Hypotonia

Hypotonia is abnormally low resistance to passive movement in a relaxed muscle. It can arise from disorders affecting the brain, spinal cord, nerves, neuromuscular junction, or muscle.

Muscle tone: Muscle tone is the resting tension in a muscle, reflected clinically by resistance to passive movement. Hypotonia describes reduced resting resistance, though tone is distinct from muscle strength.
Muscle weakness: Muscle weakness is reduced ability to generate force during voluntary movement. Hypotonia concerns passive resistance; weakness concerns force production, though they can occur together.
Infant hypotonia: Infant hypotonia is abnormally low muscle tone identified during infancy, with causes ranging from transient conditions to serious disease. It is a common presentation that may affect posture, feeding, and motor milestones.
Down syndrome: Down syndrome is a genetic condition caused by extra chromosome 21 material, often associated with developmental differences and hypotonia. Low tone is common in infancy and can affect feeding and motor development.
Stretch reflex: The stretch reflex is an involuntary muscle contraction triggered by rapid muscle stretch. Reduced reflex contributions can accompany hypotonia when reflex pathways are impaired.
Hypertonia: Hypertonia is abnormally increased resistance to passive movement. It is the opposite clinical finding, although tone can vary with movement speed and cause.
Developmental delay: Developmental delay is slower-than-expected acquisition of skills in one or more areas of childhood development. Persistent hypotonia can coexist with delayed motor development and prompt broader assessment.
Spinal muscular atrophy: Spinal muscular atrophy is an inherited motor neuron disease that causes progressive muscle weakness and atrophy. It can present with hypotonia and weakness from loss of lower motor neurons.
Muscle spindle: A muscle spindle is a sensory receptor that detects changes in muscle length and the speed of stretching. Its sensory feedback helps regulate the reflex activity that contributes to tone.
Spasticity: Spasticity is velocity-dependent increased muscle resistance associated with upper motor neuron dysfunction. Unlike hypotonia, spasticity produces increased resistance that grows with faster passive movement.

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