Hypotonia

Hypotonia, also known as floppy infant syndrome, refers to a generalized or localized decrease in muscle tone, which may or may not be associated with muscle weakness. Muscle tone, responsible for maintaining stable posture, is the resistance a muscle offers to passive stretching: even at rest, a muscle maintains a certain tension and rigidity that limits its elasticity and provides resistance to passive movement. Muscle weakness, on the other hand, is the reduction in the maximum force that can be generated.

Hypotonia can be congenital, appearing at birth, or acquired, developing later. In most cases, hypotonia in infants is a benign condition that resolves on its own.

Based on its origin, hypotonia is classified into the following types:

• Central or non-paralytic hypotonia: caused by a lesion in the central nervous system. This is the most common type and primarily affects the cervical and paraspinal muscles.
• Peripheral or paralytic hypotonia: results from lesions in the peripheral nervous system. It is associated with muscle weakness and predominantly affects proximal muscles.
• Mixed hypotonia: affects both the central and peripheral systems.

Symptoms

The most common symptoms of hypotonia, which become more noticeable as the baby grows, include:

• Difficulty latching onto the breast, sucking, swallowing, or chewing.
• Trouble holding up the head, sitting upright without support, or rolling over when lying on the stomach.
• Presence of abnormal postures.
• Increased joint mobility: joints extend beyond the normal range.
• Decreased or absent movement.
• Resting position on the back with limbs loose and extended.
• Developmental delay: reaching milestones such as crawling or walking later than usual.
• Shallow breathing.
• Difficulty speaking.
• Mouth hanging open with the tongue protruding.
• Muscles feel soft and floppy when touched or held.
• Possible lethargy or seizures.

Causes

Hypotonia is a nonspecific sign associated with various disorders:

Causes of central hypotonia:

• Hypoxic-ischemic encephalopathy before or immediately after birth. This is the most common cause.
• Intracranial hemorrhage.
• Chromosomal abnormalities, such as Down syndrome, Prader-Willi syndrome, or Marfan syndrome.
• Central nervous system infections.
• Spinal cord injury during birth.
• Congenital metabolic disorders, such as aminoacidopathies.

Causes of peripheral hypotonia:

Affects motor neurons, peripheral nerves, the neuromuscular junction, or muscles.

• Spinal muscular atrophy type I.
• Pontocerebellar atrophy type I.
• Poliomyelitis.
• Peripheral nerve abnormalities (polyneuropathies).
• Neonatal myasthenia gravis.
• Congenital myasthenia.
• Congenital myopathies.
• Metabolic myopathies.
• Mitochondrial myopathies.
• Infant botulism.
• Congenital muscular dystrophy.
• Congenital myotonic dystrophy.

Causes of mixed hypotonia:

• Mitochondrial encephalopathies.
• Leukodystrophies.
• Myelopathies.
• Muscular dystrophies with brain involvement.

Risk Factors

The main risk factors for hypotonia in infants include:

• Genetic risks related to parental consanguinity.
• Threatened miscarriage during pregnancy.
• Risk of premature birth.
• Infections during pregnancy.
• Use of harmful substances during pregnancy.
• Problems during labor: preterm birth, asphyxia, acute fetal distress, or amniotic fluid aspiration.
• Low birth weight.
• Hyperbilirubinemia.
• Seizures.
• Sepsis.
• Intraventricular hemorrhage.

Complications

In addition to frequently associated psychomotor delays, hypotonia can lead to bone misalignments resulting in skeletal deformities. Additionally, the primary risk of hypotonia is that it may be a symptom of a severe or even life-threatening condition, such as spinal muscular atrophy, mitochondrial encephalopathies, or Marfan syndrome.

Prevention

Neonatal hypotonia can be potentially prevented by taking proper care during pregnancy, protecting against infections, maintaining a healthy diet, and avoiding alcohol or drug consumption.

Which Doctor Treats Hypotonia?

Neonatal hypotonia is assessed and treated in the pediatric neurology unit.