Hypotonia is a condition of abnormally low muscle tone (the amount of tension or resistance to movement in a muscle), often involving reduced muscle strength.1 Hypotonia is not a specific neurological disorder, but a potential manifestation of many different diseases and disorders that affect motor nerve control by the brain or muscle strength (Table 30-1). Recognizing hypotonia, even in early infancy, is usually relatively straightforward, but diagnosing the underlying cause can be difficult and often unsuccessful. The long-term effects of hypotonia on a child’s development and later life depend primarily on the severity of the muscle weakness and the nature of the cause. Some disorders have a specific treatment but the principal treatment for most hypotonia of idiopathic or neurological cause is physical therapy and/or occupational therapy to help the child compensate for the neuromuscular disability.

Floppy Infant Syndrome

Since hypotonia is most often diagnosed during infancy, it is also known as “floppy infant syndrome” or “infantile hypotonia” (Figure 30-1). Hypotonic patients may display a variety of objective manifestations that indicate decreased muscle tone. Infants with hypotonia have a floppy or “rag doll” appearance because their arms and legs hang by their sides and they have little or no head control. Motor skills delay is often observed, along with hypermobile or hyperflexible joints, drooling and speech difficulties, poor reflexes, decreased strength, decreased activity tolerance, rounded shoulder posture, with leaning onto supports, and poor attention and motivation. The extent and occurrence of specific objective manifestations depends upon the age of the child, the severity of the hypotonia, the specific muscles affected, and sometimes the underlying cause. Hypotonic infants often have difficulty feeding, as their mouth muscles cannot maintain a proper suck-swallow pattern, or a good breast-feeding latch. Hypotonia does not affect intellect. However, depending on the underlying condition, some children with low tone may develop language and social skills later in childhood.

Developmental Delay

Children with normal muscle tone are expected to achieve certain physical abilities within an average time-frame after birth (Tables 30-1 and 30-2). Most low-tone infants have delayed developmental milestones, but the length of delay can vary widely. Motor skills are particularly susceptible to the low-tone disability. They can be divided into two areas, gross motor skills and fine motor skills, both of which are affected. Hypotonic infants are late in lifting their heads while lying on their stomachs, rolling over, lifting themselves into a sitting position, remaining seated without falling over, balancing, crawling, and walking. Fine motor skills delays occur in grasping a toy or finger, transferring a small object from hand to hand, pointing out objects, following movement with the eyes, and self-feeding.

Speech difficulties can result from hypotonia. Low-tone children learn to speak later than their peers, even if they appear to understand a large vocabulary, or can obey simple commands. Difficulties with muscles in the mouth and jaw can inhibit proper pronunciation, and discourage experimentation with word combination and sentence-forming. Since the hypotonic condition is actually an objective manifestation of some underlying disorder, it can be difficult to determine whether speech delays are a result of poor muscle tone or some other neurological condition, such as mental retardation, that may be associated with the cause of hypotonia.

Muscle Tone versus Muscle Strength

The low muscle tone associated with hypotonia must not be confused with low muscle strength. In body building, good muscle tone is equated with good physical condition, with taut muscles, and a lean appearance, whereas an out-of-shape, overweight individual with fleshy muscles is said to have “poor tone.” Neurologically, however, muscle tone cannot be changed under voluntary control, regardless of exercise and diet. True muscle tone is the inherent ability of the muscle to respond to a stretch. For example, by straightening the flexed elbow of an unsuspecting child with normal tone, the biceps will quickly contract as a way of protection against possible injury. When the perceived danger has passed, the muscle then relaxes, and returns to its normal resting state. The child with low tone has muscles that are slow to initiate a muscle contraction, contract very slowly in response to a stimulus, and cannot maintain a contraction for as long as his or her “normal” peers. Because these low-toned muscles do not fully contract before they again relax, they remain lax and fail to maintain a full muscle contraction over time.

Diagnosis

Diagnosing hypotonia in a child includes obtaining family medical history and a physical examination (Table 30-3). Symptoms may vary depending on severity and cause (Table 30-4). The initial examination should be focused on determining whether the hypotonia is a result of an upper motor neuron (central) or lower motor neuron (peripheral) disturbance. Tests and laboratory investigations include computerized tomography (CT) scans, magnetic resonance imaging (MRI) scans, electroencephalogram (EEG), blood tests, genetic testing (chromosome karyotype and tests for specific gene abnormalities), metabolic CSF studies, electromyography muscle tests, or muscle and nerve biopsy in order to determine the cause (Table 30-5). Mild or benign hypotonia is often diagnosed by physical and occupational therapists through a series of exercises designed to assess developmental progress, or observation of physical interactions. Since a hypotonic child has difficulty deciphering spatial location, the child may have some recognizable coping mechanisms, such as locking the knees while attempting to walk. A common sign of low-tone infants is a tendency to observe the physical activity of those around them for a long time before attempting to imitate, due to frustration over early failures. Developmental delay can indicate hypotonia.