Chapter 17: Neonatal Hypotonia

HISTORICAL PERSPECTIVE

It is difficult to find any reference to neonatal hypotonia before the beginning of the 20th century. This may be in part related to the very high infant mortality rate and the fact that for most of history children were not considered of much value until they had reached the age of 7 or so, when they had survived the vicissitudes of childhood and could start working.¹ With the advent of the Industrial Revolution and the fundamental changes in medicine that accompanied it, the first reports of infantile hypotonia were published (for an excellent summary of this history, refer to Victor Dubowitz’s seminal work on the floppy infant²). Since the 1970s, and likely as a result of the exponential improvements in diagnostic tools, the scientific publications dedicated to hypotonia have steadily accumulated (Figure 17-1).

The earliest reports² of congenital hypotonia are probably those by Werdnig in 1891 and Hoffman in 1893. They independently described infants, apparently healthy at birth, who subsequently developed hypotonia and respiratory failure. Both of these authors described the pathology associated with spinal muscular atrophy (SMA). In 1900, Oppenheim described a series of infants born hypotonic and hyporeflexic who subsequently appeared to improve with no obvious residual pathology. He suggested the term myatonia congenita, later renamed amyotonia congenita. In 1957, Walton suggested the term benign congenital hypotonia to refer to cases in which the prognosis was “mild and may be overcome in time.”³ His major point was to distinguish patients with a grim prognosis, as is the case in SMA, from those who faired better. Although the choice of the term benign was an unfortunate one that has led to considerable confusion, the diagnostic challenge that he was facing is still the dilemma that faces today’s clinician. That is, what is the pathology behind the apparent hypotonia, and perhaps more importantly, what is the prognosis? The object of this chapter is to offer a rational approach to the diagnostic workup of the hypotonic infant.

DEFINITION: HYPOTONIA VS WEAKNESS

The first and perhaps most important step in evaluating a hypotonic infant is to ask the question: Is the infant truly hypotonic, or does the perceived hypotonia reflect weakness? The difference can be subtle but is of the essence since this difference is the first step in elucidating the etiology of the observed phenotype. Tone is generally defined as resistance to passive movement.⁴ Tone can be further divided into postural or truncal tone (proximal muscle groups) and phasic or peripheral tone (extremities). In contrast, weakness refers to lack of muscle strength.⁵ Therefore, in its simplest terms, strength can be thought of as the ability to generate movement and tone the ability ...