Hypotonia

(medicine) Decrease of normal tonicity or tension, especially diminution of intraocular pressure or of muscle tone.

Definition

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.

Description

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.

Demographics

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

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."

Treatment team

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.

Treatment

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.

Clinical trials

Prognosis

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.

Resources

BOOKS

Volpe, Joseph J. Neurology of the Newborn, 4th ed. Philadelphia: W. B. Saunders Company, 2001.

Weiner, William J. and Christopher G. Goetz, eds. Neurology for the Non-Neurologist, 4th ed. Philadelphia: Lippincott Williams & Wilkins, 1999.

OTHER

The National Institute of Neurological Disorders and Stroke. NINDS Hypotonia Information Page. (March 26, 2003). http://www.ninds.nih.gov/health_and_medical/disorders/hypotonia.htm.

Thompson, Charlotte E. "Hypotonia, Benign Congenital" National Organization for Rare Disorders Report. (2003). http://www.rarediseases.org.

ORGANIZATIONS

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

Definition

Hypotonia, or severely decreased muscle tone, is seen primarily in children. Low-toned muscles contract very slowly in response to a stimulus and cannot maintain a contraction for as long as a normal muscle. Hypotonia is a symptom that can be caused by many different conditions.

Description

Hypotonia, also called floppy infant syndrome or infantile hypotonia, is a condition of decreased muscle tone. The low muscle tone can be caused by a variety of conditions and is often indicative of the presence of an underlying central nervous system disorder, genetic disorder, or muscle disorder. Muscle tone is the amount of tension or resistance to movement in a muscle. It is not the same as muscle weakness, which is a reduction in the strength of a muscle, but it can co-exist with muscle weakness. Muscle tone indicates the ability of a muscle to respond to a stretch. For example, if the flexed arm of a child with normal tone is quickly straightened, the flexor muscle of the arm (biceps) will quickly contract in response. Once the stimulus is removed, the muscle then relaxes and returns to its normal resting state. A child with low muscle tone has muscles that are slow to start a muscle contraction. Muscles contract very slowly in response to a stimulus and cannot maintain a contraction for as long as a normal muscle. Because low-toned muscles do not fully contract before they again relax, they remain loose and very stretchy, never achieving their full potential of sustaining a muscle contraction over time.

Hypotonic infants, therefore, have a typical "floppy" appearance. They rest with their elbows and knees loosely extended, while infants with normal muscle tone tend to have flexed elbows and knees. Head control is usually poor or absent in the floppy infant with the head falling to the side, backward, or forward. Infants with normal tone can be lifted by placing hands under their armpits, but hypotonic infants tend to slip between the hands as their arms rise unresistingly upward. While most children tend to flex their elbows and knees when resting, hypotonic children hang their arms and legs limply by their sides. Infants with this condition often lag behind in reaching the fine and gross motor developmental milestones that enable infants to hold their heads up when placed on the stomach, balance themselves, or get into a sitting position and remain seated without falling over. Hypotonia is also characterized by problems with mobility and posture, lethargy, weak ligaments and joints, and poor reflexes. Since the muscles that support the bone joints are so soft, there is a tendency for hip, jaw, and neck dislocations to occur. Some hypotonic children also have trouble feeding and are unable to suck or chew for long periods. Others may also have problems with speech or exhibit shallow breathing. Hypotonia does not, however, affect intellect.

Demographics

No demographic information as of 2004 was available for hypotonia, since it is a symptom of an underlying disorder. However, a study conducted in year 2000 by the University of Illinois provides some insights. The study followed 243 infants with hypotonia for three to seven years. By the age of three, about 30 percent had minimal problems and 46 percent had significant impairments, while 24 percent of the infants were normal. Hypotonic infants who matured into children with minimal disabilities were highly likely to have poor motor coordination at age three (78%). About 25 percent had learning problems or language delay; 20 percent had borderline cognition or attention deficits; and 66 percent had two or more of these characteristics.

Causes and Symptoms

Hypotonias are often of unknown origin. Scientists believe that they may be caused by trauma; environmental factors; or by other genetic, muscle, or central nervous system disorders. The National Institutes of Health list the following common causes of hypotonia:

• Down syndrome: a chromosome abnormality, usually due to an extra copy of the twenty-first chromosome.
• Myasthenia gravis: a neuromuscular disorder characterized by variable weakness of voluntary muscles, which often improves with rest and worsens with activity. The condition is caused by an abnormal immune response.
• Prader-Willi syndrome: a congenital disease characterized by obesity, severe hypotonia, and decreased mental capacity
• Kernicterus: also called Rh incompatibility, a condition that develops when there is a difference in Rh blood type between that of the mother (Rh negative) and that of the fetus (Rh positive).
• Cerebellar ataxia: a movement disorder which with its sudden onset, often following an infectious viral disease, causes hypotonia.
• Infant botulism: a type of botulism, in which Clostridium botulinum bacteria grow within an infant's digestive tract, producing a toxin which is potentially life-threatening.
• Familial dysautonomia: also called Riley-Day syndrome, an inherited disorder that affects the function of nerves throughout the body.
• Marfan syndrome: an inherited disorder of connective tissue (tissue that adds strength to the body's structures), affecting the skeletal system, cardiovascular system, eyes, and skin.
• Muscular dystrophy: a group of disorders characterized by progressive muscle weakness and loss of muscle tissue.
• Achondroplasia: a disorder of bone growth that causes the most common type of dwarfism.
• Trisomy 13: a syndrome associated with the presence of a third number 13 chromosome.
• Sepsis: a severe, life-threatening illness caused by overwhelming infection of the bloodstream by toxin-producing bacteria.
• Aicardi syndrome: a rare genetic disorder characterized by infantile spasms (jerking), absence of the corpus callosum (the connection between the two hemispheres of the brain), mental retardation, and lesions of the retina of the eye or optic nerve.
• Canavan disease: an inherited metabolic disorder characterized by degeneration of the white matter of the brain.
• Congenital hypothyroidism: a disorder that results from decreased thyroid hormone production.
• Hypervitaminosis D: a condition that appears several months after excessive doses of vitamin D are administered.
• Krabbe disease: an inherited disorder characterized by a deficiency of the enzyme galactosylcereamidase, resulting in destruction of myelin, the fatty material that surrounds and insulates many of the nerves.
• Metachromatic leukodystrophy: an inherited disease characterized by the absence of the enzyme arylsulfatase A, which causes a material called cerebroside sulfate to accumulate in cells, which is toxic to cells, especially to the cells of the nervous system.
• Methylmalonic academia: an inherited metabolic disorder, usually diagnosed in infancy, which causes the accumulation of methylmalonic acid in the body and can lead to severe metabolic disturbances.
• Rickets: a childhood disorder involving softening and weakening of the bones, primarily caused by lack of vitamin D, calcium, or phosphate.
• Spinal muscular atrophy type 1 (Werdnig-Hoffman): a group of inherited diseases causing progressive muscle degeneration and weakness, eventually leading to death.
• Tay-Sachs disease: a genetic disorder found predominantly in Ashkenazi Jewish families results in early death.
• Vaccine reaction: any injury or condition that occurs as a result of a vaccination.

The following are common symptoms associated with hypotonia. Each child may experience different symptoms, depending on the underlying cause of the hypotonia:

• decreased muscle tone; muscles feel soft and doughy
• ability to extend limb beyond its normal limit
• failure to acquire motor skill developmental milestones (such as holding head up without support from parent, rolling over, sitting up without support, walking)
• feeding problems (inability to suck or chew for prolonged periods)
• shallow breathing
• mouth hangs open with tongue protruding (underactive gag reflex)

When to Call the Doctor

Normally developing children tend to develop motor skills, posture control, and movement skills by a given age. Motor skills are divided into two categories. Gross motor skills include the ability of an infant to lift its head while lying on the stomach, to roll over from its back to its stomach. Normally, by a given age, a child develops the gross motor skills required to get into a sitting position and remain seated without falling over, crawl, walk, run, and jump. Fine motor skills include the ability to grasp, transfer an object from one hand to another, point out an object, follow a toy or a person with the eyes, or to feed oneself. Hypotonic children are slow to develop these skills, and parents should contact their pediatrician if they notice such delays or if their child appears to lack muscle control, especially if the child previously seemed to have normal muscle control.

Diagnosis

Hypotonia is normally discovered within the first few months of life. Since it is associated with many different underlying disorders, the doctor will accordingly seek to establish a family history as well as the child's medical history. A physical examination will be performed, usually including a detailed nervous system and muscle function examination. The latter may be performed with instruments, such as lights and reflex hammers, and usually does not cause any pain to the child. Most of the disorders associated with hypotonia also cause other symptoms that, when taken together, suggest a specific disorder and cause for the hypotonia. Specific diagnostic tests used will vary depending on the suspected cause of the hypotonia. Typical medical history questions include:

• When was the hypotonia first noticed?
• Was it present at birth?
• Did it start suddenly or gradually?
• Is the hypotonia always the same or does it seem worse at certain times?
• Is the child limp all over or only in certain areas?
• What other symptoms are present?

The following diagnostic tests may also be used:

• Blood tests.
• Creatine kinase (CK) test: elevated CK level in blood indicating muscles are damaged or degenerating.
• Computerized tomography scan (CT scan): a diagnostic imaging procedure that uses a combination of x-rays and computer technology to produce cross-sectional images. CT scans help physicians evaluate bone and muscle structures.
• Magnetic resonance imaging (MRI): a diagnostic procedure that uses a combination of large magnets, radio frequencies, and a computer to produce detailed images of organs and structures within the body.
• Electromyogram (EMG): a test used to evaluate nerve and muscle function.
• Electroencephalogram (EEG): a test that measures the electrical activity in the brain. An EEG measures brain waves through small button electrodes that are placed on the child's scalp.
• Spinal tap: also called lumbar puncture, measures the amount of pressure in the spinal canal and/or to remove a small amount of cerebral spinal fluid (CSF) for testing. Cerebral spinal fluid bathes the brain and spinal cord.
• Karyotype: a test that performs a chromosomal analysis from a blood test, used to determine whether the hypotonia is the result of a genetic disorder.
• Muscle biopsy: a sample of muscle tissue removed and examined under a microscope. Hypotonia can be assessed because muscle fibers have a smaller diameter than that of normal muscle.

Treatment

When hypotonia is caused by an underlying condition, that condition is treated first, followed by symptomatic and supportive therapy for the hypotonia. Physical therapy can improve fine motor control and overall body strength. Occupational and speech-language therapy can help breathing, speech, and swallowing difficulties. Therapy for infants and young children may also include sensory stimulation programs. Specific treatment for hypotonia is determined by the child's physician based on the following:

• the child's age, overall health, and medical history
• the extent of the condition
• the underlying cause of the condition
• the child's tolerance for specific medications, procedures, or therapies
• expectations for the course of the condition
• parent opinion or preference

No specific treatment is required to treat mild congenital hypotonia, but children with this problem may periodically need treatment for common conditions associated with hypotonia, such as recurrent joint dislocations. Treatment programs to help increase muscle strength and sensory stimulation programs are developed once the cause of the child's hypotonia is established. Such programs usually involve physical therapy through an early intervention or school-based program among other forms of therapy.

Hypotonic children are often treated by one or more of the following specialists:

• Developmental pediatrician: a pediatrician with specialized training in children's social, emotional, and intellectual development as well as health and physical growth. He or she may conduct a developmental assessment which will determine any delays the child has and to what extent the delay is present.
• Neurologist: a physician who has trained in the diagnosis and treatment of nervous system disorders, including diseases of the brain, spinal cord, nerves, and muscles. Neurologists perform neurological examinations of the nerves of the head and neck; muscle strength and movement; balance, ambulation, and reflexes; and sensation, memory, speech, language, and other cognitive abilities.
• Geneticist: a specialist in genetic disorders. He or she starts with the detailed history of the family's background, looks at the child's features and orders blood tests to look at the 46 chromosomes and possibly at specific genes on those chromosomes.
• Occupational therapist (OT): a professional who has specialized training in helping to develop mental or physical skills that help accomplish daily living activities, with careful attention to enhancing fine motor skills. In a developmental assessment, the occupational therapist assesses the child's fine motor skills, coordination, and age-appropriate self-help skills (eating with utensils, dressing, etc.).
• Physical therapist (PT): a professional trained in assessing and providing therapy to treat developmental delays using methods such as exercise, heat, light, and massage. In a developmental assessment, the physical therapist assesses the ability and quality of the child's use of legs, arms, and complete body by observing the display of specific gross motor skills as well as observing the child in play.
• Speech/language pathologist (SLP): a professional who is trained in assessing and treating problems in communication. Some SLPs are also trained to work with oral/motor problems, such as swallowing, and other feeding difficulties resulting from hypotonia.

Nutritional Concerns

In some hypotonic infants, sucking is weak and in some cases not present at all. They do not act hungry or show interest in feeding. Special techniques and procedures are then required to provide adequate nutrition, such as special nipples, manipulation of mouth and jaw, and on rare occasions, insertion of a gastrostomy tube.

Prognosis

The outcome in any particular case depends largely on the nature of the underlying disease. Hypotonia can be life long, but in some cases, muscle tone improves over time. Children with mild hypotonia may not experience developmental delay, although some children acquire gross motor skills (sitting, walking, running, jumping) more slowly than most. Most hypotonic children eventually improve with therapy and time. By age five, they may not be the fastest child on the playground, but many will be there with their peers and will be holding their own. Some children are more severely affected, requiring walkers and wheelchairs and other adaptive and assistive equipment.

Prevention

As of 2004 there was no prevention for hypotonia. However, measures of prevention are increasingly possible in the early 2000s for several underlying disorders.

Parental Concerns

Parents of an hypotonic child must follow the treating physician's orders for treatment of the underlying cause. They must exercise special care when lifting and carrying the hypotonic infant to avoid causing an injury to the child. If lifted under the armpits, the hypotonic infant's arms will raise with no resistance and easily slip between the hands.

Another source of concern that parents face is addressing the special needs of their hypotonic child. The world of typical children can be a difficult place for a hypotonic child, and it is tempting to isolate the child. It is not easy to go to a playgroup of toddlers when a child's latest milestone is getting from the floor into a sitting position while the other children are running across the room. There are resources for parents to help their child become as able and independent as he or she can possibly be, and the family physician is a good resource for advice.

See also Bayley Scales of Infant Development; Muscular dystrophy.

Resources

Books

Amiel-Tison, Claudine, et al. Neurological Development from Birth to Six Years: Guide for Examination and Evaluation. Baltimore, MD: Johns Hopkins University Press, 2001.

Preedy, Victor R., and Timothy J. Peters. Skeletal Muscle: Pathology, Diagnosis, and Management of Disease, 3rd ed. Edited by Kenneth J. Ryan. Albuquerque, NM: Health Press, 2002.

Periodicals

Carboni, P., et al. "Congenital hypotonia with favorable outcome." Pediatric Neurology 26, no. 5 (May 2002): 383–86.

Heilstedt, H. A., et al. "Hypotonia, congenital hearing loss, and hypoactive labyrinths." American Journal of Medical Genetics3, no. 3 (August 2002): 238–42.

Richer, L. P., et al. "Diagnostic profile of neonatal hypotonia: an 11-year study." Pediatric Neurology 25, no. 1 (July 2001): 32–37.

Thompson, C. E. "Benign congenital hypotonia is not a diagnosis." Developments in Medical Child Neurology 44, no. 4 (April 2002): 283–84.

Trifiro G., et al. "Neonatal hypotonia: don't forget the Prader-Willi syndrome." Acta Paediatrica 92, no. 9 (September 2003): 1085–89.

Pomerance, H. H., et al. "Infant with inadequate feeding and weight gain, progressive respiratory difficulty, hypotonia, and weakness, with onset at birth." American Journal of Medical Genetics 94, no. 1 (September 2000): 68–74.

Organizations

Child Development Institute (CDI). 3528 E. Ridgeway Road, Orange, California 92867. Web site: www.childdevelopmentinfo.com.

Genetic and Rare Diseases Information Center. PO Box 8126, Gaithersburg, MD 20898–8126. Web site: www.rarediseasesinfo.nih.gov.

March of Dimes Birth Defects Foundation. PO Box 3006, Rockville, MD 20847. Web site: www.marchofdimes.com.

Muscular Dystrophy Association. 3300 East Sunrise Drive, Tucson, AZ 85718–3208. Web site: www.mdausa.org.

National Institute of Child Health and Human Development (NICHD). 31 Center Drive, Rm. 2A32, MSC 2425, Bethesda, MD 20892–2425. Web site: www.nichd.nih.gov.

National Institute of Neurological Disorders and Stroke (NINDS). PO Box 5801, Bethesda, MD 20824. Web site: www.ninds.nih.gov.

National Organization for Rare Disorders (NORD). PO Box 1968, 55 Kenosia Avenue, Danbury, CT 06813–1968. Web site: www.rarediseases.org.

Web Sites

"Hypotonia." Family Village. Available online at www.familyvillage.wisc.edu/lib_hypot.htm (accessed October 18, 2004).

"What is Benign Congenital Hypotonia?" Benign Congenital Hypotonia Site. Available online at www.lightlink.com/vulcan/benign/aboutbch.htm (accessed October 18, 2004).

[Article by: Monique Laberge, Ph.D.]

A condition characterized by deficient muscle tone.

Abnormally decreased tonicity or strength.

• ocular h. — low intraocular pressure.

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