Hypotonia (Low Muscle Tone)

Tone vs strength

Muscle tone is the resting tension in a muscle, felt as resistance when someone else moves the limb. Muscle strength is the force a person can produce. A child can have low tone but normal strength, which usually points to a brain-related (central) cause. Low tone with real weakness points more to nerve or muscle disease.

Signs of hypotonia

In babies

• poor head control and head lag when pulled to sit
• slipping through your hands when lifted under the arms
• arms and legs lying flat and turned outward, a "frog-leg" position
• draping over your hand like an upside-down "U" when held tummy-down
• weak suck, feeding difficulties and drooling
• weak cry or reduced facial expression
• delayed rolling, sitting, crawling and walking

In older children

• delayed motor milestones and clumsiness
• loose, very flexible joints
• poor posture, rounded shoulders, and fatigue with activity
• difficulty with fine motor skills such as writing
• speech articulation problems from low tone in the mouth muscles

Causes

Benign congenital hypotonia

Some children have low tone with no underlying disease found. They may be late to walk but usually catch up, and the low tone improves with age, though some remain loose-jointed and clumsy. This is a diagnosis made only after other causes have been excluded.

Diagnosis

A pediatrician or pediatric neurologist will take a detailed history of pregnancy, birth, development and family history, and examine tone, strength, reflexes and development. Tests depend on the suspected cause:

• blood tests: thyroid function, creatine kinase (CK) for muscle disease, glucose, metabolic screening
• genetic testing: chromosomal microarray, testing for specific conditions such as SMA and Prader-Willi syndrome, and exome or genome sequencing
• MRI of the brain (and sometimes spine)
• nerve conduction studies and EMG
• muscle biopsy in selected cases
• newborn screening results: SMA is now included in newborn screening in all US states

Treatment

There is no medicine that raises muscle tone directly. Treatment focuses on the cause and on building function:

• Disease-specific treatments: gene therapy or other medicines for SMA (most effective when started early), thyroid hormone for hypothyroidism, antitoxin for infant botulism
• Physical therapy: improves strength, posture, balance and motor milestones
• Occupational therapy: fine motor skills, play and daily activities
• Speech and feeding therapy: sucking, swallowing and speech
• Orthotics and equipment: ankle-foot braces, supportive seating, and mobility aids
• Early intervention programs: coordinated developmental support from infancy

Outlook

The outlook depends on the cause. Benign congenital hypotonia generally improves. Children with genetic syndromes or cerebral palsy often make steady progress with therapy. Progressive nerve and muscle diseases need ongoing specialist care, but treatments for conditions like SMA have dramatically improved outcomes.

Frequently asked questions

What is hypotonia?

Hypotonia means low muscle tone, so muscles feel soft and floppy and offer little resistance when moved. It is a sign of an underlying condition affecting the brain, spinal cord, nerves or muscles, rather than a disease by itself.

What are the signs of hypotonia in babies?

Babies may have poor head control, feel floppy when held, slip through your hands when lifted, lie with arms and legs flat and turned out, feed poorly, and be late to roll, sit or walk.

What causes floppy baby syndrome?

Common causes include genetic conditions such as Down syndrome and Prader-Willi syndrome, brain injury or cerebral palsy, spinal muscular atrophy, muscle diseases, infections and hypothyroidism. Sometimes no cause is found, and the child improves with time.

Can hypotonia be cured?

Low tone itself cannot be cured with medicine, but many children improve with therapy and age. Some causes, such as hypothyroidism, infant botulism and spinal muscular atrophy, have specific treatments that can greatly improve tone and strength.

Does hypotonia affect intelligence?

Not necessarily. Hypotonia is about muscle tone, not intelligence. Some underlying causes, such as certain genetic syndromes or brain conditions, can also affect learning, while others, such as many muscle diseases or benign hypotonia, do not.