Discussion
Respiration is regulated by two separate neural mechanism – voluntary and automatic. 7 The automatic process is regulated by the respiratory centers in the brainstem. The chemoreceptors in the carotid and aortic bodies send signals to the medulla which innervates the respiratory muscles. In awake state there is a continuous voluntary mechanism to maintain ventilation. During sleep the voluntary mechanism is lost and anatomical and functional integrity become more important.8
Here in this case our patient had a healthy life with no prior history of any chronic respiratory illness until he developed a viral pneumonia. He developed acute type 2 respiratory failure requiring noninvasive ventilation. His arterial blood gas characteristics were suggestive of alveolar hypoventilation. His respiratory failure did not improve even after the treatment of pneumonia. Persistent unexplainable respiratory failure and bradypnea made us think of an alternate diagnosis of central cause for hypoventilation which was supported by his new onset occasional difficulty in swallowing. MRI substantiated our diagnosis. His hypoventilation was masked until he had a normal life. Central hypoventilation became evident when he had increased demand of ventilation due acute illness.
Type-I is the most common form of ACM and has been estimated to occur in 1 in 1000 births and mostly asymptomatic. ACMs cases are often detected coincidently while undergoing unrelated investigations.9 Although ACM is still listed as a rare disorder by the Office of Rare Diseases Research (ORDR), this classification is based on outdated data, mostly before the MRI era. The introduction of MRI has increased the frequency of ACM case discoveries.
The prevalence rate of Type-I ACM is ranged between 0.1-0.5% and less predominant in males. 10 Genetic basis for Type-I ACM has been often suggested. 11,12 Recent studies favor the presence of genetic linkage to chromosomes 9 and 15.13 Other researchers suggested that Type-I ACM is a disorder of primary para-axial mesodermal origin, which could affect the formation of axial skeletal defect and different neurological anomalies.14
Type-I ACM is a common structural disorder characterized by motor deficiency, sensory loss, lower cranial palsy, and cerebellar syndrome. 15-18 Neurological symptoms associated with Type-I ACM are including mild to severe sensory deficit, lower cranial nerve palsies, headache, neck pain, and ataxia. Respiratory-related signs and symptoms seems to be unusual19 or properly unmasked by other conditions,17 and often associated with central alveolar hypoventilation 17,19 which is usually the trigger for clinicians to consider ACM not the respiratory failure, and this is typically what led us to make this observation. To the authors’ knowledge, respiratory failure as the first manifestation of ACM has only been reported once. 15