DESCRIPTION
Muscle tone indicates how much muscles resist outside forces (passive movement). “Hypo-” means low, so “hypotonia” translates to low tone. It is a medical term meaning decreased muscle tone.
Even when relaxed, muscles have a small amount of contraction. This gives them a springy feel. It means they have some resistance to passive movement. In hypotonia, this resistance is lower than normal.
Low muscle tone is not the same as muscle weakness. However, the two conditions can coexist.
Muscle weakness.
Muscle weakness refers to decreased ability to voluntarily and actively move muscles against resistance. Weakness may arise from any portion of the motor unit including:
- Brain
- Spinal cord
- Peripheral nerves
- Muscles
Acute (sudden) weakness is often caused by:
- Severe brain or spinal cord trauma
- Brain tumors
- Seizures
- Infections
- Autoimmune processes
Chronic causes of weakness include:
- Neurodegenerative disorders
- Cerebral palsy
- Endocrine abnormalities
- Brain tumors
- Congenital skeletal muscle disorders
Muscle tone.
Muscle tone is regulated by special signals. These travel from the brain to the muscles through the nerves. These signals tell the muscles to contract. Hypotonia is caused by problems with signals in any of the following:
- Brain
- Spinal cord
- Nerves
- Muscles
Hypotonia comes in two forms:
1. Benign congenital hypotonia.
This condition is present from birth. It stands alone. It is caused by differences in how the fetus developed. Infants with this form of the disorder are hypotonic at birth or shortly after. Later on, they have normal tone. They also have no other signs of brain dysfunction.
2. Secondary hypotonia.
This occurs when hypotonia is associated with another condition. That condition usually causes progressive loss of muscle tone. For instance, it can be associated with:
- A central nervous system (brain and spinal cord) disorder
- A primary neuromuscular disorder
- A genetic disorder
The first step is determining the cause of a child’s low muscle tone. Then, treatment and therapy can begin.
CAUSES
General Causes
Hypotonia is caused by problems with the nervous system. These problems can occur for different reasons. General causes can include:
- Trauma
- Environmental factors
- Genetics
- Muscle diseases
- Central nervous system disorders
- Metabolic diseases
More than half of all cases in infants and children are due to genetic variations or metabolic diseases.
Specific Causes
The nervous system is important to maintaining normal muscle tone. The specific causes of hypotonia tend to affect either the central nervous system (CNS) or the peripheral nervous system (PNS).
Central Hypotonia
The CNS is made up of the brain and spinal cord. Problems with the CNS cause 60% to 80% of all hypotonia cases in infants and children. These cases are known as central hypotonia.
Doctors might suspect central hypotonia if they spot certain clues, such as:
- Malformations of the organs
- Normal or brisk reflexes
- Fisting of the hands
Central hypotonia could be related to:
- Cerebral palsy
- Chromosomal disorders, such as Prader-Willi syndrome or Lowe syndrome
- Brain injuries during the newborn period or infancy
- Spinal cord disorders usually associated with the birth process
Peripheral Hypotonia
The PNS is made up of nerves that travel to the muscles. Problems with the PNS cause 15% to 30% of all hypotonia cases in infants and children. These cases are known as peripheral (or motor unit) hypotonia.
Doctors might suspect peripheral hypotonia if they spot certain clues, such as:
- Absent or slow reflexes
- Muscle twitches
Peripheral hypotonia could be related to:
- Muscular diseases, such as Duchenne muscular dystrophy and Becker muscular dystrophy
- Metabolic disorders
- Neuromuscular junction disorders, such as congenital myasthenia and infantile botulism
- Disorders of the skeleton
- Spinal cord disorders
LABORATORY INVESTIGATIONS
Diagnosing hypotonia can require assessing:
- Motor and sensory skills
- Balance and coordination
- Mental status
- Reflexes
- Nerve function
Diagnostic tests may include:
- Magnetic resonance imaging (MRI) of the brain or spinal cord
- Electromyography (EMG) to evaluate nerve and muscle function
- Electroencephalogram (EEG) to measure electrical activity in the brain (if there is concern about seizures)
Genetic testing and metabolic testing can also be helpful. This might mean testing how an infant or child metabolizes:
- Sugars
- Fats
- Proteins
Diagnostic tests are usually selected based on:
- A child’s clinical history
- Neurologic exams
- Systemic diseases, such as heart or lung diseases
Publications
Care Pathway for Therapeutic Interventions for Central Hypotonia
Comprehensive information and practical tools for children with hypotonia, ages 0 to 6 years. Compiled by The American Academy for Cerebral Palsy and Developmental Medicine (AACPDM).
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