ACTH and cortisol responses to CRH in acute, subacute, and prolonged critical illness: a randomized, double-blind, placebo-controlled, crossover cohort study

Patients suffering from critical illnesses typically reveal high plasma (free)cortisol concentrations and low-normal plasma adrenocorticotropic hormone (ACTH). The absence of elevated plasma ACTH, particularly in patients with severe infections, has been interpreted as caused by inflammation or hypoperfusion-induced damage to cells of the hypothalamus whereby synthesis of corticotropin-releasing hormone (CRH) and arginine vasopressin (AVP) is hampered [1‐4]. Shock or inflammation could also directly damage the anterior pituitary gland [4]. Also, direct inhibition at the hypothalamus and/or pituitary level by various drugs have been suggested [2, 4, 5]. However, an alternative explanation could be that high circulating free cortisol levels, brought about by suppressed cortisol binding proteins and by reduced cortisol breakdown [6], exert negative feedback inhibition at the pituitary and/or the hypothalamic level, as such lowering ACTH, CRH, and AVP expression/secretion [7]. Nevertheless, during critical illness, ACTH secretion is not completely suppressed unlike what is observed with high doses of exogenous glucocorticoids or in patients with adrenal Cushing’s syndrome [8, 9]. This could be explained by other concomitant central activation, such as via stress induced AVP increase which could potentiate CRH effects [9‐12]. Also, during the first weeks of critical illness, the frequency of the ACTH and cortisol pulses was found to be normal, whereas pulse amplitudes were lower than normal [13]. However, a recent study has shown that a central suppression of ACTH is present during critical illness [14]. Suppressed ACTH sustained over an extended period of time could predispose to adrenocortical atrophy and dysfunction [15].

Differentiation between hypothalamic lesions, damage to the pituitary corticotropes and adrenal/ectopic causes of Cushing’s syndrome can be done by analyzing plasma ACTH (and cortisol) responses to an intravenous CRH bolus injection [16]. If during critical illness, the hypothalamus would be acutely damaged by shock or inflammation, and the anterior pituitary gland would be intact, one would expect augmented/prolonged ACTH responses [17]. If the pituitary would be acutely damaged by shock or inflammation, suppressed ACTH responses would be expected from the early phase onward [17]. Alternatively, if ACTH is suppressed by feedback inhibition at the level of the pituitary and hypothalamus, as in patients with adrenal/ectopic Cushing’s syndrome or on high doses of glucocorticoids, the ACTH responses to a CRH injection expectedly depend on the duration of hypercortisolism, with initially normal ACTH responses to CRH injection followed by lowered ACTH responses in the prolonged phase of illness [18]. Although the test is commonly used in the setting of Cushing’s syndrome, only few studies have been performed in critically ill patients, none of which investigated the impact of duration of illness [19‐22].

We hypothesized that a longer duration of elevated circulating free cortisol, brought about by suppressed cortisol binding proteins and by reduced cortisol breakdown, reduces ACTH responses to a CRH injection specifically in the prolonged phase of critical illness, irrespective of the presence of sepsis/septic shock and irrespective of risk of death. To test this hypothesis, we performed a randomized, double-blind, placebo-controlled crossover cohort study to compare the ACTH (and cortisol) responses to a synthetic human CRH-analogue, in the acute, subacute and prolonged phases of critical illness with those of healthy subjects, in relation to presence of sepsis/septic shock and risk of death.

In the presence of low/normal baseline plasma ACTH and increased plasma (free)cortisol concentrations, incremental ACTH responses to CRH in patients in the acute phase of critical illness were normal, whereas ACTH responses became ± 55% lower than normal in the later phases, irrespective of the presence of sepsis/septic shock or risk of death. Interestingly, the total cortisol responses to CRH were always lower than in healthy subjects whereas the free cortisol responses were always normal, in line with increased cortisol distribution volume during critical illness [6, 14]. The time courses of the ACTH responses to CRH were thus compatible with prolonged feed-back inhibition exerted by elevated free cortisol, rather than with hypothalamic and/or pituitary cell damage, similarly as seen with prolonged exposure to exogenous glucocorticoids [23]. These findings generate the hypothesis that CRH could offer potential for prevention of central hypoadrenalism in ICU patients who require intensive care for several weeks, for whom it has been shown that free cortisol levels are no longer elevated [14]. The absence of hemodynamic instability in response to the CRH injections in the patients of this study is an important safety aspect for future studies.

The observation of a normal ACTH response to CRH in the first few days of critical illness, similarly as documented by Schroeder et al. [20], argues against a damaged hypothalamus or pituitary by hypoperfusion or inflammation [4]. The finding that presence of sepsis or septic shock did not affect ACTH responses at any time during the course of critical illness further supports this interpretation. The 55% lowering of the ACTH responses to CRH in the subacute and prolonged phase of critical illness corroborates sustained feedback inhibition by elevated circulating free cortisol and is in line with the previously documented suppressed nocturnal pulsatile ACTH secretion during critical illness [13]. Indeed, a similar degree of suppression of the ACTH response to CRH has been reported for patients after surgical treatment for Cushing’s syndrome and for patients after withdrawal of ≥ 2 weeks of therapeutic glucocorticoid treatment [18, 26]. The suppressed ACTH responses to CRH observed in the subacute/prolonged phases of critical illness is compatible with low endogenous CRH and/or low vasopressin signaling [27], that both can be suppressed by high circulating levels of glucocorticoids [7]. Of note, baseline plasma ACTH concentrations were not completely suppressed and slightly increased over time. This is in line with earlier observations [6, 14] and suggests that during critical illness, specific central stimulatory pathways are still activated [9‐12]. During health, hypothalamic CRH-neurons co-express CRH and AVP, which synergistically activate distinct signaling pathways within pituitary corticotropes [28]. It is well known that AVP is only a weak direct stimulator of ACTH but a much more powerful synergizer of CRH [7], and thus AVP action may be required for a normal ACTH response to exogenous CRH [29]. Vice versa, experiments in CRH knockout mice have shown that ACTH secretion depends on CRH [23, 30]. Reactivation of hypothalamic CRH secretion is indeed crucial for the reactivation of ACTH secretion after withdrawal of chronic glucocorticoid treatment [23]. Downregulation of CRH expression, via activating the glucocorticoid receptor, can be brought about by elevated free cortisol and/or by high circulating levels of bile acids that have previously shown to characterize subacute and prolonged critical illness [31, 32]. Also, a sustained endotoxin challenge could reduce CRH expression, although the observed comparable responses in patients with sepsis and in those without sepsis does not support a primary role for endotoxin or cytokines [33]. A postmortem study of human patients who died from septic shock after an illness of approximately 1 week, reported reduced ACTH mRNA levels in the pituitary gland [1]. This suppressed ACTH gene expression occurred in the absence of a compensatory rise in the expression of CRH and vasopressin in the hypothalamus and without altered expression of the CRH-receptor 1 and the vasopressin-receptor (V1b), supporting our current findings [1]. The results of the current study however cannot rule out a direct pituitary defect due to effects of inflammation and/or hypoxia selectively in the more prolonged phases of illness.

Remarkably, in all patients, irrespective of the duration of illness, total cortisol responses to CRH were lower than normal whereas free cortisol responses were always normal. This is in line with a recent study of long-stay patients who received weekly short ACTH stimulation tests for 4 weeks in the ICU, that revealed uniformly low incremental total cortisol responses but normal incremental free cortisol responses, explained by low plasma binding and increased cortisol distribution volume [14]. In the current study, with increasing duration of critical illness, both total and free cortisol responses tended to further decrease. This could be partially explained by the suppressed ACTH release in response to CRH and/or by the onset of decline of adrenocortical function. Indeed, appropriate ACTH signaling is essential to maintain integrity and function of the adrenal cortex [34]. A post-mortem study of adrenal glands harvested from patients who had been critically ill for several weeks showed loss of zonational structure, lipid droplet depletion, and suppressed ACTH-regulated gene expression [15]. Suppressed ACTH secretion could thus negatively affect adrenal function in long-stay ICU patients [13, 35]. Such a negative effect of suppressed ACTH could also explain why critically ill patients beyond the fourth week in the ICU were recently shown to have circulating total and free cortisol levels that were not higher than those of healthy subjects, despite their severe illness and high risk of death [14]. One week after ICU discharge on the regular ward, survivors had higher than normal plasma ACTH and total and free cortisol levels, although they were recovering. This further suggested a central adrenocortical suppression during the ICU phase, which could predispose long-stay ICU patients to central adrenal insufficiency.

A first limitation of this study is that, for obvious reasons, no hypothalamic and pituitary tissues were available for quantification of expression of CRH, vasopressin, ACTH, and of the CRH-receptor 1 and vasopressin-receptor. This should be done in validated animal models of prolonged critical illness [36]. A second limitation is that one cannot exclude additional suppression at the hypothalamic level from analgo-sedative drugs that are used throughout ICU stay, of which opioids are the main component [37]. Indeed, intra-operative opioids and prolonged opioid use for chronic pain have shown to lower plasma ACTH concentrations [38‐42]. Furthermore, in healthy subjects, morphine blunts the ACTH response to CRH injection at a supra-pituitary level [43]. However, given the normal ACTH responses to CRH, observed during the acute phase, when opioid doses are usually higher than in the later phases, an important role of opioids is unlikely. Third, plasma free cortisol was calculated from plasma total cortisol with the Coolens method adapted for individual albumin and CBG concentrations, and not measured with ultra-filtration and equilibrium dialysis which could have induced some bias [44]. However, as also plasma total cortisol concentrations were always higher in patients, and given the clear decrease in both albumin and CBG, increased plasma free cortisol is obvious. Finally, we observed that throughout the acute, subacute and prolonged phases of critical illness, ACTH responses were not predictive for patient outcome. However, earlier smaller studies performed either in the acute-subacute [21] or prolonged [22] phases reported higher peak ACTH, but not AUC ACTH, responses in non-survivors than in survivors, a finding that could be at least partially biased by the variation in the day of death. The strengths of the study were the randomized, double-blind, placebo-controlled crossover design, which allowed to compare matched patients in different phases of critical illness while minimizing confounders.

Our findings open perspectives for novel strategies to protect long-stay ICU patients against the risk of developing adrenal insufficiency. If the lack of priming of the corticotropes by CRH would be responsible for reduced ACTH expression and secretion, providing CRH could potentially allow (re)activation of ACTH synthesis and release in response to any fall in cortisol and could hereby prevent adrenal atrophy in the prolonged phase of illness [45]. It has been shown that continuous infusion of CRH can reactivate ACTH secretion with preservation of circadian rhythmicity and pulsatility [46]. Studies of CRH infusion in the critically ill should probably initiate this intervention rather early, when the corticotropes are still fully responsive to CRH. If corticotropes remain sensitive to feedback inhibition, CRH infusion may not result in too high plasma cortisol and would respect any eventual tissue-specific regulation of cortisol action, which are important safety aspects. In the current study, no side effects of a CRH bolus were noted. However, caution is warranted given that CRH has also been involved in anxiety disorders, depression, memory and learning [47, 48], and is able to increase catecholamines and heart rate [49]. If a direct pituitary defect would be present in the prolonged phases of illness, which we could not exclude, CRH will not be able to prevent this.

In conclusion, the results of the CRH tests did not support the presence of shock/inflammation-induced hypothalamic and/or pituitary damage in critically ill patients. Instead, the consequences of prolonged feedback inhibition exerted by elevated (free)cortisol are compatible with suppressed ACTH responses to CRH in the prolonged phases of critical illness. These findings raise the hypothesis that CRH infusion could prevent the development of a central adrenal insufficiency in long-stay ICU patients, which should be further investigated.