of stifness and have difficulty in releasing the grip.
Pathogenesis and Genetics:
- Autosomal dominant inheritance
- Expansion of a short repeat in the DNA sequence (CTG in one gene or CCTG in another gene)
- This expansion affects the mRNA for the dystrophila myotonia-protein kinase (DMPK) affecting eventual level of protein product.
- variation in fiber size
- striking increase in the number of internal nuclei
- ring fiber with a subsarcolemmal band of cytoplasm that appears distinct from the center of the fiber
- irregular mass of sarcoplasm extending outward from the ring
- age at onset: 20 to 40 years
- atrophy of face muscles and ptosis ensue leading to typical facial appearance
- frontal balding, gonadal atrophy, cardiomyopathy, smooth muscle involvement
- decreased IgG and an abnormal glucose tolerance test response
- dementia in some cases
- decreased life expectancy
SHORT DESCRIPTON OF REMAINING TYPES:
Limb-girdle muscular dystrophy (LGMD):
- LGMDs all show a similar distribution of muscle weakness, affecting both upper arms and legs.
- The recessive LGMDs are more frequent than the dominant forms, and usually have childhood or teenage onset.
- The dominant LGMDs usually show adult onset.
- Some of the recessive forms have been associated with defects in proteins that make up the dystrophin-glycoprotein complex.
- Within 20 years, walking becomes difficult or impossible.
- Sufferers typically live to mid to late adulthood.
- Death from LGMD is usually due to cardiopulmonary complications.
Facioscapulohumeral muscular dystrophy (FSHD):
- Initially affects the muscles of the face, shoulders, and upper arms with progressive weakness.
- Symptoms usually develop in the teenage years.
- The pattern of inheritance is autosomal dominant, but the underlying genetic defect is poorly understood.
- Most cases are associated with a deletion near the end of chromosome 4.
- Walking, chewing, swallowing, and speaking problems can occur.
- About 50% of sufferers can walk throughout their lives. Sufferers usually live a normal life span.
- Congenital muscular dystrophies progress slowly and affect males and females.
- The two forms that have been identified : a)Fukuyama and b) congenital muscular dystrophy with myosin deficiency.
- Fukuyama congenital muscular dystrophy causes abnormalities in the brain and often seizures.
- Later cause muscle weakness at birth or in the first few months of life, along with severe and early contractures (shortening or shrinking of muscles that causes joint problems).
- This form of muscular dystrophy appears in men and women in their 40s, 50s, and 60s.
- It progresses slowly, causing weakness in the eye and face muscles, which may lead to difficulty swallowing.
- Weakness in pelvic and shoulder muscles may occur later.
- Choking and recurrent pneumonia may occur.
- This group of rare diseases affects adult men and women.
- It causes weakness and wasting of the distal muscles (those farthest from the center) of the forearms, hands, lower legs, and feet.
- It is generally less severe, progresses more slowly, and affects fewer muscles than other forms of muscular dystrophy.
- This rare form of muscular dystrophy appears from childhood to the early teens and affects only males.
- It causes muscle weakness and wasting in the shoulders, upper arms, and lower legs.
- Life-threatening heart problems are common and can also affect carriers (those who have the genetic information for the disease but do not develop the full-blown version including mothers and sisters of sufferers).
- Muscle shortening (contractures) occurs early in the disease.
- Weakness can spread to chest and pelvic muscles.
- The disease progresses slowly and causes less severe muscle weakness than some other forms of muscular dystrophy.