Multiminicore disease

Multiminicore disease is a disorder that primarily affects muscles used for movement (skeletal muscles). This condition causes muscle weakness and related health problems that range from mild to life-threatening.

Researchers have identified at least four forms of multiminicore disease, which can be distinguished by their characteristic signs and symptoms. The forms of multiminicore disease are the classic form, the progressive form with hand involvement, the antenatal form with arthrogryposis, and the ophthalmoplegic form.

The classic form accounts for about 75 percent of cases of multiminicore disease. This form causes muscle weakness beginning in infancy or early childhood. The muscles of the torso and neck (axial muscles) are most affected with arm and leg muscles less so. Muscle weakness causes affected infants to appear "floppy" (hypotonic) and they may have feeding problems early in life. Muscle weakness can delay the development of motor skills such as sitting, standing, and walking. In this form, the muscles of the ribcage and spine become stiff. In addition, the muscles needed for breathing are weak. This combination of muscle weakness and stiffness leads to severe or life-threatening respiratory problems. Almost all children with the classic form develop an abnormal curvature of the spine (scoliosis), which appears during childhood and steadily worsens over time.

The progressive form with hand involvement causes muscle weakness and looseness of the joints (joint laxity) in the arms and hands. Individuals with this form may experience muscle pain (myalgia) or extreme fatigue in response to physical activity (exercise intolerance). This form accounts for about 10 percent of cases of multiminicore disease.

The antenatal form with arthrogryposis is characterized by stiff, rigid joints throughout the body (arthrogryposis) and distinctive facial features. Weakness in the muscles needed for breathing can result in breathing problems for affected individuals. This form also accounts for about 10 percent of cases of multiminicore disease.

The ophthalmoplegic form of multiminicore disease is characterized by paralysis of the eye muscles (external ophthalmoplegia). This can lead to abnormal eye movements and droopy eyelids (ptosis). This form of the condition can also cause weakness in the muscles close to the center of the body (proximal muscles), such as those of the upper arms and legs. The ophthalmoplegic form accounts for 5 to 10 percent of cases of multiminicore disease.

Many people with multiminicore disease also have an increased risk of developing a severe reaction to certain drugs used during surgery and other invasive procedures. This reaction is called malignant hyperthermia. Malignant hyperthermia occurs in response to some anesthetic gases, which are used to block the sensation of pain, either given alone or in combination with a muscle relaxant that is used to temporarily paralyze a person during a surgical procedure. If given these drugs, people at risk of malignant hyperthermia may experience a rapid increase in heart rate (tachycardia) and body temperature (hyperthermia), abnormally fast breathing (tachypnea), muscle rigidity, breakdown of muscle fibers (rhabdomyolysis), and increased acid levels in the blood and other tissues (acidosis). The complications of malignant hyperthermia can be life-threatening unless they are treated promptly.

Multiminicore disease gets its name from small, disorganized areas called minicores, which are found in skeletal muscle cells of many affected individuals. These abnormal regions can only been seen when muscle tissue is viewed under a microscope. Minicores are often present in cells with few or no mitochondria, which are the energy-producing centers within cells. Although the presence of minicores can help doctors diagnose multiminicore disease, it is unclear how they are related to muscle weakness and the other features of this condition.