A FLOPPY INFANT
History
You are asked to review a baby on the postnatal ward who has not been feeding well. It is a 1-day-old girl, who is the first baby of a 23-year-old Scottish woman. She was born at 39 weeks’ gestation and the pregnancy was uncomplicated. The baby was delivered with a ventouse and weighed 3.2 kg (25th–50th centile). The mother reports that the baby has not been very active since she was born and that she has hardly latched on to the breast.
Examination
The baby looks alert and is not dysmorphic. Head circumference is 34.5 cm (50th centile) and the anterior fontanelle is normal. She makes almost no antigravity movements and lies in a ‘frog leg’ posture. Respiratory, cardiovascular and abdominal examinations are unremarkable. She appears to make conjugate eye movements in all directions, there is no facial asymmetry and she has normal facial expression. Red reflexes are normal. Tone appears to be symmetrically reduced in the upper and lower limbs, and tendon reflexes cannot be elicited. When she is held vertically, she feels like she is slipping downwards, and she is unable to raise her head at all when held horizontally under her abdomen. The sucking reflex is very weak and there appears to be fasciculation of the tongue.
Questions
• What can be concluded from the neurological examination?
• What additional questions should the mother be asked?
• What is the most likely diagnosis and what are the other possible causes of a floppy infant?
Medicine
This baby has a very abnormal neurological examination. The key findings are severe hypotonia and weakness with preservation of facial and eye movements, absent deep tendon reflexes, poor sucking reflex and fasciculation of the tongue. These features indicate a process affecting the lower motor neurones of the entire spinal cord and selective cranial nerve motor nuclei (IX and XII). In this case it is likely that there is an underlying genetic cause for the baby’s condition, so further questions should be asked to try to substantiate this. Parental consanguinity, history of previous pregnancies ending in miscarriage or stillbirth, other family members with medical problems and pregnancies in other family members should be asked about. In the current pregnancy, there may have been reduced fetal movements. The parents should be asked in detail about whether they have any medical problems, and direct questions about symptoms of neuromuscular disease, including weakness, fatiguability and muscle cramps. In assessing any floppy infant, it is important to establish whether an acquired cause may be present, so an assessment should be made of the maternal drug history and of any drugs given during labour, whether any brain or spinal cord injury may have been sustained during delivery, and any risk factors for sepsis or features of metabolic disease. An infant can be floppy due to ‘central’ or ‘peripheral’ (neuromuscular) causes. In the former, deep tendon reflexes are present, and in the latter they are absent and the child has marked weakness. Central causes are much more common than peripheral causes. In this case, the clinical findings are of a peripheral cause and the most likely diagnosis is infantile spinal muscular atrophy (SMA type 1). SMA is an autosomal recessive condition, with type 1 affecting about 1 per 10 000 live births. Sixty per cent of affected infants will be floppy at birth. Motor neurone degeneration leads to progressive weakness and death usually occurs from respiratory complications, e.g. pneumonia, within the first year of life.
Causes of a floppy
infant ‘Central’ – hypotonic but not ‘Peripheral’– hypotonic, weak, areflexic significantly weak
Chromosomal disorder, Infantile spinal muscular atrophy e.g. Down’s syndrome, Prader-Willi syndrome
Brain injury, e.g. hypoxic-ischaemic Congenital myasthenia encephalopathy
Brain infection or sepsis Congenital myotonic dystrophy
Metabolic disturbance, Congenital myopathy e.g. hypoglycaemia
Drug exposure, e.g. pethidine Congenital muscular dystrophy
KEY POINTS
• An infant can be floppy due to central or peripheral (neuromuscular) causes.
• Assessment of muscle power and deep tendon reflexes helps to classify the likely causes.
• Central causes of hypotonia are the most common.
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