Behavioural and neurological disturbances associated with hypoxic brain damage

Hypoxia reveals posterior thalamic, cerebellar, midbrain, and limbic deficits in congenital central hypoventilation syndrome

Congenital central hypoventilation syndrome (CCHS) patients show deficient respiratory and cardiac responses to hypoxia and hypercapnia, despite apparently intact arousal responses to hypercapnia and adequate respiratory motor mechanisms, thus providing a model to evaluate functioning of particular brain mechanisms underlying breathing. We used functional magnetic resonance imaging to assess blood oxygen level-dependent signals, corrected for global signal changes, and evaluated them with cluster and volume-of-interest procedures, during a baseline and 2-min hypoxic (15% O(2), 85% N(2)) challenge in 14 CCHS and 14 age- and gender-matched control subjects. Hypoxia elicited significant (P < 0.05) differences in magnitude and timing of responses between groups in cerebellar cortex and deep nuclei, posterior thalamic structures, limbic areas (including the insula, amygdala, ventral anterior thalamus, and right hippocampus), dorsal and ventral midbrain, caudate, claustrum, and putamen. Deficient responses to hypoxia included no, or late, changes in CCHS patients with declining signals in control subjects, a falling signal in CCHS patients with no change in controls, or absent early transient responses in CCHS. Hypoxia resulted in signal declines but no group differences in hypothalamic and dorsal medullary areas, the latter being a target for PHOX2B, mutations of which occur in the syndrome. The findings extend previously identified posterior thalamic, midbrain, and cerebellar roles for normal mediation of hypoxia found in animal fetal and adult preparations and suggest significant participation of limbic structures in responding to hypoxic challenges, which likely include cardiovascular and air-hunger components. Failing structures in CCHS include areas additional to those associated with PHOX2B expression and chemoreceptor sites.

Profound hypoxia in Papio anubis and Macaca mulatta--physiological and neuropathological effects. I. Abrupt exposure following normoxia. II. Abrupt exposure following moderate hypoxia

Lightly anaesthetized and spontaneously breathing P. anubis (PA) and M. mulatta (MM) inhaled at ambient pressure 3.2% oxygen (identical to 37,500 ft or 11,430 m) from air and also after pre-exposure to 14% oxygen (identical to 10,000 ft or 3,048 m). The EEG, ECG, respiratory rate, arterial and cerebral venous sinus pressures, end-tidal pO2 and pCO2 and body temperature were recorded. Arterial and cerebral venous sinus blood gases, pH and pyruvate and lactate contents were estimated. Before hypoxia, MM showed a relative hyperventilation. Profound hypoxia, from air, ended with the "last breath" at 89--205 sec in PA and at 93--570 sec in MM. Brain damage was restricted to one MM (4 exposures). Profound hypoxia after exposure to 14% oxygen ended with the "last breath" at 87--210 sec in PA and at 120 sec--94 min (including 9 exposures over 5 min) in MM. Brain damage was restricted to one MM ("last breath" at 94 min). In the two MM with brain damage there was evidence of reduction in cerebral perfusion near the end of profound hypoxia. Brain damage in one animal contrasts with the frequent and often severe brain damage in MM after equivalent sub-atmospheric decompressions preceded by exposure to moderate altitude (10,000 ft).

Subatmospheric decompression: neurological and behavioural studies

Several studies in animals over the past decade have shown that prolonged exposures to pressures within the range 226 mm Hg to 160 mm Hg (30,000 to 37,500 ft) are likely to lead to brain damage. This often results in neurological and behavioural disturbance, which may be subtle and reversible or gross and ultimately fatal. The appearance of these impairments is often delayed until several hours or even days after exposure. Immediate survival does not necessarily ensure recovery. In contrast, decompression to pressures below 160 mm Hg or above 226 mm Hg are unlikely to have adverse effects if the exposure is survived. The most probable outcomes of such decompressions are death or uneventful recovery.