Infantile hypotonia

• Infantile (floppy infant) hypotonia is reduced resistance to passive movement in a baby, noticed as floppiness, poor head control and difficulty against gravity.
• Roughly 60 to 80 percent of hypotonia in young children is central in origin, from brain, brainstem or cervical cord problems, and the rest is peripheral, from the motor unit. ^[1]
• A useful bedside rule: an alert infant with normal sleep-wake cycles but profound weakness and reduced or absent tendon reflexes points to a peripheral cause; a depressed or irritable infant with preserved strength but poor tone and normal or brisk reflexes points to a central one. ^[2]
• Examination focuses on head lag on pull-to-sit, vertical (axillary) suspension, ventral (prone) suspension, tendon reflexes and tongue fasciculations, together with dysmorphic features and examination of the mother. ^[3]
• Investigations are guided by whether the problem looks central or peripheral: creatine kinase, genetic tests, electromyography and nerve conduction studies, imaging, and, when needed, muscle or nerve biopsy. ^[3]
• Early red flags are respiratory insufficiency, poor suck and swallow, congenital contractures and absent reflexes, because they can signal a severe neuromuscular disorder with a poor prognosis. ^[3]

By the end of this page you should be able to:

1. Define hypotonia and explain the difference between tone and strength.
2. Divide the causes of infantile hypotonia into central, peripheral and combined groups, and give common examples of each. ^[2]
3. Recognise the clinical features that separate a central from a peripheral cause.
4. Elicit and interpret head lag, vertical suspension, ventral suspension and tendon reflexes in a floppy infant.
5. Plan a sensible investigation strategy using creatine kinase, genetic tests, electromyography, neuroimaging and biopsy.
6. Produce a diagnostic flowchart for a floppy infant and say when to refer.

Tone can be resting (phasic) or active (postural/antigravity). Practical bedside tests probe both. Several maneuvers are commonly used to identify hypotonia in infants: pull-to-sit (head lag), scarf sign, vertical or axillary suspension (slip through hands), and ventral or horizontal suspension (rag doll posture). ^[1] Reduced tone in the neck and trunk (axial) differs from reduced tone in the limbs (appendicular), and noting which is affected helps localize the problem. ^{[1] [3]}

Hypotonia is a symptom, not a diagnosis. It tells you that the systems that normally maintain resting and active muscle tone are not working properly, but it does not tell you why. Those systems run from the cerebral cortex and brainstem down through the spinal cord, anterior horn cells, peripheral nerves, neuromuscular junctions and muscle fibres. Dysfunction at any level can make an infant floppy.

Central hypotonia results from damage above the level of the spinal motor neuron and accounts for roughly 60 to 80 percent of cases in young children. ^[1] It includes encephalopathies, genetic syndromes, structural brain abnormalities and delayed development without overt brain pathology. ^[1] Peripheral or motor-unit hypotonia includes damage to the anterior horn cells, peripheral nerves, neuromuscular junction or muscle. ^[1] Some conditions affect both levels, for example hypoxic-ischaemic encephalopathy and certain metabolic or degenerative disorders. ^[2]

Tone usually develops and supports posture before voluntary movement appears, so parents often notice a baby who slips through their hands or cannot hold the head up, rather than one with isolated weakness.

Listing causes by the level of the nervous system that is affected keeps the differential organized.

• Hypoxic-ischaemic encephalopathy and other perinatal brain injuries. Infants with cerebral palsy due to hypoxic-ischaemic injury often present initially with generalized hypotonia, with hypertonia developing in the limbs over time while the trunk and neck may remain hypotonic. ^[1]
• Intracranial haemorrhage or infection, including congenital infections such as toxoplasmosis, rubella, cytomegalovirus and herpes, which raise the likelihood of central nervous system dysfunction. ^[3]
• Structural malformations of the brain or brainstem, neuronal migration defects and disorders of the posterior fossa. ^[3]
• Genetic and chromosomal syndromes such as Down syndrome (trisomy 21) and Prader-Willi syndrome, in which hypotonia is a constant feature together with characteristic facial and body features, reduced deep tendon reflexes, feeding difficulties and hypogonadism. ^[3]
• Hypothyroidism, congenital. In a delayed-diagnosis cohort, hypotonia was found in 72 percent of children with congenital hypothyroidism, alongside constipation and cretinoid facies with macroglossia. ^[5]
• Metabolic and mitochondrial disease, including inborn errors of metabolism. Suspect these when a previously well infant develops hypotonia after a period of normality, or when there is multisystem involvement. ^[3]
• Systemic illness such as sepsis, electrolyte disturbance and prematurity, which can produce transient hypotonia. ^[1]

• Spinal muscular atrophy (SMA). An autosomal recessive disorder caused by loss or mutation of the SMN1 gene on chromosome 5q13, with a prevalence estimated between 1 in 6000 and 1 in 10 000 births and a carrier frequency of roughly 1 in 40 to 1 in 50. ^[4] The severe infantile form (type 1, onset before 6 months) presents as floppy infant syndrome with severe generalized hypotonia, poor suck and swallow from facial and bulbar weakness, and common tongue atrophy and fasciculations; infants cannot control the head or sit. ^[4]
• Infant botulism. An age-limited illness in which Clostridium botulinum is ingested and produces toxin in the intestine. It usually presents between six weeks and one year of age, most often between 2 and 26 weeks, with constipation as the first symptom followed by listlessness, ptosis, facial weakness, poor feeding and descending flaccid paralysis. ^[2] In 20 percent of cases honey or corn syrup is the source. ^[2]
• Congenital myopathies and congenital muscular dystrophies, in which creatine kinase and muscle pathology help separate subtypes. ^[3]
• Congenital myotonic dystrophy, usually inherited from an affected mother. Examining the mother for myotonia and facial weakness can provide the clue, because the infant's hypotonia is often severe. ^[3]
• Disorders of the neuromuscular junction, including transient neonatal myasthenia from maternal myasthenia gravis, congenital myasthenic syndromes and aminoglycoside or magnesium toxicity, which share facial diplegia, ptosis, feeding difficulty and generalized weakness. ^[2]
• Peripheral neuropathies, which may be associated with very slow nerve conduction velocities, arthrogryposis and, in some cases, a markedly elevated cerebrospinal fluid protein. ^[3]
• Metabolic myopathies such as Pompe disease, suggested by cardiac enlargement on a chest radiograph. ^[3]

Several diseases damage both levels, so that a baby can have dysmorphic features or a depressed consciousness together with profound weakness and absent reflexes. Examples include hypoxic-ischaemic encephalopathy, lipid storage and lysosomal disorders, mitochondrial disease and infantile neuroaxonal degeneration. ^[2] Always reconsider the classification when the picture does not fit neatly into one column.

• Immediately dangerous and treatable: infant botulism with respiratory compromise, hypothyroidism (irreversible neurodevelopmental harm if treatment is delayed), sepsis or severe electrolyte disturbance, and some inborn errors of metabolism. ^[2][3][5]
• Urgent but staged: spinal muscular atrophy type 1, for which modern disease-modifying therapy is most effective before irreversible motor neuron loss. ^[4]
• Important for counselling and surveillance: Down syndrome, Prader-Willi syndrome and the congenital myopathies, where the emphasis is on feeding support, developmental therapy and monitoring. ^[3]

Other alarm features include a need for mechanical ventilation soon after birth, Apgar scores that were low with birth asphyxia suggesting central injury, a history of poor fetal movements or polyhydramnios, and later onset of hypotonia after an initial period of normality, which points towards an inborn error of metabolism. ^[3]

Take a structured history because it often narrows the possibilities before the baby is examined. ^[3]

• Pregnancy and delivery: reduced fetal movements, polyhydramnios, breech presentation, parental age, consanguinity, exposure to drugs or teratogens, maternal diabetes or epilepsy, maternal muscle weakness or myotonia, and delivery complications with a low Apgar score. Reduced movements and a short umbilical cord point to a neuromuscular disorder present before birth. ^[3]
• Postnatal course: timing of onset of hypotonia, degree and progression of weakness, feeding and respiratory difficulties, apnoea, seizures, and response to stimulation.
• Family history: neuromuscular disease, developmental delay, premature death and repeated miscarriages, suggesting prenatal-onset, chromosomal, metabolic or muscle disease. ^[3]
• Medications and exposures: maternal magnesium sulfate, aminoglycoside treatment in the infant, and possible honey or corn syrup ingestion. ^[2]
• Development: delayed head control, poor antigravity movement, difficulty sucking and swallowing.

A physical examination of the mother is valuable when congenital myotonic dystrophy or neonatal myasthenia is suspected: look for myotonia, facial weakness and fatigable ptosis or arm weakness on sustained abduction. ^[3]

Examine generally first. Note dysmorphic features, skin findings, contractures, chest shape and cardiac size. Look for characteristic facies, macroglossia and large fontanelles in hypothyroidism, and for the facial appearance and hypogonadism of Prader-Willi syndrome. ^[3][5]

Neurological examination then separates central from peripheral causes (see the table below). The key signs are:

• Posture: a frog-legged posture with the hips abducted and externally rotated and flaccid extension of the arms is typical of hypotonia. ^[3]
• Head lag (pull-to-sit): marked lag of the head when the baby is pulled from supine to sitting, and head bobbing when sitting, indicate poor neck and trunk tone. ^[1][3]
• Vertical (axillary) suspension: hold the infant under the arms and lift; a hypotonic baby slips through the hands or shows little resistance, and scissoring or crossing of the legs suggests spasticity rather than simple hypotonia. ^[1][3]
• Ventral (prone) suspension: when held horizontally face down, the normal infant lifts head and limbs; the hypotonic infant drapes like a rag doll. ^[1][3]
• Spontaneous movements: paucity of antigravity movement suggests weakness, whereas infants with pure central hypotonia can have relatively preserved power. ^[1][2]
• Tendon reflexes: normal or brisk reflexes point to a central lesion; reduced or absent reflexes point to a lower motor neuron or muscle problem. ^[2][3]
• Fasciculations: tongue fasciculations strongly suggest spinal muscular atrophy, although normal random tongue movements can be mistaken for fasciculations. ^[3]
• Extraocular movements: sparing of eye movements in anterior horn cell disease versus ptosis and ophthalmoparesis in neuromuscular junction disorders. ^[3]
• Postural reflexes: generally preserved despite few spontaneous movements in central hypotonia, but absent or diminished in peripheral hypotonia; the Moro reflex may be exaggerated in some acute encephalopathies. ^[3]

The distinction is not always pure: conditions such as hypoxic-ischaemic encephalopathy, lysosomal and mitochondrial disease and infantile neuroaxonal degeneration can produce features of both. ^[2]

Let the clinical picture, and especially the central versus peripheral impression, guide the work-up. ^[3] A schematic approach is: clinical assessment first, then targeted investigations, with genetic and metabolic tests selected to the phenotype.

• Creatine kinase (CK). Helpful for muscle disorders such as congenital muscular dystrophies, metabolic myopathies and some congenital myopathies. Draw CK before electromyography or muscle biopsy, because these procedures can cause a falsely elevated CK. ^[3] CK can be raised about 10-fold for up to one week after a normal vaginal delivery, and may be even higher in acidosis, for example after severe birth asphyxia. ^[3]
• Thyroid function tests (TSH and free T4) in any floppy infant with lethargy, feeding difficulty, constipation, prolonged jaundice, large fontanelle, macroglossia or suspected hypothyroidism. ^[5]
• Metabolic screening (lactate, carnitine, acylcarnitine profile, amino acids, urine organic acids) when there is multisystem involvement or late-onset hypotonia after a period of normality. ^[3]
• Blood count, electrolytes and inflammatory markers to exclude sepsis, electrolyte disturbance and acidosis. ^[3]

Karyotype detects chromosomal duplications, deletions and trisomies such as trisomy 21. ^[3] Molecular genetic tests are selected to the phenotype: deletion testing for spinal muscular atrophy (SMN gene on chromosome 5q) and trinucleotide-repeat testing for myotonic dystrophy. ^[3][4] In Prader-Willi syndrome, genetic imprinting should be checked. ^[3] Genetic evaluation is recommended early when hypotonia is unexplained. ^[4]

Cranial and spinal imaging with ultrasound, computed tomography or magnetic resonance imaging is most useful in suspected central hypotonia, where it can show structural malformations, neuronal migration defects, posterior fossa and brainstem abnormalities, and features suggestive of mitochondrial or metabolic disease. ^{[3] [2]}

Nerve conduction studies and electromyography assess the lower motor unit. Electromyography is particularly helpful for spinal muscular atrophy and neuromuscular junction disorders such as botulism and congenital myasthenic syndromes. ^[3] Nerve conduction studies are reliable after about 32 weeks of gestation; slow velocities or conduction block favour peripheral nerve disease, and neuromuscular junction disorders. ^[3]

Muscle or nerve biopsy, with immunohistochemistry and electron microscopy, may be needed even when electrophysiology is normal, because it is the method of choice for distinguishing myopathies from muscular dystrophies and can guide molecular testing. ^[3] Biopsy is invasive and decisions should involve the multidisciplinary team.

A chest radiograph can show cardiac enlargement suggesting cardiomyopathy or thin ribs from reduced fetal breathing movements, an important clue to antenatal neuromuscular disease. ^[3] Lumbar puncture is useful when neuromuscular infection or particular degenerative conditions are suspected, including a markedly elevated cerebrospinal fluid protein. ^[3]

The cause-specific treatments for each peripheral and central disease are covered in their own topics. The supportive measures that matter for almost every floppy infant are these:

• Respiration: many affected infants need prolonged mechanical ventilation and careful chest physiotherapy to clear secretions. ^[3]
• Feeding and nutrition: feed initially by nasogastric tube, with a gastrostomy if swallowing remains unsafe; monitor weight closely because excess weight gain worsens weakness. ^[3]
• Positioning and physiotherapy: regular physiotherapy prevents contractures and limb deformity. ^[3]
• Anaesthesia precautions: children with neuromuscular disease are sensitive to muscle relaxants and susceptible to malignant hyperthermia, so these agents should be used only when necessary and implicated drugs avoided. ^[3]
• Cause-directed therapy: levothyroxine 10 to 15 micrograms per kilogram per day orally for congenital hypothyroidism, started as soon as possible and preferably within the first two weeks of life; ^[5] meticulous supportive care, with attention to nutrition, ventilation and nursing, for infant botulism, for which antibiotics are not helpful; ^[2] and early referral and multidisciplinary care for spinal muscular atrophy. ^[4]
• Parental counselling: discuss prognosis, the appropriateness of resuscitation and the genetic implications, sensitively and early. ^[3]

Low-resource note: where imaging, electrophysiology or genetic testing are limited, a careful history and examination, basic blood tests and simple chest radiography still allow a useful central versus peripheral classification and safe supportive care; refer early when ventilation or feeding support is not available locally.

Refer any floppy infant to paediatric neurology early, and the same day when there is respiratory insufficiency, feeding failure, congenital contractures, absent reflexes or a suspicious family history. ^[3] Early referral is especially important when a diagnosis of spinal muscular atrophy is possible, because disease-modifying treatment is time-critical, and when infant botulism is suspected, because progression can be rapid. ^[2][4] Arrange follow-up with respiratory and therapy services and genetic counselling. ^[3]

• Spinal muscular atrophy
• Infant botulism
• Down syndrome
• Prader-Willi syndrome
• Congenital myopathies
• Congenital myotonic dystrophy
• Congenital hypothyroidism
• Hypoxic-ischaemic encephalopathy
• Inborn errors of metabolism