Iron stores, periodic leg movements, and sleepiness in obstructive sleep apnea

Most clinical sleep studies are performed for suspected obstructive sleep apnea (OSA), yet one-quarter to one-half show periodic leg movements (PLMs), for reasons that remain unknown. Several other disparate sleep disorders also increase the risk for PLMs. We examined the novel hypotheses that OSA as a representative sleep disorder could promote lower body iron stores, as reflected by serum ferritin levels, and, through downstream effects on dopaminergic transmission, increase PLMs and daytime sleepiness.

Subjects were recruited as they underwent laboratory-based polysomnography for suspected OSA. Serum ferritin levels were measured the next morning. Each subject completed an Epworth Sleepiness Scale and a brief questionnaire to assess for restless legs syndrome (RLS).

The frequency of apneic events showed no association with serum ferritin levels, before or after adjustment for age, sex, body mass index, and likely RLS (each p value > 0.3). Serum ferritin levels did not predict the frequency of PLMs (p = 0.7) or Epworth scores (p = 0.8). Iron deficiency as a dichotomous variable, determined by ferritin levels less than < 50 microg/L or in combination with low transferrin saturation or mean corpuscular volume, showed similar results. In exploratory analyses, contrary to expectations, lower minimum oxygen saturation and increased sleep-stage shifts predicted increased rather than decreased ferritin levels (p = 0.03 and p = 0.02, respectively).

Results of this study, powered to detect small to moderate effect sizes, strongly suggest that OSA does not cause lower serum ferritin levels, which, in turn, cannot explain PLMs or daytime sleepiness in these patients.