What Is Hypotonia?

Medically Reviewed by Dan Brennan, MD on December 01, 2022
3 min read

Hypotonia is a medical word for low muscle tone. If your baby has it, they will likely feel limp in your arms, like a rag doll. That’s why it’s also called floppy infant syndrome.

Doctors can diagnose the condition in the first few minutes of life. They do routine checks of newborns’ muscle tone at 1 minute and 5 minutes after birth. Sometimes hypotonia shows up a bit later, but it will usually be noticeable by 6 months of age.

Poor muscle tone tends to signal a problem with the brain, spinal cord, nerves, or muscles. But physical therapy and other treatments can help your child build stronger muscles and better coordination.

Most babies enter the world with good muscle tone. It lets them flex and flail their little limbs. Newborns with hypotonia won’t have strong arm and leg movements.

As they get older, “floppy” babies will miss important milestones, like being able to lift their heads when they’re on their tummies. Other common symptoms include:

  • Poor head control. When your baby can’t control their neck muscles, their head will fall forward, backward, or to the side.
  • Feeling limp, especially when you lift them. If you pick them up with your hands beneath their armpits, their arms may raise without resistance -- as if they could slip through your hands.
  • Arms and legs hang straight. Babies usually rest with their arms and legs flexed -- there’s a slight bend in their elbows, hips, and knees. But children with hypotonia don’t.

Sometimes, the condition can cause problems with sucking and swallowing. Also, your child’s joints may be extremely flexible, as if they are double-jointed.

Floppy infant syndrome can happen for no clear reason -- what doctors call benign congenital hypotonia. But more often, it’s related to another health problem. There are many causes. A few are:

  • Brain damage due to lack of oxygen right before or after birth
  • Problems with the way the brain formed in the womb
  • Disorders that affect nerves
  • Achondroplasia
  • Spinal cord injury
  • Cerebral palsy
  • Severe infections

Hypotonia isn’t always a sign of a major problem. When babies are born too early, they may have poor muscle tone because their bodies haven’t had enough time to develop properly. In this case, things should get much better as weeks and months pass. You just need to make sure your baby is meeting milestones and getting any treatment they need.

Because many things can cause hypotonia, it may take some time to figure out what’s behind your child’s condition. The doctor will want to learn about your family’s medical and genetic history and give your baby a physical exam. They may check their:

  • Motor skills
  • Sensory skills
  • Balance
  • Coordination
  • Mental status
  • Reflexes

The doctor also may order several tests, such as:

  • CT or MRI scan of the brain
  • Blood tests
  • Electromyography (EMG) to measure how well the nerves and muscles work
  • Electroencephalogram (EEG) to measure electrical activity in the brain
  • Spinal tap, which can measure the pressure inside the spine and let the doctor get a sample of the fluid around the spinal cord for testing
  • Muscle biopsy, when the doctor gets a sample of your child’s muscle tissue to study under a microscope
  • Genetic tests

The doctor also will want to know whether there were any problems before the baby was born or during delivery.

Once the doctor figures out the cause of your child’s hypotonia, they will try to treat that condition first. For example, they can prescribe medicine to treat an infection that caused their muscle problems. But sometimes, there’s no cure for the problem that causes hypotonia. If an inherited condition caused it, your child will have that condition for life.

No matter the cause of hypotonia, your child can have therapy to strengthen their muscles and improve coordination. There are many options, including:

  • Sensory stimulation programs: These help babies and young kids respond to sight, sound, touch, smell, and taste.
  • Occupational therapy: This will help your child get fine motor skills, which are (or will be) essential for daily tasks.
  • Physical therapy: Like occupational therapy, it can help your child get more control of their movements. It can also improve strength and muscle tone over time. 
  • Speech-language therapy: Helps with problems breathing, speaking, and swallowing.

A child with benign congenital hypotonia may not need any therapy, although they may need to see a doctor for related problems, such as joint dislocation.