Information on the following diseases can be found in the Related Disorders section of this report:

• Werdnig-Hoffmann Disease (Infantile Spinal Muscular Atrophy)
• Nemaline Myopathy (Rod Myopathy)
• Associated Congenital or Infantile Disorders (General)
• .

The symptoms of the various disorders formerly called benign congenital hypotonia are low muscle tone (hypotonia) of voluntary skeletal muscles and diminished resistance of the muscles to stretching. Associated findings include excessive flexibility or an increased range of motion of the joints (hypermobility). The hypotonia is associated with muscle weakness. Deep tendon reflexes may be diminished (hypoactive). These reflexes are involuntary, automatic responses to stimuli. An involuntary contracture of a muscle should occur in response to a brief tap on its tendon, the fibrous connective tissue joining the muscle to bone.

Children diagnosed with one of the conditions formerly called benign congenital hypotonia generally have delays in gross motor skills such as crawling, walking, running, and jumping. These are skills that require the use and coordination of large muscle groups. In children with hypotonia, gross motor delays may be associated with abnormal or clumsy walking (gait disturbances), speech delays and other problems.

Most of the disorders tend to improve over time. In some instances, there may be significant improvement during early to mid childhood; in others, symptoms may persist into adulthood. Typically, hypotonia decreases but does not disappear entirely. In addition, there may be associated problems in some of the disorders. Curvature or scoliosis of the spine is one of the most important to be watched for, as the children get older.
.

The specific causes of the group of neuromuscular disorders formerly known as benign congenital hypotonia are being researched. Specific gene deletions have been found for several. Also, each of the disorders except for congenital myotonic dystrophy has characteristic muscle biopsy findings. This is key to distinguishing among the myopathies and for diagnosis. For example, in myotubular myopathy, one of the congenital myopathies, the muscle biopsy shows fetal-like muscle fibers.
.

Both males and females can have the disorders. The hypotonia is usually evident at birth (congenital) or during the first few months of life. It can also develop later on, however. The frequency of some of the disorders is not known.
.

The following disorders can result in hypotonia at birth or in the first few months.

Werding-Hoffmann disease is a progressive, neuromuscular disorder inherited as an autosomal recessive trait. It is also known as infantile spinal muscular atrophy or type I SMA. The disorder is characterized by degeneration of groups of nerve cells (motor nuclei) within the spinal cord (anterior horn cells). Motor neurons are nerve cells that transmit nerve impulses from the spinal cord to muscle. The disease may have its onset during fetal development (in utero) or may be apparent at birth (congenital). It also may occur within the first few months. Characteristic findings are decreased muscle tone (hypotonia) or “floppiness” at birth, general weakness, wasting (atrophy) of voluntary skeletal muscles and absence of reflexes.

Infants who appear to develop normally during the first few months may then develop weakness in the pelvic, trunk and shoulder girdle area but gradually the weakness spreads to affect almost all the voluntary muscles. Infants with Werdnig-Hoffmann disease lack head control, are unable to roll over or support their weight and tend to lie relatively still with little or no movement. In addition, they may have difficulty sucking, swallowing, and breathing; have an increased susceptibility to respiratory infections (pneumonia) and develop other complications that can be life-threatening during the first months of life or within the first two years without mechanical respiratory support.

Infants who appear to have normal development until six months or more of age can then develop muscle weakness. They usually are able to sit and some can support their weight in a standing position. This is called Type II spinal muscular atrophy.

Nemaline rod myopathy is a muscular disorder (myopathy) usually apparent during infancy or early childhood. The disorder is characterized by diminished muscle tone (hypotonia) with generalized weakness of skeletal muscles; diminished or absent tendon reflexes; feeding and swallowing difficulties; and/or an increased susceptibility to respiratory infections leading to potentially life-threatening complications. Many of the children have an usually narrow body, a high arched palate, narrow face, abnormal curvature of the spine (kyphoscoliosis) and/or other symptoms. The disorder derives its name from thread or rod-like substances within the muscle fibers. Nemaline myopathy is inherited as either an autosomal dominant or autosomal recessive trait. (For more information, choose nemaline myopathy as your search term in the rare disease database).

The other principal disorders that cause hypotonia at birth, or in the first three months, are central core disease, minicore disease, myotubular myopathy, and congenital myotonic dystrophy. Infantile polymyositis, Prader-Willi’s syndrome, congenital fibre type disproportion, minimal change myopathy and other rare types with subcellular structural change can also cause hypotonia.
.

Diagnosis
The congenital muscle diseases are diagnosed by a thorough clinical examination, identification of characteristic physical findings, and a complete patient and family history. Specialized tests should include one for creatine phosphokinase (CPK or CK). These are enzymes from muscle found in the blood. An EMG or test to record electrical activity of the muscles is rarely needed in infants and children. The diagnostic evaluation, without a family history of a disorder, must always include a muscle biopsy performed under a local anesthetic because of the risk of a condition called malignant hyperthermia. Advanced imaging studies of the brain and spinal cord, such as computerized tomography and magnetic resonance imaging, are rarely needed.

Treatment
The treatment of congenital muscle disorders may involve the coordinated efforts of a team of medical professionals including a pediatrician, neurologist, (physicians who specialize in the treatment of neurological disorders), orthopedists, (physicians who diagnose and treat disorders of the bones, joints and muscles), physiatrists who prescribe special equipment for patients, pulmonologists or pulmonary doctors, physical therapists, occupational therapists, speech therapists, and other health care professionals.

Disease management should include careful monitoring of the hypotonia as well as ongoing evaluation to help prevent and ensure prompt detection and treatment of any complications. The conditions may improve with time. However, follow-up of children should continue into adulthood. Family-centered early intervention services are often recommended for infants and children. These may include an individual early education program, physical, occupational, and/or special therapy.
.

TEXTBOOKS
Thompson CE. Raising a Child With A Neuromuscular Disorder. Oxford University Press, New York and London. 1999.

Behrman RE, et al., eds. Nelson Textbook of Pediatrics. 15th ed. Philadelphia, PA: W.B. Saunders Company; 1996:1745.

Menkes JH., au., Pine JW, et al., eds. Textbook of Child Neurology. 5th ed. Baltimore, MD: Williams & Wilkins; 1995:819-820, 845-47.

REVIEW ARTICLE
Miller VS, et al. Neonatal hypotonia. Semin Neurol. 1993;13:73-83.

JOURNAL ARTICLES
Thompson CE. Benign congenital hypotonia is not a diagnosis. Dev Med Child Neurol. 2002;44:283-86.

Duger T, et al. The assessment of Bruininks-Oseretsky test of motor proficiency in children. Pediatr Rehabil. 1999;3:125-131.

Cohen SM. Congenital hypotonia is not benign. Early recognition and intervention is the key to recovery. MCN Am J Matern Child Nurs. 1998;23:93-98.

Parush S, et al. Developmental correlates of school-age children with a history of benign congenital hypotonia. Dev Med Child Neurol. 1998;40:448-452.

Joensen F. Benign congenital hypotonia. Ugeskr Laeger. 1997;159:1752-1754. Comment in: Ugeskr Laeger. 1997;159:4281.

Shuper A, et al. Benign congenital hypotonia. A clinical study in 43 children. Eur J Pediatr. 1987;146:360-364.

Zalzberg J, et al. Benign congenital hypotonia: a disease or syndrome? Harefuah. 1985;108:104-105.

Farkas-Bargeton E, et al. Delay in the maturation of muscle fibers in infants with congenital hypotonia. J Neurol Sci. 1978;39:17-29.