Diseases that affect the motor unit may affect the phrenic-diaphragmatic motor unit, upper airway motor unit, or both. The degree of involvement of both units may be the same or one unit may be more involved than the other.
Phrenic-diaphragmatic motor unit diseases produce central apnea by lack of effective diaphragmatic contraction. Upper airway motor unit diseases produce obstructive apnea because the normal contraction of the upper airway muscles that prevent the narrowing of the upper airway (which occurs as the result of the negative pressure created by diaphragmatic contraction) does not occur. Diseases that affect both the phrenic-diaphragmatic and upper airway motor units may present with central, obstructive, or mixed apnea. Apnea due to phrenic-diaphragmatic or upper airway motor unit diseases occur more frequently during active sleep. During active sleep there is a physiologic generalized hypotonia due to hyperpolarization of the anterior horn motor neurons that affects primarily the intercostal muscles. Diseases of the phrenic-diaphragmatic and cranial upper airway motor units may involve the motor neuron, nerves, myoneural junction, or muscles.
The only motor neuron disease that may present with apnea in the neonatal period are Spinal Muscular Atrophy with Respiratory Distress Type 1 and Werdnig-Hoffman disease.
Neonates with Spinal Muscular Atrophy with Respiratory Distress Type 1 look the same as neonates with Spinal Muscular Atrophy (Werdnig-Hoffmann disease). The features that distinguish spinal muscular atrophy respiratory distress type 1 are diaphragmatic weakness, distal limb wasting, distal arthrogryposis and bulky proximal fingers (Figure 32.1). Diaphragmatic weakness may lead to apnea and respiratory distress. Spinal Muscular Atrophy with Respiratory Distress Type 1 is due to a mutation in the gene encoding inmunoglobulin mu-binding protein 2. This gene is located on chromosome 11.

A	B

Figure 32.1.— [A] The bulky proximal digits (prominent fat pad) contrast with the wasted appearance of more distal regions of the fingers. [B] Right diaphragmatic paralysis.

Most neonates with Werdnig-Hoffmann disease do not have apnea because the disease tends to spare the phrenic center even when severe generalized hypotonia is present. When apnea occurs, it is usually central, but obstructive and mixed apnea may occur. The diagnosis of Werdnig-Hoffman disease is confirmed by DNA testing. More about...132