Hypotonia means "low tone," and refers to a physiological state in which a muscle has decreased tone, or tension. A muscle's tone is a measure of its ability to resist passive elongation or stretching.
Hypotonia is more a description than a diagnosis. It is most often seen in newborns (congenital) and infants, but it may persist through adolescence into adulthood. Another name for infantile hypotonia is "floppy baby syndrome." This refers to the tendency of a hypotonic infant's arms, legs, and head to "flop," or dangle loosely, when they are picked up or moved. In the past, the term "benign congenital hypotonia" was used for many cases in which no obvious cause for the hypotonia could be detected. Better diagnostic techniques and increased knowledge of neuromuscular disorders, however, have resulted in much less frequent use of this term.
Hypotonia is the most common muscular abnormality seen in neonatal (newborn) neurological disorders. It affects males and females equally, and shows no preponderance in any particular ethnic group or race. An increase in the occurrence of hypotonia in recent years is correlated with increased survival rates of infants born significantly premature, since these children are at increased risk for neurological problems.
Causes and symptoms
The causes of hypotonia are varied and numerous. Some involve trauma to, or diseases of, the brain or spinal cord (CNS), while others affect the peripheral nerves, neuromuscular junction, or the muscles themselves. A disorder of the nervous system is a neuropathy, while a muscle disease is a myopathy. A neuromuscular condition is one in which a neurological disorder results in associated muscular symptoms.
CNS trauma and infection are perhaps the most common cause of hypotonia, both in infants and in children. Insult to the brain may occur prenatally (before birth), perinatally (around the time of birth), or postnatally (after birth).
Prenatal CNS damage may be caused by certain maternal/fetal infections, maternal diseases, problems with the placenta or umbilical cord, or maternal use of harmful substances such as alcohol or certain drugs. Most congenital brain malformations, however, have no discernible cause and are likely due to chance maldevelopment of a very complex organ. Perinatal asphyxia/hypoxia (lack of oxygen to the baby's brain) occurs less frequently than is commonly believed, but does present a risk for CNS damage that can result in hypotonia. The greatest risk for asphyxia/hypoxia is from complicated and/or premature deliveries. Infants who are born healthy may sustain post-natal brain injury if they suffer from breathing difficulties, develop an infection in the lining of the brain (see Meningitis), or suffer some other type of physical trauma or abuse.
While it is less common, hypotonia may develop in an adult. This is again most often the result of CNS trauma or disease, usually affecting the cerebellum. The primary function of the cerebellum is control of balance and coordination, including maintaining passive tension/tone of the muscles, such as muscular control required for standing.
A number of different genetic disorders are associated with hypotonia, and may affect the nerves (and by extension the muscles), or the muscles only. Most genetic conditions are generalized (affecting multiple muscle groups) and progressive. Some genetic conditions are hereditary
(autosomal recessive or X-linked recessive) and some are sporadic (chromosomal disorders). Hereditary conditions would typically imply a 25% recurrence risk for siblings on the affected child, while the chance for another child with the same chromosomal abnormality is usually about 2–3%.
In addition to low muscle tone, infants with hypotonia may also exhibit excessive flexibility of the joints (hypermobility), decreased deep tendon reflexes (e.g., tapping the knee joint produces little or no muscle jerk), and difficulties with sucking and swallowing. Children in whom hypotonia persists often show delays in gross motor skills such as sitting up, crawling, and walking. They may also have difficulties with coordination and exhibit speech delays. In some cases, symptoms may persist into adulthood. Hypotonia itself is not associated with decreased intellectual development, but the underlying cause may pose significant risks for developmental delay and mental retardation.
Diagnosis of the cause of hypotonia may involve a number of different medical methods, procedures, and tests. These include:
- A complete prenatal (before birth) and perinatal (around the time of birth) history. Along with this a complete family medical history should be obtained.
- A physical examination to determine the degree of hypotonia and the muscles affected
- An electromyelograph (EMG), measures muscle response to electrical stimulation
- A nerve conduction velocity (NCV), measures a nerve's ability to transmit electrical impulses to and from the muscle
- Electroencephalogram (EEG), a test that measures the electrical activity in the brain
- A muscle biopsy to analyze the microscopic structure of affected muscle
- Biochemical tests on muscle tissue and blood
- Genetic tests to look for possible sporadic (chance occurrence) or hereditary genetic errors affecting the brain, nerves, and/or muscles
- Imaging studies (CT scan or MRI) of the brain and spinal cord
Determining which tests to use depends on the clinician's judgment of what is most likely to be the underlying cause of the hypotonia. This in turn is based upon the history and physical findings. In some cases, different doctors will order different tests based upon their area of expertise. There is always a possibility that a diagnosis will not be determined. The term for hypotonia without a diagnosis is "idiopathic," which literally means "unknown cause."
Along with normal pediatric care, specialists who may be involved in the care of a child with hypotonia include developmental pediatricians (specialize in child development), neurologists, neonatologists (specialize in the care of newborns), geneticists, occupational therapists, physical therapists, speech therapists, orthopedists, pathologists (conduct and interpret biochemical tests and tissue analysis), and specialized nursing care. Depending on the cause and progression of hypotonia, treatment and evaluation may be needed throughout life.
Unlike the wide array of potential causes of hypotonia, treatment options for low muscle tone are somewhat limited. In very severe cases, treatment may be primarily supportive, such as mechanical assistance with basic life functions like breathing and feeding, physical therapy to prevent muscle atrophy and maintain joint mobility, and measures to try and prevent opportunistic infections such as pneumonia. Treatments to improve neurological status might involve such things as medication for a seizure disorder, medicines or supplements to stabilize a metabolic disorder, or surgery to help relieve the pressure from hydrocephalus (increased fluid in the brain). If the neurologic condition is untreatable, physical and occupational therapy may help to improve muscle tone, strength, and coordination.
Recovery and rehabilitation
In all cases, frequent or periodic monitoring of muscle tone and performance, along with neurological status, should be done to determine if the hypotonia is worsening, static, or improving. Effective recovery and rehabilitation can only be achieved if an accurate status of the condition is known. Since muscle weakness often accompanies hypotonia, efforts to improve muscle strength may also improve low muscle tone. Some individuals with persistent symptoms may need assistance with mobility, such as a walker or wheelchair. Occupational and physical therapy can assist individuals in developing alternative methods for accomplishing some everyday tasks they may find difficult. Speech therapy is primarily directed at young children to help them develop language skills early, but can be beneficial at any age if the muscles of the face and throat are hypotonic.
Determining a prognosis depends on determining a diagnosis for hypotonia. Some genetic conditions are fatal in infancy, while others result in permanent disability and mental retardation. For those few genetic metabolic disorders that are treatable, improvement may be dramatic, or minimal. Outcomes for hypotonia caused by CNS trauma or infection depend on the severity of neurologic damage. Mild trauma obviously has the best chance for improvement and recovery, but even significant neurologic deficits may improve over time.
Most individuals with a nongenetic form of hypotonia will improve to some degree. From a broad perspective, some individuals with hypotonia will respond very little or not at all to any treatment method attempted, while in others the condition will resolve on its own; each case is unique.
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Thompson, Charlotte E. "Hypotonia, Benign Congenital" National Organization for Rare Disorders Report. (2003). <http://www.rarediseases.org>.
March of Dimes Birth Defects Foundation. 1275 Mamaroneck Avenue, White Plains, NY 10605. 888-663-4637; Fax: 914-428-8203. <http://www.marchofdimes.com>.
Muscular Dystrophy Association. 3300 East Sunrise Drive, Tucson, AZ 85718-3208. 800-572-1717; Fax: 520-529-5300. <http://www.mdausa.org/>.
National Institute of Child Health and Human Development Clearinghouse. PO Box 3006, Rockville, MD 20847. 800-370-2943. <http://www.nichd.nih.gov>.
National Organization for Rare Disorders (NORD). P.O. Box 1968, 55 Kenosia Avenue, Danbury, CT 06813-1968. 203-744-0100; Fax: 203-798-2291. <http://www.rarediseases.org>.
Scott J. Polzin, MS, CGC