Hypotonia, or abnormally low muscle tone, is by itself not a disorder but a symptom of an enormous array of issues - many of which can be difficult to diagnose accurately. Even in the absence of a specific underlying diagnosis, however, children with hypotonia can benefit from clinical intervention.
Hypotonia can result from damage to the brain, spinal cord, nerves, or muscles, or may be a result of genetic, muscular, or central nervous system (CNS) disorders. The condition appears independently from muscle weakness, although the two may coexist in some disorders, such as motor neuron disease or multiple sclerosis. Young children with hypotonia appear “floppy” and may present with inappropriate head lag, astasis, hypermobility, decreased deep tendon reflexes, and problems sucking or swallowing. Older children with hypotonia may exhibit delays in gross motor skills or coordination or problems with ligament and joint laxity, respiratory control, posture, or speech.
Demographically, hypotonia affects both genders equally and is no more likely to occur in one particular ethnic or racial group than another. It is one of the most common muscular abnormalities diagnosed in newborns with disorders such as Down Syndrome, Cerebral Palsy, Prader-Willi syndrome, and Tay-Sachs disease yet, in some connective tissue disorders or certain muscular dystrophies, low muscle tone may not be revealed until later in life. Hypotonia is an associated symptom among many children with autism spectrum disorders, but typically improves over time and responds to clinical intervention.
Neuromuscular specialist for children Thomas O. Crawford, MD, who treats patients at Johns Hopkins Children’s Center in Baltimore, MD, estimates more than 95% of patients he sees for hypotonia can be evaluated with a comprehensive history and physical examination.
“In central issues, muscle tone may be diminished, but there is a discrepancy between tone during power of motion and the resting tone. That’s an important distinction. If I find a child has more vigorous muscle power than tone, that leads me to concentrate on CNS issues,” Crawford explained.
“If a child is weak as well as hypotonic from a central cause, his face is less animated than, for example, a child with spinal muscular atrophy, but in children with congenital myopathies, their faces may look unresponsive but the children are not,” Crawford said.
In the 1970s, neurologists began classifying types of hypotonia, but “benign congenital hypotonia” remained a diagnosis when no cause could be found. Improvements in identifying genetic, neuromuscular, and connective tissue disorders now provide more detailed answers, opening the door to appropriate treatment responses. Still, some families live with the frustration of knowing hypotonia exists without discovering its underlying cause.
In some cases, idiopathic hypotonia resolves within the first few years of childhood, though minor cognitive impairments or developmental delays may persist. Hypotonia caused by hormonal or metabolic disorders, such as rickets or congenital hypothyroidism, must be specifically screened for but can be easily treated.
Ronald D. Cohn, MD, chief of the Division of Clinical and Metabolic Genetics at The Hospital for Sick Children in Toronto, Canada, and an internationally recognized specialist in the genetics and clinical care of children with hypotonia, helped design a diagnostic algorithm to streamline hypotonia assessment for clinicians, distinguishing primary involvement of the upper motoneuron (central hypotonia) versus the lower motoneuron and motor unit (peripheral hypotonia) to indicate, for example, the need for magnetic resonance imaging.
Cohn and genetic counselor Emily C. Lisi, MS, have concluded that hypotonia can be a symptom of more than 600 genetic disorders, with still more waiting to be identified.
“A staged diagnostic approach categorizing patients as having peripheral, central, or combined hypotonia is the most efficient to providing a rational work-up. Establishing a diagnosis is crucial for prognosis, management, and treatment strategies and for ascertaining an accurate recurrence risk for future offspring,” they wrote in a 2011 study.6
Hypotonia in chromosomal abnormalities
“Individuals with Down syndrome are at risk for foot alignment problems due to hypotonia and ligamentous laxity. Both characteristics contribute to joint hypermobility,” said senior physical therapist Patricia C. Winders, PT, director of therapies at the Anna and John J. Sie Center for Down Syndrome at Children’s Hospital Colorado in Aurora.
“The ligaments do not hold the bones together tightly for optimal alignment and function. The joints of the foot have excessive flexibility, which causes instability and inefficient mechanics when standing, walking, running, and jumping,” she said. “The child cannot use his strength effectively because the muscles are not aligned for efficient activation. Since his strength does not generate efficient power, he uses more energy during each skill and fatigues more quickly. The consequences of faulty alignment and mechanics range from impaired performance to pain, which can result in limitations in walking. Since walking will be vital for his entire lifetime, it is very important to be proactive in promoting optimal alignment and function, beginning when he learns to walk.”
The goal of physical therapy for an individual with Down syndrome is to achieve maximal physical potential and to build a body that is fit and functional throughout his or her life, Winders said.
“Because of physical problems [hypotonia, ligamentous laxity, and decreased strength], he is prone to develop compensations, which are ways he adapts to make up for the physical problems. Some compensations, if allowed to persist, will eventually result in inefficient and painful movement patterns that will compromise his function as an adult,” she said. “Physical therapy needs to focus on minimizing the compensations that will lead to impairment of motor functioning and on building the posture, strength, and movement patterns that he will need as an adolescent and adult.”
Hypotonia is a common feature of genetic disorders associated with developmental delay. Children with Angelman syndrome have gait ataxia with tremulous limbs, but may not always have hypotonia. Neonates with Prader-Willi syndrome (PWS), however, have severe hypotonia, evidenced by lethargy and weak or no sucking. As children grow, their gross motor skills are typically delayed; they may sit at 12 months and walk at 24 months, though in some cases walking may be delayed until they are aged 4 or 5 years.
The insatiable appetite that is the hallmark of PWS does not typically present before preschool age, but obesity rates begin to soar during preschool years. Scoliosis; hip dysplasia; respiratory control issues; early risks of osteoporosis; short stature; short, wide feet that require extra care in fitting shoes properly; and the stress of a restricted dietary and home environment pose challenges for the families of children with PWS and their healthcare providers.10
In 2010, Korean researchers found a high prevalence of spinal deformity, limb malalignment, and foot abnormality in PWS, regardless of age or obesity. They urged paediatric orthopedic surgeons to evaluate PWS patients annually for these conditions because of their possible concealment by obesity.
Early intervention with occupational and physical therapy and lifelong strength training and aerobic conditioning are critical to addressing overall health status, yet therapists must be aware that many patients with PWS have decreased sensitivity to pain. Any evidence of pain should be promptly addressed, as it may suggest a serious but masked underlying problem, such as a fracture or abnormality.
Cohn suggests children with mild hypotonia and features such as joint hypermobility, pectus excavatum, pes planus, or cardiac abnormalities be carefully evaluated, as he believes patients with connective tissue disorders who have less severe forms of hypotonia remain dramatically underdiagnosed.
Crawford, too, cautions that, beyond the parameters of classic presentations of obvious congenital disorders of infancy, hypotonia may be missed.
“Children with classic Duchenne muscular dystrophy are not hypotonic as infants and, as a consequence, that diagnosis may not be considered until later. Some pediatricians might not think of Duchenne dystrophy for a boy who at 15 months of age manifests motor and cognitive delay. But weakness doesn’t show up until later. In many of those cases, we are missing diagnoses,” Crawford said. “Any boy not walking by 16 months should have a CK [creatine kinase] screening. In the case of Duchenne, the result will be five digits even though the child may show no signs of weakness.”
Congenital hypotonia may be seen in tandem with joint hypermobility or ligamentous laxity, but, excluding obvious dysmorphic presentations, hypermobility may not be diagnosed until children are school-aged and present with arthralgia, back pain, abnormal gait, or joint deformity. Knees, elbows, wrists, metacarpophalangeal joints, and ankles are most commonly involved, according to British arthritis researchers, who collected data during a three-year period from pediatric rheumatology and hypermobility clinics. Nearly half of the study participants were described in their clinical history as “clumsy,” and more than a third showed signs of poor coordination in early childhood.
Dutch paediatric physical therapists performing a retrospective study concluded that one-third of children with generalized joint hypermobility presented with severe delays in motor development, though there was no significant association between the number of hypermobile joints and the age of independent walking.Joint hypermobility, if associated with hypotonia, may be indicative of Marfan syndrome, Ehlers-Danlos syndrome, osteogenesis imperfecta, or other mild variants of these and other musculoskeletal or connective tissue disorders.
In a 2005 study, physical therapists and occupational therapists found that the specific characteristics of hypotonia included increased flexion, hypermobile joints, round shoulder posture, decreased strength, low activity tolerance, delays in motor development, and poor attention and motivation.
Some studies acknowledge that objective measurement of the degree of hypotonia in an individual poses a challenge because, historically, therapists have defined resistance subjectively based on their methods and areas of expertise. Reliable tools, such as the 2011 Segmental Assessment of Trunk Control (SATCo),which measures discrete levels of trunk control in children with motor disabilities, are being developed.
Billi Cusick, PT, MS, C/NDT, COF, of Telluride, CO, uses SATCo in her practice.
“In my experience managing the alignment and movement disorders that are common in children with hypotonia, the functioning alignment of the joints and the somatosensory system are critical areas of concern,” Cusick told LER.
She pointed to a recent systematic review that determined children with benign joint hypermobility syndrome demonstrated significantly poorer proprioception compared to children without the disorder.
“Mechanoreceptors in load-bearing joints and the skin on the plantar foot deliver sensory information about limb position and weight bearing when they are stimulated,” Cusick explained. “When joint surfaces are malaligned due to laxity in supporting ligaments, those receptors lose appropriate contact, and, presumably, their messages to the central nervous system are compromised. Lax joints fall to end range when loaded, where it appears that the sensory receptors finally detect the functioning positions. This lack of adequate and timely sensory information is evident in postural deviations, such as a wide-based stance and gait, excessive spinal lordosis, anterior pelvic tilt, knee hyperextension, and foot pronation seen commonly in children with hypotonia. For these children, such postural deviations are normal, and they persist without intervention.”
Intervention with physical therapy and orthoses in childhood is key to preventing or managing pain in adolescence and adulthood commonly associated with joint hypermobilility and hypotonia, even if an underlying diagnosis is not determined. Persistent postural deviations and foot joint laxity typically deform the feet and interfere with independent mobility and endurance as body size and weight increase, Cusick said, citing a 2011 study by Wolf et al that recommended managing the pain with prolonged therapy and general conditioning, with special emphasis on improving strength and proprioception.17
Cusick not only has decades of experience treating children clinically, she has used her expertise with her own daughter, Ting, who arrived from China aged 12 months and unable to assume an all-fours position, crawl, get into and out of a sitting position, or pull to kneeling or standing.
“Her hips were very weak,” Cusick said. “She had never taken weight on her knees or feet before, so I began to help her put weight through them in small increments and in a variety of postures. I used a jumper seat suspended on a spring and later fitted her in a pair of support shorts to keep her hips from sliding into full abduction when she attempted to assume all fours.”
Ting was soon crawling and pulling to stand, but her feet were profoundly pronated, her foot ligaments were lax, and her wide stance imposed further pronatory forces on her forefeet. Cusick fitted her with heel cups and sturdy flat-soled sneakers, and Ting was soon cruising. At age 5 years, she began gymnastics; at 6, she started soccer; and at 15, Ting is now a competitive cheerleader.
“Although Ting’s feet are aligned and competent, because she has a tendency toward joint laxity that is typical of the Chinese population, she continues to wear plantar orthotic inserts that protect her feet and knees from the wear that commonly occurs with excessive pronatory strain,” Cusick said.
In caring for patients with low muscle tone or ligament laxity, Cusick relies on management strategies that improve functioning joint alignment and raise the level of - and improve the quality of -sensory input in daily life.
As Cohn and Lisi underscored, an inability to define an underlying diagnosis for low muscle tone should not interfere with the ability to manage hypotonia in patients of any age. Symptomatic treatment can and needs to be tailored, they wrote, to create lifelong strategies vital to maintaining strength, reducing pain, and fostering independence.