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based on outdated data, mostly before the MRI era.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