Introduction
Cushing syndrome (CS) is characterized by chronic exposure to excessive glucocorticoid, and thus classified into adrenocorticotropic hormone (ACTH)-dependent and ACTH-independent CS. ACTH-independent CS accounts for approximately 15–20% of CS, primarily due to the unilateral adrenal tumor such as adrenocortical adenoma (ADA) and adrenal carcinoma that accounts for 10 and 5%, respectively. The rare causes of ACTH-independent CS include primary bilateral macronodular adrenal hyperplasia (BMAH), primary pigmented nodular adrenal disease (PPNAD), and McCune–Albright syndrome [
1].
PPNAD is rarely encountered and accounts for 0.6–1.9% of CS [
2]. It can occur isolated or as a component of the Carney complex (CNC). CNC is a hereditary multiple neoplasia syndrome characterized by skin lentigines, myxomas, and endocrine tumors, first described by J. Aidan Carney in 1985 [
3]. According to Stratakis et al., 95% patients with PPNAD fulfilled the diagnostic criteria of CNC [
4]; however, other reports on the incidence rate of CNC in patients with PPNAD are limited. The pathological feature of PPNAD constitutes of multiple pigmented cortical nodules and atrophy of the internodular cortex. The typical radiological features of PPNAD may present as multiple nodules of bilateral adrenal glands; however, in most cases, the appearance of adrenal glands with PPNAD might be normal or presented with macronodules [
5] that are indistinguishable from BMAH, which is characterized by large adrenal nodules, >5 cm. Thus, occasionally, the preoperative diagnosis through imaging is misleading and rather challenging [
5,
6]. Therefore, a new method with high sensitivity and specificity that can discriminate PPNAD from other adrenal diseases is an urgent prerequisite.
Several reports have mentioned an anomalous increase in the glucocorticoid level in patients with PPNAD [
7‐
9]. Stratakis et al. reported a paradoxical increase in urinary free cortisol (UFC) in response to dexamethasone during the Liddle test. Moreover, an increase of more than 50% in 24 h UFC on day 6 could distinguish PPNAD patients from those with other primary adrenal disorders causing CS with a 69.2% sensitivity and 80.0% specificity [
4]. Dexamethasone suppression tests (DST) are widely used in the diagnosis of CS. Low-dose dexamethasone suppression test (LDDST) serves as the definitive test in the confirmation of CS, while the high-dose dexamethasone suppression test (HDDST) differentiates between pituitary-dependent and non-pituitary-dependent forms of CS. Nevertheless, the usefulness of DST in the diagnosis of PPNAD is yet to be elucidated. In addition, since the level of cortisol differs among races, whether an increase in >50% of 24 h UFC in the Liddle test could also be applied to Asian people is yet to be investigated.
Herein, we analyzed data from 25 patients with PPNAD and control groups consisting of 27 patients with BMAH and 84 with ADA were admitted to the Peking Union Medical College Hospital (PUMCH) from 2001 to 2016. To our knowledge, this was the largest single-center study of PPNAD in Asia. LDDST and HDDST were conducted, and comparisons were made among the three groups. We used the ratio of 24 h UFC post-dexamethasone to prior-dexamethasone, for the first time, to determine the cut-off value for the evaluation of PPNAD.
Discussion
PPNAD occurs as a rare cause of CS, accounting for 0.6–1.9% of all such patients [
2]. CNC is an autosomal dominant inherited and multiple neoplasia syndrome characterized by skin tumors and pigmented lesions, cardiac myxomas, schwannomas, breast adenomas, bone lesions and various endocrine disorders caused by tumors of the pituitary and thyroid glands, pancreas, and/or gonads [
10,
11]. A total of 26–60% of the CNC patients exhibited PPNAD, and it is the most common endocrine tumor associated with CNC [
3,
10,
12]. In our study, 36.0% PPNAD patients had the components of CNC, and all of them presented spotty pigmentation in the skin or the mucosa, five females demonstrated the hypoechoic nodules of the thyroid gland and/or breast, and a male developed a cyst in the head of the epididymis. Nevertheless, the commonly reported manifestation, cardiac myxoma [
13], was not found in our patients. Stratakis et al. reported that 20/21 patients (95%) showed PPNAD that occurred as a component of CNC [
4]. Such an enormous difference in the incidence rate of CNC in patients with PPNAD between the current study (36%) and that by Stratakis et al. (95%) might be attributed to the ethnic differences. Additionally, the sample size of both studies was not sufficiently large, which leads to inevitable bias to the results. Further investigations with expanded sample size need to be undertaken in order to clarify the genetic distinctions among various races.
DST is commonly used in the diagnosis and differential diagnosis of ACTH-dependent CS. LDDST or 1 mg overnight DST is used for screening the presence of CS in the patient, while HDDST is used to distinguish CS from other ACTH-dependent CS; >50% suppression of cortisol concentration indicates CS. However, limited studies are available concerning the administration of dexamethasone in ACTH-independent CS since the baseline of serum ACTH level is relatively low. Several reports have mentioned a paradoxical increase in the glucocorticoid level after administration of dexamethasone in patients with PPNAD [
14‐
17]. Silverman et al. reported a patient with CS secondary to bilateral nodular adrenocortical hyperplasia, in whom urinary 17-OHCS could not be suppressed by a large dose of dexamethasone, and the adrenal pathology demonstrated cortical hyperplasia with multiple small nodules [
14]. In 1999 Stratakis et al. found that UFC levels on day 6 of the Liddle test had the highest accuracy in the diagnosis of PPNAD, and an increase of >50% UFC levels could discriminate 9/13 patients with PPNAD from other primary adrenocortical disorders [
4]. In the current study, we also found that after DST, the 24 h UFC levels in patients with PPNAD increased, while in the other two groups the 24 h UFC levels remained unchanged or even decreased, which suggested the usefulness of DST in the diagnosis of PPNAD. In patients with ADA, the 24 h UFC levels were slightly decreased after DST, which might be partially attributed to the suppressibility of those patients whose basal 24 h UFC values were within the normal range.
Herein, we used the ratio of 24 h UFC post-dexamethasone to prior-dexamethasone, for the first time, in the diagnosis of PPNAD. After LDDST, the ratio of 24 h UFC Post/Pre (L) in the PPNAD group was significantly higher than that in the ADA group, while it could not differentiate PPNAD from BMAH. After administration of high-dose dexamethasone, a statistically significant difference between PPNAD and BMAH was noted, indicating the diagnostic value of DST, especially the HDDST could identify PPNAD from both BMAH and ADA. Our study revealed the ratio of 24 h UFC after LDDST to the baseline value >1.16 with 64.0% sensitivity and 77.9% specificity. Similarly, when the value of 24 h UFC (Post-H-Dex)/UFC (Pre-H-Dex) is >1.08, the sensitivity and specificity increased to 84.0 and 75.6%, respectively. Compared to the previous study using 24 h UFC of >50% increase on day 6 of the Liddle test, the degree of UFC level was slightly lower in our study. This phenomenon might be attributed to the theory that the proportion of patients with isolated PPNAD was different between our study and that by Stratakis et al. The molecular mechanism underlying the paradoxical response to dexamethasone in patients with PPNAD was due to an increased expression of glucocorticoid receptor in PPNAD nodules [
17]. The CNC is primarily caused by germline mutations in the protein kinase A regulatory subunit 1A (
PRKAR1A) gene [
18‐
22]. Nevertheless, several studies demonstrated that patients with isolated PPNAD exhibited specific molecular genetic abnormalities with mutations in phosphodiesterase
PDE11A,
PDE8B, and the PKA catalytic subunit
PRKACA gene [
23‐
25]. The mutations in various gene loci may differentially influence the cAMP/PKA pathway in patients with isolated PPNAD and CNC, leading to variable levels of UFC.
Nevertheless, the present study exhibited some limitations. Since we mainly focused on the clinical study regarding the usefulness of DST in the diagnosis of PPNAD, the genetic mutations were not assessed in the participants, which made it impossible to compare the genetic abnormities between patients with isolated PPNAD and those with CNC. Furthermore, due to the retrospective nature of the current study, some clinical data were missing, which could lead to a potential bias in our study.
In conclusion, DST, especially HDDST, is greatly beneficial in the early diagnosis of PPNAD. When the radiological examination could not provide valuable information, DST could detect the patients with subclinical, atypical PPNAD-associated manifestations and distinguish PPNAD from other adrenal diseases. With respect to the Chinese population, the cut-off value of 24 h UFC (Post-H-Dex)/(Pre-H-Dex) was 1.08 with 84.0% sensitivity and 7.6% specificity. However, the cut-off values of 24 h UFC levels between different races need to be investigated further.