Floppy Infant Approach
Approach to a floppy infant. After an introduction to a floppy infant, we will discuss the courses and then go into the important points in history, physical examination, and investigations. The definition of a floppy baby is one with generalized hypotonia, as evidenced by unusual posturing. You may find the child in a frog-like posture, with the hips abducted and externally rotated, and the knees flexed. There is reduced resistance to passive movements, and an increased range of joint movement. In a critically ill child, emergency treatment and airway management takes precedence over making a definitive diagnosis. Narrow down the differentials by determining if the hypotonia is of central or peripheral cause. First rule out intercurrent illness and sepsis.
A critically ill child may not have normal tone, and treating the infection should take priority. Don't forget to check for prematurity. The causes are broadly divided into central versus peripheral causes. I will describe how to clinically differentiate between the two in a short while.
Looking under central causes, hypothyroidism is an easily corrected cause and should be screened for. Other central causes including chromosomal disorders, of which Trisomy 21 is common. Metabolic disease, cerebral malformations, evolving cerebral palsy, infections like TORCH infection, trauma, and the use of drugs, especially sedative drugs. When thinking about peripheral causes, think through the anatomy of the peripheral motor unit. Stop at the anterior horn cell, which can be affected in spinal muscular atrophy, or poliomyelitis.
Move down the peripherial motor unit. The abnormality could lie in the peripheral nerve. These include acquired and hereditary causes. It could also be localized to the neuromuscular junction, as in Myasthenia Gravis. And finally, in the muscles, as inducing muscular dystrophy or congenital myopathies. Always ask into history, when was the hypotonia first noticed? Is it of new onset? Importantly, is there any developmental regression? Is the involvement of the hypotonia equal in the truncal regions as in the limbs? Remember to take a very thorough developmental history, as well. In the antenatal history, ask about poor fetal movements, breach presentation, or the use of maternal sedative drugs. Was the birth process a traumatic one? Was there any evidence of birth anoxia? Take a direct family history, asking about weakness.
Check for parental consanguinity. This is relevant especially when suspecting diseases of autosomal recessive inheritance. Remember to ask about the presence of any previous early deaths, or recurrent miscarriages. On systemic review, ask about feeding difficulties, as well as difficulty breathing, as well as the presence of any fever or seizures. Introduce yourself and shake the hand of the parents.
Observe for any obvious abnormalities. You may just be able to pick up signs of myotonic dystrophy or myasthenia gravis in the parents. In the child, observe for dysmorphic features. Measure the occipital frontal circumference. Look at the facial features. Is the child alert or obtunded in appearance? Examine the eyes. Look for ptosis, as well as the presence of any ophthalmoplegia. Look for the presence of any interventions already in place.
These may include nasogastric tube for feeding or oxygen supplementation, or even noninvasive ventilator support like c-pap. Always check the tongue for fasciculations. Inspect the posturing in detail. Also look at the way the child is breathing. Is there an abnormal chest shape, or is paradoxical breathing present? Next do the 180 degree maneuver. Start with the infant in the supine position.
Inspect for any anti-gravity movements. Pull to sit. Look for head lag, which should be minimal by the time the child is three months old. Is the child able to sit unsupported? By six months, an infant and can sit with a rounded back with assistance, while by 10 months, an infant should be able to sit with a straight back independently. Hold the child by the axilla and pull the child up, attempting to wait there. Does the infant slip through your hands? Is there any lower limb scissoring that would suggest evolving cerebral palsy? Hold the child in horizontal ventral suspension. If the child was hyotonic, there would be an inverted U sign, as the infant is able to maintain posture in the horizontal plane. Finally, place the child in the prone position with the hands opposed to the bed.
On either side of the trunk, the child should be able to push up on arms halfway by three months, fully extending the arms by six months, and crawl by nine months. Do a full neurological examination to localize the lesion. Do not that infants with hypotonia of central origin tend to be less weak. They have either normal or hyperreflexia, and persistence of primitive reflexes. On the other hand, lower motor neuron lesions cause a child to be very weak, with a paucity of anti-gravity movements. And these infants also have hyporeflexia. Recognize the classical patterns. Is the weakness more significant in the proximal or distill muscle groups? Is there associated facial involvement? Finally, don't forget to check for the presence of hepatosplenomegaly, or cardiac failure.
These may be present in metabolic causes. Investigations depend on your assessment whether this is of central or peripheral cause. Investigations for central hypotonia. Thyroid function test. Neuroimaging. Karyotype in the presence of dysmorphisms.
TORCH screen. And specific metabolic tests, like serum ammonia, serum amino acids, and urine organic acids. For peripheral hypotonia, investigations include CK levels, or creatine kinase levels, electromyography, muscle biopsy, and nerve conduction studies. Also look for specific genetic abnormalities in specific cases, like SMN gene deletion in suspected cases of spinal muscular atrophy.
Remember you do serial neurological examinations and monitor the progress of the child's weakness. Watch also for respiratory and feeding difficulties. Importantly, rule out common and easily correctable conditions first. Ensure the patient is stable before doing further tests to obtain a definitive diagnosis. Always observe for anti-gravity movements. If the baby appears to have relatively good power, this is more likely to be hypotonia secondary to a central cause. Check the occipital frontal circumference.
If the baby's microcephalic, this points to a central cause. Remember to take a thorough family history. And finally, narrow down the differential by first determining if the hypotonia is central or peripheral in origin.
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