Secretion pattern of melatonin after head injury

Introduction

The circadian rhythm of melatonin secretion is a most reliable biochemical marker of the endogenous biological rhythm of the organism. Plasma melatonin levels are low during the day (1–5 pg/ml) and normally increase to about 50–100 pg/ml at night. The objective of the present study was to investigate any potential disturbances of melatonin's secretion pattern after head injury.

Materials and methods

The sample consisted of eight subjects (seven males and one female) admitted at the Intensive Care Unit after head injury. Mean age ± SD of subjects was 41 ± 17.91 years, mean APACHE II Score was 15.5 ± 3.75, mean Glasgow Coma Score was 7.11 ± 3.14, mean duration of stay in the ICU was 23.44 ± 12.23 days, and mortality rate was 11.1%. All patients were under sedation (Fentanyl and Propofol) and mechanical ventilation. Various medications were administered as appropriate. Blood samples for melatonin determination were collected via an arterial line at 08.00, 12.00, 16.00, 18.00, 21.00, 24.00, 03.00 and 06.00 for the first 2 days after admission. Core body temperature was also recorded through an esophageal thermometer at hourly intervals. All patients were under constant ambient light (< 600 lux) and none of them received β-adrenergic blockers. Plasma melatonin was assayed using a RIA method.

Results

The peak mean melatonin concentrations were recorded during the dark photoperiod and the lowest mean concentrations were recorded during daylight hours (Table 1).

Table 1

In overall a clear circadian secretion pattern of melatonin was observed during the first two post-admission days, with higher melatonin levels during the dark period (24.00–06.00) and lower levels during daylight hours (08.00–18.00). However, only four patients exhibited a typical diurnal variation of plasma melatonin, whereas the other four did not.

No consistent core body temperature pattern was observed during the study period and individual temperature fluctuations were not related to the corresponding melatonin changes.

Conclusions

Melatonin secretion appeared reduced in subjects treated in the ICU after head injury. However, its circadian secretion pattern was not disturbed in half of them during the first two post-admission days, indicating an intact endogenous circadian generating system. Furthermore, the diurnal secretion pattern of melatonin appeared to be desynchronized with the rhythm of core body temperature.