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Muscular dystrophy refers to a group of genetic, hereditary muscle diseases that cause progressive muscle weakness. Muscular dystrophies are characterized by progressive skeletal muscle weakness, defects in muscle proteins, and the death of muscle cells and tissue. Nine diseases including Duchenne, Becker, limb girdle, congenital, facioscapulohumeral, myotonic, oculopharyngeal, distal, and Emery-Dreifuss are always classified as muscular dystrophy but there are more than 100 diseases in total with similarities to muscular dystrophy. Most types 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.
In the 1860s, descriptions of boys who grew progressively weaker, lost the ability to walk, and died at an early age became more prominent in medical journals. In the following decade, French neurologist Guillaume Duchenne gave a comprehensive account of thirteen boys with the most common and severe form of the disease, which now carries his name—Duchenne muscular dystrophy.
It soon became evident that the disease had more than one form.
|Becker's muscular dystrophy||300376||DMD|
|Congenital muscular dystrophy||Multiple||Multiple||
|Duchenne muscular dystrophy||310200||DMD||
|Distal muscular dystrophy||254130||DYSF||
|Emery-Dreifuss muscular dystrophy||310300, 181350||EMD, LMNA||
|Facioscapulohumeral muscular dystrophy||158900||DUX4||
|Limb-girdle muscular dystrophy||Multiple||Multiple||
|Myotonic muscular dystrophy||160900, 602668||DMPK, ZNF9||
|Oculopharyngeal muscular dystrophy||164300||PABPN1|
These conditions are inherited, and the different muscular dystrophies follow various inheritance patterns
The best-known type, Duchenne muscular dystrophy (DMD), is inherited in an X-linked recessive pattern, meaning that the mutated gene that causes the disorder is located on the X chromosome, one of the two sex chromosomes, and is thus considered sex-linked. In males (who have only one X chromosome) one altered copy of the gene in each cell is sufficient to cause the condition. In females (who have two X chromosomes) a mutation must generally be present in both copies of the gene to cause the disorder (relatively rare exceptions, manifesting carriers, do occur due to dosage compensation/X-inactivation). Males are therefore affected by X-linked recessive disorders much more often than females. A characteristic of X-linked inheritance is that fathers cannot pass X-linked traits to their sons. In about two thirds of DMD cases, an affected male inherits the mutation from a mother who carries one altered copy of the DMD gene. The other one third of cases probably result from new mutations in the gene. Females who carry one copy of a DMD mutation may have some signs and symptoms related to the condition (such as muscle weakness and cramping), but these are typically milder than the signs and symptoms seen in affected males. Duchenne muscular dystrophy and Becker's muscular dystrophy are caused by mutations of the gene for the dystrophin protein and lead to an overabundance of the enzyme creatine kinase. The dystrophin gene is the second largest gene in mammals.
|Diseases||History and Physical||Diagnostic tests||Other Findings|
|Motor Deficit||Sensory deficit||Cranial nerve Involvement||Autonomic dysfunction||Proximal/Distal/Generalized||Ascending/Descending/Systemic||Unilateral (UL)
or Bilateral (BL)
No Lateralization (NL)
|Onset||Lab or Imaging Findings||Specific test|
|Adult Botulism||+||-||+||+||Generalized||Descending||BL||Sudden||Toxin test||Blood, Wound, or Stool culture||Diplopia, Hyporeflexia, Hypotonia, possible respiratory paralysis|
|Infant Botulism||+||-||+||+||Generalized||Descending||BL||Sudden||Toxin test||Blood, Wound, or Stool culture||Flaccid paralysis (Floppy baby syndrome), possible respiratory paralysis|
|Guillian-Barre syndrome||+||-||-||-||Generalized||Ascending||BL||Insidious||CSF: ↑Protein
|Clinical & Lumbar Puncture||Progressive ascending paralysis following infection, possible respiratory paralysis|
|Eaton Lambert syndrome||+||-||+||+||Generalized||Systemic||BL||Intermittent||EMG, repetitive nerve stimulation test (RNS)||Voltage gated calcium channel (VGCC) antibody||Diplopia, ptosis, improves with movement (as the day progresses)|
|Myasthenia gravis||+||-||+||+||Generalized||Systemic||BL||Intermittent||EMG, Edrophonium test||Ach receptor antibody||Diplopia, ptosis, worsening with movement (as the day progresses)|
|Electrolyte disturbance||+||+||-||-||Generalized||Systemic||BL||Insidious||Electrolyte panel||↓Ca++, ↓Mg++, ↓K+||Possible arrhythmia|
|Organophosphate toxicity||+||+||-||+||Generalized||Ascending||BL||Sudden||Clinical diagnosis: physical exam & history||Clinical suspicion confirmed with RBC AchE activity||History of exposure to insecticide or living in farming environment. with : Diarrhea, Urination, Miosis, Bradycardia, Lacrimation, Emesis, Salivation, Sweating|
|Tick paralysis (Dermacentor tick)||+||-||-||-||Generalized||Ascending||BL||Insidious||Clinical diagnosis: physical exam & history||-||History of outdoor activity in Northeastern United States. The tick is often still latched to the patient at presentation (often in head and neck area)|
|Tetrodotoxin poisoning||+||-||+||+||Generalized||Systemic||BL||Sudden||Clinical diagnosis: physical exam & dietary history||-||History of consumption of puffer fish species.|
|Stroke||+/-||+/-||+/-||+/-||Generalized||Systemic||UL||Sudden||MRI +ve for ischemia or hemorrhage||MRI||Sudden unilateral motor and sensory deficit in a patient with a history of atherosclerotic risk factors (diabetes, hypertension, smoking) or atrial fibrillation.|
|Poliomyelitis||+||+||+||+/-||Proximal > Distal||Systemic||BL or UL||Sudden||PCR of CSF||Asymmetric paralysis following a flu-like syndrome.|
|Transverse myelitis||+||+||+||+||Proximal > Distal||Systemic||BL or UL||Sudden||MRI & Lumbar puncture||MRI||History of chronic viral or autoimmune disease (e.g. HIV)|
|Neurosyphilis||+||+||-||+/-||Generalized||Systemic||BL||Insidious||MRI & Lumbar puncture||CSF VDRL-specifc||History of unprotected sex or multiple sexual partners.|
|Muscular dystrophy||+||-||-||-||Proximal > Distal||Systemic||BL||Insidious||Genetic testing||Muscle biopsy||Progressive proximal lower limb weakness with calf pseudohypertrophy in early childhood. Gower sign positive.|
|Multiple sclerosis exacerbation||+||+||+||+||Generalized||Systemic||NL||Sudden||↑CSF IgG levels
|Clinical assessment and MRI ||Blurry vision, urinary incontinence, fatigue|
|Amyotrophic lateral sclerosis||+||-||-||-||Generalized||Systemic||BL||Insidious||Normal LP (to rule out DDx)||MRI & LP||Patient initially presents with upper motor neuron deficit (spasticity) followed by lower motor neuron deficit (flaccidity).|
|Inflammatory myopathy||+||-||-||-||Proximal > Distal||Systemic||UL or BL||Insidious||Elevated CK & Aldolase||Muscle biopsy||Progressive proximal muscle weakness in 3rd to 5th decade of life. With or without skin manifestations.|
Natural History, Complications and Prognosis
- The severity of disability depends on the type of muscular dystrophy. All types of muscular dystrophy slowly get worse, but how fast this happens varies widely.
- Some types of muscular dystrophy, such as Duchenne muscular dystrophy, are deadly. Other types cause little disability and people with them have a normal lifespan.
- Cardiomyopathy with heart failure
- Decreased ability to care for self
- Decreased movement
- Respiratory failure
- Mental impairment (varies)
Principal symptoms include:
- Mental retardation (only present in some types of the condition)
- Muscle weakness that slowly gets worse
Arrythmia may be present.
- Loss of muscle mass (wasting)
- Joint contractures (club foot, claw hand and others)
- Calf pseudohypertrophy
Early in the disease process, creatine phosphokinase (CPK) levels are 50-300 times greater than normal levels, but the levels tend to decrease as the muscle mass decreases.
May show right ventricular strain pattern.
Myopathic disease has these defining EMG characteristics:
- A decrease in duration of the action potential
- A reduction in the area to amplitude ratio of the action potential
- A decrease in the number of motor units in the muscle (in extremely severe cases only)
- The diagnosis of muscular dystrophy is based on the results of a muscle biopsy. In some cases, a DNA blood test may be all that is needed.
- The optimal site for biopsy is the vastus lateralis muscle.
- There is no known cure for muscular dystrophy. Inactivity (such as bed-rest and even sitting for long periods) can worsen the disease.
- Physical therapy and orthopedic instruments (e.g., wheelchairs, standing frames) may be helpful.
- Physical therapy to prevent contractures (a condition when an individual with a muscular dystrophy grows and the muscles don't move with the bones and can easily be slowed down and/or make the individual's body straighter by daily physical therapy), orthoses (orthopedic appliances used for support) and corrective orthopedic surgery may be needed to improve the quality of life in some cases.
- The cardiac problems that occur with Emery-Dreifuss muscular dystrophy and myotonic muscular dystrophy may require a pacemaker.
- The myotonia (delayed relaxation of a muscle after a strong contraction) occurring in myotonic muscular dystrophy may be treated with medications such as quinine, phenytoin, or mexiletine.
A grid computing-based research project called "Help Cure Muscular Dystrophy" was launched on December 19, 2006 by Décrypthon (a collaboration between French Muscular Dystrophy Association, French National Center for Scientific Research and IBM).
MYO-029 is an experimental myostatin inhibiting drug developed by Wyeth Pharmaceuticals for the treatment of muscular dystrophy. Myostatin is a protein that inhibits the growth of muscle tissue, MYO-029 is a recombinant human antibody designed to bind and inhibit the activity of myostatin. A 2005/2006 trial was completed by Wyeth in Collegeville, PA. As of April 2007, the results of the study have not yet been made public, but it is one of the few known drugs in development for the treatment for muscular dystrophy.
National research and support in the United States
Within the United States, the three primary federally funded organizations that focus on Muscular Dystrophy include the National Institute of Neurological Disorders and Stroke (NINDS), National Institute of Arthritis and Musculoskeletal and Skin Diseases (NIAMS), and National Institute of Child Health and Human Development (NICHD).
On December 18, 2001 the MD CARE Act was signed into law and amends the Public Health Service Act to provide research for the various muscular dystrophies. This law also established the Muscular Dystrophy Coordinating Committee to help focus research efforts through a coherent research strategy.
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