E. Idiopathic apnea of prematurity.
The vastmajority of apneic episodes occurr in infants who are immature and haveno organic disease. It is classified as idiopathic apnea of prematurity. It is a natural consequence of immaturity. It is also expected thatinfants will outgrow apnea of prematurity as they mature. This diagnosiscan be made only after exclusion of all other causes of apnea. (MarchalF. et al. Cl.in Perinatol. 1987 ; 14 : 509-529).
Because, no identifiable cause is determined, idiopathic apnea of prematurity is most likely the cause of apnea in this infant.

Pathogenesis of Idiopathic apnea of prematurity

    There is no single physiologic or chemical explanation completely describes these apneic episodes. Several factors singly or together make the premature infant more susceptible to apnea and their explanations are listed as follow:
    1. Hypoxemia causes respiratory depression in neonate instead of sustained hyperventilation as in adult. Because, it is centrally mediated and not overridden by stimulation from peripheral chemoreceptors. (Rigatto H, BradyJ.Pediarics 1977; 50: 219)
    2. Hypercapnic ventilatory response is diminished in apneic premature infant. It may be secondary to central neural mechanism or respiratory muscle failure. Central respiratory chemoreceptor activity is less well developedin infants < 33 weeks gestation and in infants who are less than 3 weeks postnatal age. (Rigatto H. et al. Pediatrics 1975; 55: 614-620).
    3. Pharyngeal hypotonia and failure of upper airway respiratory muscles (genioglossus, ala nasi) to contract during inspiration my compromise upper airway patency and cause obstructed apnea. (Carlo WA. et al. Pediatrics 1983; 72:338, Thach BT, Stark Ar. J. Pediatr 1979; 94: 295.)
    4. Apnea is most common during active sleep (REM). Because during this stage, respiration is irregular, rib collapses, lung volume drops 30% and PaO₂ falls. [Henderson-Smart D, Reed D. In Sleep Apnea Syndromes. Guilleminault C, Dement WC (eds.) 1978, Alan R Liss, New York.]
    5. Delayed auditory evoked responses is exhibited in apneic infant. Because, there are fewer dendritic synaptic connection in brainstem, asssociatedwith instability of respiratory control. (Henderson,-Smart DJ. etal. N.Engl J Med. 1983; 308: 353-357)