Hyperadrenocorticism (HAC) in dogs and cats is a well-described disease in literature and one of the most common endocrinopathies in dogs. There is little information concerning adrenal gland size enlargement under trilostane therapy. Trilostane is competitive inhibitor of 3-beta-hydroxysteroid dehydrogenase [1] and is the most commonly used modality in treatment of canine HAC. The level of cortisol production inhibition is dosage dependent and reversible. Few studies have been published describing ultrasonography in dogs with pituitary dependent HAC treated with trilostane [1], sonographic changes of adrenal glands under trilostane therapy [2] and contrast enhanced ultrasound of adrenal glands in dogs with pituitary dependent Cushing’s syndrome [3]. This prospective study was performed to evaluate whether there is a correlation between adrenal gland enlargement and positive reaction to trilostane therapy given twice daily. Presently, endocrine tests and clinical examination are used to evaluate and establish the correct dose of trilostane in dogs. The aim of this study is to evaluate an additional objective parameter that can help to decide whether to adjust or not the dosage of trilostane. Additionally, with this modality, suspicion of possible concurrent disease could be confirmed if the dog is not reacting to therapy as expected, but laboratory tests are within normal limits.

All the patients that were included in our study matched criteria of adrenal gland enlargement established in the recent studies [4]. Increase in adrenal gland size was already described in previous reports [1] [2] [3] and the focus in our study was at correlation between increase in adrenal gland size and improvement of clinical symptoms of HAC. To the author’s knowledge this is the first time that this correlation was observed. Establishing diagnosis of HAC can be challenging if the clinical symptoms are mild, or in patients with concurrent diseases [5]. Measurement of basal cortisol plasma level is not a useful screening tool for diagnostic of HAC [6] [7], because of elevation of endogenous cortisol levels in patients with non-adrenal diseases, chronical inflammation and stress [8] [9]. Endocrine tests should be performed if there is an indication according to clinical signs, CBC (complete blood count), biochemistry and urine analyses [6] [10]. In our case all patients showed strong indication for performing endocrine tests according to their clinical signs and blood work. The LDDS (low dose dexamethasone suppression) test is considered as the screening test of choice if iatrogenic HAC is not suspected [6] [11] [12] and in our study it was performed as a first choice test. ACTH stimulation tests were done if high clinical indication of HAC was present, but LDDS test showed negative results. One dog was suspected to have an atypical onset of HAC. Atypical Cushing’s syndrome is suspected in a patient with enlarged adrenal glands who is showing historical, routine laboratory and clinical signs of Cushing’s but the endocrine test remains normal [13] [14]. An ACTH stimulation test was done and post ACTH plasma concentration of 17 OH progesterone was increased. Good response to trilostane therapy, adrenal enlargement and excessive stimulation confirmed our suspicion of atypical onset of HAC. Trilostane is considered as a medical treatment of choice for dogs with HAC [15] [16] and it was used in this study as well. The dosage of 0.5 mg/kg twice daily for PD and 1 mg/kg twice daily for AD was the initial dose for all patients, which matches the current recommendation that can be found in literature [17]. All dogs but one received trilostane twice daily, but once daily usage represents a good alternative and initial dose should be 1 - 2 mg/kg [17] [18]. The ACTH stimulation test represents until today the most used test for monitoring of HAC in dogs [19]. There are several protocols and recommendations when to perform a test based on time after drug administration [15] [20] [21] [22] in our case we have performed ACTH stimulation tests two to three hours after administration of trilostane, since that is the time when nadir of cortisol serum concentration occurs [21] [23] [24]. Two dogs were considered as suitable candidates for the “pre pill” test instead of ACTH stimulation test. Recent studies showed better correlation of the “pre pill” test with clinical signs and owners reports about patient’s condition than the ACTH stimulation test [25] [26]. If difficulties during blood sampling occurred (for example excessive movement, vocalizing, etc.) as suggested in recent studies [27] the stress factor was included in interpretation of test results. After every endocrine test ultrasound examination of both adrenal glands was done. After two weeks of trilostane therapy all patients but one showed increase in adrenal size. On second ultrasound examination that was performed adrenal glands increased in size by every patient. Adrenal enlargement during second ultrasound examination was similar to already described in literature [3]. We believe that the level of adrenal enlargement is in direct correlation with clinical improvement receiving trilostane therapy. It is already suggested that adrenal enlargement during trilostane therapy occurs because of suppression of the negative feedback mechanism affecting production of cortisol in adrenal glands [2] and that ACTH not trilostane causes changes in adrenal gland tissue [28]. A possible theory that connects adrenal gland enlargement and clinical improvement could be that better suppression cortisol production causes higher production of endogenous ACTH [29] and therefore increase in adrenal gland size [30], but additional studies to support this theory are needed. It is important to be mentioned that adrenal gland enlargement is not only observed in patients with cushing’s syndrome, but as well in patients with primary hyperaldosteronism, pheochromocytoma, incidentaloma, acromegaly and sometimes in patients with severe chronic or acute diseases (pyometra, severe gastro-enteritis, glomerulonephritis, etc.). Because in this study low dose of trilostane was used as initial dosage, no dog showed any sign of hypoadrenocorticism and therefore no data is available on adrenal gland enlargement in those cases. It might be interesting to see if in those cases adrenal glands increase in their size even more than in dogs that are showing improvement in clinical signs. Evaluation of adrenal gland enlargement might be a useful tool in monitoring of HAC patient, especially in the ones with concurrent diseases that are showing poor to no improvement of clinical sings of Cushing’s syndrome. With this additional tool (to clinical examination and endocrine tests) we could determine better whether or not poor clinical improvement is because of insufficient dosage of trilostane or caused by some concurrent disease and gain more confidence in meeting the decision of adjusting the dosage of therapy. Possible combination of novel methods of monitoring Cushing disease (measuring haptoglobin-Hp, serum alanine aminotransferase-ALT, gamma-glutamyl transferase-γGT) [16] with assessment of adrenal gland enlargement (together with clinical examination and endocrine tests) could as well be considered. Limitation of using abdominal ultrasound as a monitoring tool could be: demand for skilled sonographer and difficulties in obtaining measurements at the same position every time. The biggest advantage in using sonography for monitoring of HAC, especially in patients that do not improve under trilostane therapy, is the possibility of diagnosing concurrent diseases (pancreatic changes, increased thickness of bladder wall, etc.) that could be responsible for lack of clinical improvement.

Our study has several limitations. The number of dogs included in our study is small and the statistical correlation is lacking. A study on larger population of dogs would overcome this limitation. Secondly, some of the patients showed up later on date for scheduled appointments which could interfere with the level of change in adrenal glands size. The third limitation of our study might be that we didn’t include control group of the dogs with diagnosed Cushing’s syndrome that didn’t received treatment with trilostane to observe level of change of adrenal glands in that population.

Evaluation of adrenal gland size increase might be a useful additional tool for monitoring dogs with HAC under trilostane therapy because adrenal gland enlargement could have correlation with improvement in clinical signs under trilostane therapy. In this study, all dogs that had an increase in adrenal gland size by more than 7% showed as well clinical improvement in HAC signs. Further and additional studies on this topic are needed. Adrenal gland enlargement evaluation alone should not be used as an only monitoring tool, because increase in adrenal gland size is seen as well in patients with poor control group.

Not applicable for studies involving humans.

The authors would like to give special thanks to Dr. Nenad Suknovic of University of Geneva for technical support in data analysis.

The authors would also like to express gratitude to Anicura group for the support in publishing this paper.

The authors declare no conflicts of interest regarding the publication of this paper.

Masnov, B., Tiemessen, I. and Stosic, A. (2022) Ultrasonographic Changes of Adrenal Glands Size under Trilostane Therapy, Their Correlation with Clinical Signs and Endocrine Tests and Possibility of Introducing Ultrasound as Additional Monitoring Modality of Cushing’s Syndrome. Open Journal of Veterinary Medicine, 12, 57-70. https://doi.org/10.4236/ojvm.2022.127007