1 University Federico II Department of Nephrology2 Fondazione Pascale Department of Epidemiology Naples Italy Email: sabbatin{at}unina.it
Sir,
We read with great interest the article by Iliescu et al. [1] on the quality of sleep and the health-related quality of life in haemodialysis (HD) patients. This paper confirms the high prevalence of sleep disorders in uraemic patients, recently stressed by our group [2], focusing on the strict relationship of bad sleep quality and the health-related quality of life. Sleep quality was assessed in 89 HD patients by the Pittsburgh Sleep Questionnaire Index (PSQI) [3], a score derived by a self-rated questionnaire consisting of 19 questions grouped into seven components (03 scale), summed to yield the global index (range 021) in which higher scores indicate worse sleep quality.
We were surprised by the presence of a significant negative correlation between the PSQI of the patients and their haemoglobin (Hb) values, since in our previous study [2], in which 694 patients were surveyed on insomnia through a different questionnaire [4], we failed to find any correlation between Hb values and sleep disorders. On this basis, we have tried to verify the existence of such a relationship on a larger cohort of HD patients (n = 249, from 10 units of our geographical area, enrolled for a different study) surveyed through the same Pittsburgh Questionnaire. The inclusion criteria of our patients were similar to those requested in the study by Iliescu et al., i.e. age >18 years and time on dialysis >6 months; moreover, we did not consider patients with cancer or chronic debilitating diseases (connective tissue and chronic respiratory diseases, congestive heart failure).
Both the demographic data and the PSQI of our patients strictly overlapped those reported by Iliescu. The mean age of our patients was in fact 60.5 ± 13.8 years (compared with 60.1 ± 16.8 in Iliescus study), nor there were differences in their time on dialysis (50.8 ± 53 vs 49.4 ± 48.1 months, respectively) and in the prevalence of males vs females (62 and 66%); conversely, a slight difference was detected in Hb values, lower in our patients (10.9 ± 1.4 vs 11.6 ± 1.2 g/dl, respectively). Last, the PSQI of our patients averaged 8.5 ± 4.5, a value comparable with the one reported by Iliescu (8.7 ± 4.5).
As in Iliescus study, we first performed a bivariate correlation, but no relationship could be detected between PSQI and Hb (r - 0.09, P = 0.17, Spearman coefficient), nor between PSQI and the time on dialysis (r 0.063, P = 0.3). The second step of our analysis was the logistic regression (adjusted for gender and HD units), so the patients were divided into two categories, according to the distribution of PSQI values [1,3]: good sleepers (i.e. patients with PSQI 5, n = 78) and poor sleepers (patients with PSQI >5, n = 171). Among the several tested variables, however, a significant association was detected only for age as a predictor of PSQI in poor sleepers older than 50 years (OR 2.1, 95% CI 1.13.9), whereas Hb values did not influence the quality of sleep, although it was higher in good sleepers than in poor sleepers (11.14 ± 1.21 vs 10.65 ± 1.60 g/dl, P < 0.02).
Which is the truth?
Several factors may explain the differences in the data. One could be the small number of patients in the study by Iliescu et al., coming from a limited number of units: this could have determined a bias in the selection of patients. The bivariate analysis is a poor predictor of the relationship between sleep quality and Hb, resulting inappropriate in these kind of studies. Sleep quality, in fact, represents a complex phenomenon influenced both by demographic factors, like gender, age, time on dialysis, dialysis shift, as well as by clinical problems like the levels of blood pressure, with the use of anti-hypertensive drugs, the existence of cardiovascular diseases or the presence of secondary hyperparathyroidism associated with variable degrees of bone pain and pruritus; all these factors may condition the onset of sleep disorders [2], but have not been taken into account in both studies. This explains why no relationship between Hb and sleep quality could be demonstrated even when a more accurate analysis was performed. A third possible confounding factor is the different baseline level of Hb between our patients and those in the study by Iliescu et al.: the difference in absolute values and in their distribution (a similar standard deviation despite the numerical difference of the two populations, i.e. a largely different standard error) is not trivial, and could have influenced the analysis of the data. It is interesting to note, however, that also in our study good sleepers have Hb values significantly higher than the poor sleepers, even if they do not reach the values obtained by Iliescu et al.: this confirms that a clinical trend to a better sleep quality exists by raising Hb levels.
In summary, although the data of statistical analysis must be clearly kept in mind, to raise Hb levels above our reported values remains a possible way to challenge the bad quality of sleep and to diminish the clinical risks associated with sleep disorders [2].
Conflict of interest statement. None declared.
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