When you first observe or suspect an infant has hypotonia, you face a decision whether the condition is benign and likely to resolve over time vs. a more serious condition with an important neurologic basis.
You will be in a position to make that call in many cases in your practice. Related observations include spontaneous movement during a physical examination; basic laboratory testing; and any relevant family history of genetic-based disease. These can go a long way to guide your diagnosis. Any abnormality in growth, feeding patterns, or respiration also provides important clinical clues.
Pediatricians, depending on their experience, can determine when a "wait and watch" approach is appropriate. For example, benign hypotonia is more likely when there are no major delays in growth or motor milestones, no signs of abnormal respiration, and parents report normal feeding patterns.
Since we cannot test the strength of a 7- or 8-month-old infant by asking them to offer resistance with their arms and legs, it’s useful to observe the amount and quality of their spontaneous movements. Watch their head control when you pull the infant to sit from a supine position. Does the head lag behind at all? How vigorously does the infant kick or grasp? These signs can help examiners compare impressions of degree of weakness, if there is any, in hypotonic infants.
Parents may be unaware of any low muscle tone, so it’s important to include hypotonia on your physical examination checklist.
Ask parents about the strength of the infant during diaper or clothing changes. Often parents have a good sense of the amount of force their child displays, and it can be reassuring to make these observations in the examination room as well.
An infant with a benign form of congenital hypotonia will move her arms and legs frequently: batting, roving, and sometimes synchronous movements come and go depending on level of alertness. In contrast, an infant with hypotonia caused by a neurologic disease, such as spinal muscle atrophy, will move much less frequently and the initiation of the movement(s) is slower.
In some cases, the degree or cause of hypotonia may not be clear, and the question becomes what to do with that uncertainty. A creatine phosphokinase (CPK) assay, the most common initial laboratory test to assess low muscle tone, can be helpful, although a normal result doesn’t rule out muscle or nerve disease.
In addition, check the baby’s respiratory rate and listen for any stridor or other signs of obstruction in her breathing. Noisy breathing can suggest upper respiratory muscle involvement, in which case further work-up is warranted. How’s the head growth over time?
Testing deep tendon reflexes can be tricky. While the presence of reflexes is reassuring, absence suggests a peripheral nerve problem or anterior horn cell disease.
Ask parents about the baby’s typical feeding pattern. An infant that seems to take a long time to feed and/or has an interrupted pattern of feeding should raise your clinical suspicion about an important neurologic cause for their hypotonia.
Family history can be an independent risk factor for a more serious, inherited neurologic etiology. Many of the causes of hypotonia are genetic, and specialist input will be appropriate if you uncover relevant family history. Inquire about any first-degree relative or others with an inherited neurologic condition that could underlie the infant’s hypotonia. In some cases, a non-neurologic inherited disease such as Marfan syndrome can cause hypotonia as well.
Specialist input is indicated if you suspect a genetic basis for the hypotonia, even if the infant passes the other important elements of your evaluation (for example, normal milestones, eating, and breathing).
These patients are a pretty diverse group, and there are many infants with hypotonia who will ultimately do well. If you are unsure, it’s definitely worth tracking these patients a little closer using history and physical examination checklists. Check in with a specialist if you continue to be unsure as you move forward.
Dr. Bingham is a pediatric neurologist at Fletcher Allen Health Care and associate professor of neurology at the University of Vermont in Burlington. Dr. Bingham said he had no relevant financial disclosures.