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Carbon monoxide is endogenously produced by enzymes known as haem oxygenase (HO). The CO produced is immediately bound to blood haemoglobin as carboxyhaemoglobin (Hb-CO). HO-1, the inducible form of HO, is induced by various stimuli, including reactive oxygen species (ROS) and proinflammatory cytokines.1 Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease in humans that results in the selective death of both upper and lower motor neurones. ROS have been implicated in the mechanism of neuronal injury in ALS, based on the evidence that mutations of the superoxide dismutase (SOD) gene have been identified in patients with familial ALS, and that transgenic mice with mutated SOD genes have an ALS-like phenotype.2 Furthermore, increased oxidative damage has been found in spinal motor neurones of necropsy samples from both sporadic and familial ALS patients.2 This suggests that with the progression of neuronal injury the spinal cord in ALS patients may induce HO-1, leading to the production of CO, followed by increased concentrations of blood Hb-CO. Indeed, immunohistochemical studies have shown increased HO-1 expression in spinal motor neurones in ALS patients and animal models of ALS.3 However, blood Hb-CO in ALS patients has not been examined. We investigated arterial Hb-CO concentrations in relation to disease progression in patients with sporadic ALS (SALS) and controls.
The subjects were 21 patients with SALS (16 men, 5 women) with a mean (SD) age of 61.5 (12.8) years, and 20 healthy age matched controls (17 men, 3 women) aged 61.2 (11.8) years. The diagnosis of SALS was based on neurological history, neurological examination, and laboratory tests. When their condition was stable, the functional disability of the disease was evaluated using the ALS score developed by Norris,4 which ranges from 0 (maximum impairment) to 100 (normal). The blood samples for analysis of Hb-CO, arterial blood gas tensions, and spirometric data were obtained at the same time as the ALS score evaluation. Arterial Hb-CO concentration was measured with a spectrophotometer as previously described.5 Controls were recruited by advertisement as volunteers, and none was receiving long term drug treatment or had a history of chronic neural, muscular, or pulmonary disease. None of the patients or controls had any inflammatory diseases such as common colds, sinusitis, or bronchitis. All subjects in the study were non-smokers. We informed the subjects of the aims and content of the study, and obtained their consent for their particiation.
Age and sex did not differ significantly between controls and patients with SALS. Vital capacity and forced expiratory volume in one second were significantly lower in patients with SALS (78.2 (9.3)% of predicted (p<0.05), and 86.0 (8.0)% of predicted (p<0.01), respectively) than in control subjects (96.2 (8.6)% and 95.4 (9.8)% of predicted). As shown in the fig 1A, arterial Hb-CO concentrations in patients with SALS were higher than in the control subjects, at 1.10 (0.62)% v 0.65 (0.20)% (p<0.01 by Wilcoxon rank-sum test). Moreover, arterial Hb-CO concentrations in patients with SALS showed a significant inverse correlation with ALS score by Pearson’s correlation test (r = −0.76, p<0.05) (fig 1B). Neither arterial O2 tension (mean 89.7 (7.2) torr) nor arterial CO2 tension (mean 44.5 (11.5) torr) was significantly correlated with ALS score in patients with SALS (r = −0.05, p = 0.85; and r = 0.06, p = 0.82, respectively).
In six of the 21 patients with SALS, we were able to re-evaluate the ALS score and arterial Hb-CO concentrations six months after the first evaluation. Of the remaining 15 patients, four had died before re-evaluation, nine had changed hospital, and two discontinued attendance at our clinic for unknown reasons. In the six patients re-evaluated, arterial Hb-CO concentrations were significantly raised at the second evaluation (p<0.05), while the ALS score was significantly lower (p<0.05) (Wilcoxon matched pairs signed rank-sum test (fig 1C)). Arterial O2 and CO2 tensions were not significantly different between the first and second evaluations (90.7 (7.0) v 84.9 (10.2), p = 0.30; and 43.3 (8.0) v 38.5 (2.0) torr, p = 0.51, respectively).
In this preliminary study, we showed increases in arterial Hb-CO concentrations in patients with SALS. The increased Hb-CO concentration correlated with the severity of the disease and, within individuals, changed with the progression of the disease. In a follow up study, we were able to re-evaluate only six of the initial 21 patients. It is therefore possible that there was selection bias—for example, the capacity to produce a higher Hb-CO level might have affected mortality. A larger sample size and more frequent follow up are needed to clarify this. Arterial blood gas tensions had no significant relation to the severity of ALS in our patients, suggesting that the increase in Hb-CO level is unlikely to be a sign of early respiratory failure in SALS. Our observations might suggest that the Hb-CO produced reflects the degree of neuronal injury in ALS.
Arterial Hb-CO concentration is reported to be raised in inflammatory respiratory diseases.5 Although the level of Hb-CO concentration in SALS is equivalent to that in inflammatory respiratory diseases, laboratory data—such as C reactive protein and peripheral white blood cell count—failed to show any inflammatory evidence in SALS, in contrast to pneumonia and idiopathic pulmonary fibrosis, which cause a prominent inflammatory response.5 This suggests a different mechanism of HO-1 induction between ALS and inflammatory respiratory diseases. Arterial Hb-CO concentrations in other neurodegenerative diseases need to be investigated to clarify the disease specificity of Hb-CO elevation. Although further large cohort studies are required, arterial Hb-CO concentration may be useful for objective monitoring of disease progression in ALS.