Thursday, April 24, 2008

Skeletal and Smooth Muscle Tumors

Rhabdomyomas are rare benign tumors of skeletal muscle usually found in the area of the head and neck.

Rhabdomyosarcomas are malignant tumors of skeletal muscle. These tumors are typically found on the arms and legs. Over 85% of rhabdomyosarcomas occur in infants, children, and teenagers.

Leiomyomas are benign tumors of smooth muscle. These tumors are uncommon.

Leiomyosarcomas are malignant tumors of smooth muscle tissue. These are rare tumors in the USA.

Myasthenia gravis

Myasthenia gravis is an autoimmune neuromuscular disease caused by defective nerve transmission to the muscles. It is first characterized by drooping eyelids and slack facial muscles. Autoantibodies block the attachment of acetycholine to acetycholine receptors. This interferes with transmission of the nerve impulse.

You can find a self-paced tutorial with questions on myasthenia gravis at this NIH website.

Disease of Muscle - Myositis and Myopathy

Myositis is inflammation of muscle tissue due to injury, infection or autoimmune disease.

Myositis can be caused by infection with the parasitic nematode Trichinella spiralis. You become infected with this worm by eating poorly cooked, infected pork. The larvae produced by mating male and female worms invade muscle cells. The disease is characterized by abdominal pain, cramping, muscle pain and diarrhea.

Other infectious agents, including influenza virus, HIV, Lyme borrelia, and tapeworm can cause myositis.

Dermatomyositis is an autoimmune connective tissue disease that is characterized by inflammation of muscle and skin. This disease affects women more than men and usually occurs in people 40-60 yrs old.

Polymyositis is an ideopathic inflammatory disease that does not involve the skin. The cause is unknown.

Myopathy is a term used to describe neuromuscular diseases that are characterized by muscle weakness but are not muscular dystrophy. These diseases can be due to abnormalities in mitochondria or glycogen storage, a problem in metabolizing myoglobin or due to toxic effects of hormones or chemicals. One example is the muscle weakness or necrosis that can occur in some individuals taking cholesterol-lowering statin drugs.

Rhabdomyolysis is the sudden necrosis of skeletal muscle with the release of large amounts of myoglobin into the circulation. Myoglobin can be toxic to the kidneys and can result in their shutdown. This disease can result from any injury to the muscles, especially trauma.

Disease of Muscle - Muscular Dystrophy

For muscle to remain healthy it must be innervated by a peripheral nerve. Denervation or loss of nerve stimulation results in atrophy of the muscle or loss of muscle tissue. Muscles can also atrophy due to decreased activity. This type of atrophy is reversable with exercise.

Muscular dystrophies are a group of diseases associated with increasing muscle weakness and loss of muscle tissue. The most common form of muscular dystophy is Duchenne muscular dystophy. This disease is inherited in an x-linked recessive pattern and is caused by a defect in the gene that encodes dystrophin. This results in an absence of dystrophin.

Another formof muscular dystrophy is Becker muscular dystrophy. In this form of the disease, dystrophin is present but abnormal. This disease is also x-linked, so both forms of muscular dystrophy occur most often in males.

You can learn more about muscular dystrophy at this NIH website and at the Muscular Dystrophy Association website.

Muscle Diseases con't

Myasthenia gravis is a chronic neuromuscular disease characterized by muscle weakness. An NIH tutorial on myasthenia gravis can be found here.

Muscle tumors

Tumors of skeletal muscle are rare. Rhabdomyosarcoma is a fast growing malignant tumor found primarily in infants and children.

Rhabdomyoma is a benign tumor of striated muscle.

Leiomyoma and Leiomyosarcoma are benign and malignant tumors of smooth muscle. Leiomyomas are found in the uterus where they may be called "fibroids".

Muscular Diseases

Muscular dystrophies. These are a group of progressive genetic noninflammatory diseases characterized by muscle weakness and degeneration.

Duchenne muscular dystrophy is the most common type of MD. It is an x-linked inherited disease caused by a defect in the dystrophin gene which results in an absence of dystrophin.

Becker muscular dystrophy is a milder form of Duchenne MD. It is also due to a genetic defect in the dystrophin gene but the protein is still produced in an abnormal form.

Myositis is inflammation of the muscles.

Myositis can be a result of autoimmune reactions as in Dermatomyositis and Polymyositis.

Trichinella spiralis larva in a muscle cell

Myositis can also be caused by infection. Trichinosis is a parasitic worm infection of the muscles. It is commonly caused by eating undercooked pork.

Infection with anaerobic bacteria can also result in myositis.

Gangrene results when a wound becomes infected with the bacterium Clostridium perfringens.

Tetanus result from infection with the bacterium Clostridium tetani.

Myopathy is a term used to describe noninflammatory muscle diseases that are not muscular dystrophy.

Rhabdomyolysis is the breakdown of muscle fibers resulting in the release of myoglobin in to the circulation resulting in kidney damage.

Muscle Anatomy

The motor unit is the fundamental unit of the neuromusclular system.

It consists of:
  1. Lower motor neuron which extends from the brain stem or spinal cord via a long axon to the skeletal muscle
  2. The neuromuscular junction or motor end plate
  3. A skeletal muscle cell

Where the motor end plate and muscle join is the neuromuscular junction - a synapse. Signals transmitted down the axon to the motor end plate cause the release of acetylcholine (a neurotransmitter) into the synaptic space. Acetylcholine interacts with the muscle cell causing contraction of muscle fibers. The acetylcholine must be immediately broken down by the enzyme cholinesterase before another signal can be transmitted.

You can find an excellent review of the physiology of muscle contraction here.

See an animation of the actin myosin crossbridge in action here.

Muscle fibers are either type 1 or type 2 (A and B). Go here to read a good review on muscle fiber types and training.

Tumors of Melanocytes

Melanocytes are the cells that produce the pigment melanin. These cells are derived from the embryonic nervous system.

Nevi or moles are benign tumors of melanocytes. They are common and usually appear before age 20.

Dysplastic nevi are atypical moles. They are usually larger than common moles. Many pictures of moles here.

All nevi are important because they must be distinguished from malignant melanoma.

Malignant melanoma is the most dangerous type of skin cancer. They most commonly occure on sun-exposed skin surfaces. Half of melanomas arise spontaneously and half arise from pre-existing moles.

Tumors of the Dermis

There are many tumors that occur in subepidermal tissues. Most of them are benign tumors.

Lipomas yellow, moveable subcutaneous masses that are not premalignant.

Dermatofibroma are benign fibrous tumors due to growth of dendritic cells.

Keloids are nodular masses of exaggerated scar tissue. They do not become malignant.

Hemangiomas are abnomal overgrowths of blood vessels.

Telangiectasis is a local collection of blood vessels that are often called spider angiomas. Often associated with hormonal imbalances.

Nevus flammeus (port wine stain) is a common congenital condition composed of a colleciton of dilated dermal blood vessels.

Neurofibroma is a benign tumor of peripheral nerves. See Von Recklinghausen neurofibromatosis.

Kaposi sarcoma is a malignant tumor of vascular endothelial cells caused by a type of Herpes virus.

Tumors of the Epidermis

For more information on common types of skin cancer go here.

Epidermoid cysts are not true neoplasms. They form from occluded hair follicles.

Warts (verruca vulgaris)are common papillary growths of epidermis cause by papilloma virus infection.

Seborrheic keratosis is a non-cancerous growth of the epidermis. They are not premalignant.

Actinic keratosis is an early stage of skin cancer. They can be considered squamous carcinoma in situ.

Keratoacanthoma a common and generally considered benign tumor of sun-exposed skin.

Basal cell carcinoma is the most common skin cancer. It arises from basal cells and is associated with exposure to sunlight. It does not usually metastasize but can destroy surrounding skin.

Squamous cell carcinoma is a malignant tumor of epidermal cells (keratinocytes). Photos and more information here.

Wednesday, April 23, 2008


Acne is an inflammatory disorder of hair follicles and associated sebaceous glands.

Acne vulgaris is a common skin condition seen in teenagers. More here.

Postadolescent acne is usually seen more in women than men. It's related to hormonal fluctuations.

Rosacea has been covered in a previous post several weeks ago.

Diseases of the Dermis - Contact Dermatitis

Allergic contact dermatitis is a delayed type hypersensitivity (type 4). Also here. Click on links for photos and more information.

Allergic contact dermatitis is often called eczema. This is a descriptive word that is applied to a wide variety of skin diseases. The most common type of eczema is atopic dermatitis. It is an allergic condition that makes your skin dry and itchy. It is most common in babies and children.

Irritant contact dermatitis is caused by direct contact of certain chemicals with skin. No allergy is involved. Examples would be diaper rash or "dishpan hands".

Diseases of the Dermis - Noncontact Dermatitis

Scleroderma is a chronic, systemic, autoimmune disease of the connective tissue characterized by severe fibrosis and tightening of skin.

Photoaging is a result of too much exposure to the sun resulting in breakdown of collagen.

Stasis dermatitis is a skin condition that accompanies severe varicose veins.

Urticaria and Angiodema (see here)

Diseases of the Epidermis

Skin color is determined by the amount of melanin produced by melanocytes and deposited in basal cells.

There are several skin disorders of pigmentation. A good source of information on skin pigmentation disorders can be found here at Medline Plus (scroll down to specific disorders).

Here are some other specific disorders of pigmentation.


Freckles and moles



Some other diseases of the epidermis are:

Psoriasis Photo's here.

Seborrheic dermatitis Photos here (scroll down)

Lichen simplex chronicus


Skin in Systemic Disease

Many systemic diseases have skin manifestations. Below are links to some of them.

Skin signs of systemic disease (Cleveland Clinic)

Diabetes and skin.

Thyroid disease and skin.

HIV infection and skin. Photos here.

Blood lipid abnormalities and skin.

Systemic lupus erythematosus and skin

Sarcoidosis (scroll down to skin and click on various conditions)

Neurocutaneous syndromes

Skin Lesions 4

Bacterial infection of the skin commonly seen in children

Ichthyosis vulgaris is a common inherited skin disorder causing dry, scaly skin

Keloid scars are an overgrowth of scar tissue at the site of a healed skin injury

Moles are benign neoplasms where melanocytes replicate

Contact dermatitis caused by contact with a material that causes an allergic reaction

Poison ivy (contact dermatitis)

Wart: a small benign neoplasm caused by infection with a type of human papilloma virus that does not cause genital warts

Skin Lesions 3

Port wine stain birthmark usually present at birth

Psoriasis is a common, probably inherited skin condition probably related to an autoimmune inflammatory response

Seborrheic keartosis is a benign form of skin cancer ususally seen in those over age 40

Systemic lupus erythematosus is a chronic inflammatory autoimmune disorder

Actinic keratosis is a precancerous skin growth usually caused by sun exposure

Basal cell carcinoma is a slow growing skin cancer

Severe bedsore: an area of skin that breaks down due to the pressure caused by staying in one position too long (wheelchair, bedridden)

Early Lyme Disease lesion: an inflammatory disease caused by infection caused by the bacterium Borrelia burgdorferi (transmitted by tick bite)