A Floppy Child

A 4-month-old boy was referred for valuation of weakness. He was born after a normal pregnancy and delivery at term, with Apgar scores of 7 and 8 at 1 and 5 minutes, respectively. He presented as a floppy baby with generalized weakness. He was able to suck and swallow but with some difficulty.

His family history was contributory for a maternal female cousin who was floppy and died shortly after birth.

Examination revealed a baby lying in a froglike position ( Fig. 38-1 ). His head circumference was normal at 41 cm, and he had no evidence of ptosis or ophthalmoplegia. The pupils were normal. There were fasciculations in the tongue and diffuse muscle atrophy. Reflexes were absent throughout, and he had profound hypotonia. No Babinski signs were present. He smiled normally and cried when he was pinched. There was no visceromegaly, and the rest of the examination was normal.

What is the Differential Diagnosis?

This patient manifested as a floppy infant, a syndrome that has many causes ( Table 38-1 and Fig. 38-2 ). The absent reflexes are against a central nervous system disorder, and the lack of ophthalmoplegia and ptosis is against a defect of neuromuscular transmission. The presence of tongue fasciculations and the apparent autosomal-recessive inheritance suggest spinal muscular atrophy (SMA).

Table 38-1

Causes of Floppy Infants

Reprinted with permission from Pearn J. Autosomal dominant spinal muscular atrophy: a clinical and genetic study. J Neurol Sci . 1978;38:263–275.

Central nerve system disorders
- Cerebral palsy
- Mental retardation

Mixed (central and peripheral)
- Metachromatic leukodystrophy and other lipidosis
- Neuroaxonal atrophy
- Giant axonal neuropathy
- Fukuyama-type congenital muscular dystrophy

Anterior horn cell disease
- Infantile spinal muscular atrophy

Neuropathies
- Charcot–Marie–Tooth disease, particularly types 3 and 4

Diseases of the neuromuscular junction
- Congenital myasthenic syndromes
- Infantile botulism
- Neonatal transient autoimmune myasthenia gravis

Myopathies
- Infantile metabolic myopathies (e.g., acid maltase deficiencies or Pompe disease, infantile phosphorylase deficiency)
- Congenital muscular dystrophy
- Other congenital myopathies (e.g., central core disease, myotubular myopathy, nemaline myopathy)
- Congenital myotonic dystrophy
- Myopathy from electrolyte and endocrine abnormalities

A complete metabolic panel, complete blood count, serum creatine phosphokinase, and thyroid-stimulating hormone were normal.

What Should Be the Next Test?

With this clinical presentation, lymphocyte DNA analysis to detect deletions of chromosome 5q, typical of SMA, is important and should be the first diagnostic test when there is a positive family history. An EMG is, however, particularly useful to rule out other causes, and it was done in this case, as DNA testing was not available at that time. A muscle biopsy could also be used to rule out other causes.

An EMG Test was Performed

Motor Nerve Studies

Nerve and Site	Latency (ms)	Amplitude (mV)	Conduction Velocity (m/s)
Median Nerve R.	Normal ≤ 1.6	Normal ≥ 5	Normal ≥ 35
Wrist	2.2	1.5	–
Elbow	3.8	1.5	50

Sensory Nerve Studies

Nerve	Peak Latency Normal ≤ 2.0–2.9 (ms)	Amplitude Normal ≥ 20–70 (μV)
Median nerve R.	2.2	24

EMG Data

Nerve	Insrt Activity	Fibs	Pos Waves	Fasc	Amp	Dur	Poly	Pattern
Deltoid R.	Inc	2+	2+	Few	Norm	Norm	None	Red
Biceps brachii R.	Inc	1+	1+	None	Lg	Inc	None	Red
Vastus lateralis R.	Norm	None	None	Few	Norm	Norm	None	Red
Gastrocnemius R.	Inc	None	None	Few	Lg	Inc	None	Red
Vastus lateralis L.	Inc	1+	1+	None	Lg	Inc	None	Red