Hypotonia due to presynaptic myoneural junction disorders may result from destructive, metabolic, or dysgenetic problems (Figure 138.1).

Figure 138.1.— Salient features of generalized hypotonia due to presynaptic myoneural junction dysfunction. Arrow indicates the anatomical location of the injury (presynaptic myoneural junction); SNST: slow nerve stimulation test; RNST: rapid nerve stimulation test; C: Clostridium; DES: destructive; MET: metabolism; DYS: dysgenesis.

Infantile Botulism
Infantile botulism results from an intestinal infection by Clostridium botulinum, Clostridium baratti or Clostridium butyricium. Clostridium botulinum infection are most frequent. Frequent sources of C botulinum are honey (neonates and infants should not be given honey), soil, and dust. Infants are especially vulnerable to colonization by C botulinum the first week after breast milk is stopped.
The toxin produced by this organism in the intestine enter the blood and ultimately the toxin reaches the presynaptic component of the myoneural junction. In the presynaptic area the neurotoxin cleaves a protein that has an integral role in the neurotransmitter exocytosis process. This damage results in failure of acetylcholine release. Failure of acetylcholine release affects the nicotinic (striated muscle) and muscarinic (smooth muscle) myoneural junctions.
The clinical presentation of botulism is characterized by the onset of constipation and feeding difficulty in a previously healthy neonate. This is followed by hypotonia with decreased dynamic tone. The hypotonia involves the facial and bulbar muscles first. Pupillary reaction is usually absent or, if present, is weak. Pupillary reaction may get progressively stronger with repetitive stimulation if the light is turned on and off at a fast rate (over 20 times per second) or progressively weaker if the light is turned on and off at a slow rate (2 to 3 times per minute). This phenomena occurs because of two physiologic factors: (1) acetylcholine has a duration of action of 100 milliseconds in the synaptic cleft, and (2) the number of quanta of acethylcholine released progressively decreases with each stimulation. Hence, fast rate repetitive stimulation allows the temporal summation acetylcholine whereas slow rate repetitive stimulation do not.