Decreased tone in any muscle or muscle group, can be divided into central hypotonia affecting the core trunk muscles, axial hypotonia affecting the limbs, or specific muscle group hypotonia, such as facial hypotonia. It may be very mild and only noticed on close testing, or so profound that respiration, swallowing, and reflexes are affected.
Hypotonia in an infant can be due to primary genetic cause (e.g., Trisomy 21, Prader–Willi), neurologic abnormality (e.g., lissencephaly, hypoxic-ischemic encephalopathy, myotonic dystrophy, spinal muscular atrophy), metabolic disease (e.g., Zellweger syndrome, Pompe, congenital disorder of glycosylation), or other medical issue (e.g., prematurity, sepsis, botulism).
The search for a genetic cause of hypotonia, especially in a neonate, should include consideration of the following: