STAC3 disorder

STAC3 disorder (formerly known as Native American myopathy) is a condition that primarily affects skeletal muscles, which are muscles that the body uses for movement. People with STAC3 disorder have muscle weakness (myopathy) and poor muscle tone (hypotonia) throughout the body that typically begins at birth.

Muscle weakness underlies many of the characteristic features of STAC3 disorder. Affected individuals may have feeding and swallowing difficulties in infancy. They usually have delayed development of motor skills such as sitting, crawling, standing, and walking. Many have facial features described as "myopathic facies", which include drooping eyelids (ptosis), sunken cheeks, and a mouth often held in an open position and with the corners turned downward. Other distinctive facial features in people with STAC3 disorder can include a small lower jaw (micrognathia), an opening in the roof of the mouth (cleft palate), low-set ears that slant backward, eye openings that are shorter than average or that point downward (short or downslanting palpebral fissures), or an increased distance between the inner corners of the eyes (ocular telecanthus).

Individuals with STAC3 disorder may also be born with joint deformities that restrict movement (contractures) or develop an abnormal side-to-side or back-to-front curvature of the spine (scoliosis or kyphosis, often called kyphoscoliosis when they occur together). Affected individuals tend to be shorter than their peers and others in their family.

People with STAC3 disorder 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 drugs, which are used to block the sensation of pain, either given alone or in combination with a particular type of muscle relaxant. If given these drugs, people at risk of malignant hyperthermia may experience muscle rigidity, breakdown of muscle fibers (rhabdomyolysis), a high fever (hyperthermia), increased acid levels in the blood and other tissues (acidosis), and a rapid heart rate. The complications of malignant hyperthermia can be life-threatening unless the drugs are stopped and the symptoms are treated promptly.