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Hypercortisolism (Cushing’s Syndrome) in the dog

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  • Disciplina: Endocrinologia
  • Specie: Cane

In 1932, neurosurgeon Harvey Cushing described for the first time a strange syndrome connected with the presence of “atypical basophilic cells” in the pituitary gland. Today this disease is well known and is called Cushing’s Syndrome. When speaking specifically about the pituitary form [Pituitary dependent-hypercortisolism (PDH)] it is also called Cushing’s Disease. This syndrome is also called hyperadrenocorticism, but it is more correct to talk about hypercortisolism since hyperadrenocorticism also includes hyperaldosteronism and other forms of hyperactivity of the adrenal cortex, which are different diseases.

 

DEFINITION

Cushing’s Syndrome/Disease (spontaneous form) is represented by the clinical and laboratory alterations resulting from a chronic and pathological hypercortisolaemic state.

 

EPIDAEMIOLOGY

Cushing’s Syndrome generally occurs in subjects between 6 and 16 years of age, with an average age of around 10-11 years. In the dog it is presumed to have an incidence of 1-2 cases/1000 cases per year, while in humans the incidence is decidedly lower, with 1.2-2.4 new cases/million/year. The pituitary form is more common in subjects weighing under 20 Kg (75%) and in females (55-60%). On the other hand, adrenal tumours have a 50%  incidence in dogs weighing over 20 Kg. The breeds most at risk are Toy Poodles, Dachshunds, Beagles, Boxers, Labrador Retrievers, German Shepherds and the various Terriers.

 

ETIOPATHOGENESIS

Based on the etiopathogenesis hypercortisolism may be divided into an ACTH-dependent and an ACTH-independent form:

ACTH-DEPENDENT HYPERCORTISOLISM
Pituitary-dependent hypercortisolism (PDH) or Cushing’s Disease. Approximately 85% of dogs present this form. It is caused by an ACTH-secreting pituitary tumour. Excessive ACTH secretion results in bilateral adrenal hyperplasia and consequent hypersecretion of adrenal glucocorticoids. In the majority of cases the tumours are benign, usually microadenomas, and in 15-25% of cases macroadenomas. In 20-25% of cases these adenomas arise from the pars intermedia of the adenohypophysis (they are usually macroadenomas) while the remaining cases arise from the anterior lobe of the adenohypophysis.

Hypercortisolism caused by ectopic ACTH production. This disease is well known in humans (it accounts for around 15% of the cases of ACTH-dependent hypercortisolism) and is characterised by the production of ACTH by non-pituitary tumours (usually lung carcinomas). This causes an abnormal stimulation of the adrenal glands with consequent bilateral hyperplasia and overproduction of cortisol. This form has been recently reported in a dog.

ACTH-INDEPENDENT HYPERCORTISOLISM
Hypercortisolism due to cortisol-secreting adrenal tumours (adrenal-dependent hypercortisolism – ADH). It is caused by adenomas or carcinomas of the adrenal cortex that secrete an excessive amount of cortisol irrespective of pituitary control. The neoplasm is usually unilateral but in around 10% of the cases both glands may be involved. These neoplasms can secrete both cortisol and its precursors, as well as other molecules, for example adrenal hormones, as a result of the altered steroidogenesis.

Hypercortisolism due to the aberrant expression of hormone receptors.The expression of abnormal receptors in the adrenal glands may result in the development of hypercortisolism caused by molecules other than ACTH. A rare “diet dependent” form of hypersecretion of cortisol after meals has recently been described in a dog. The secretion of cortisol was stimulated by a gastric peptide (GIP) that acted on an aberrant receptor of the adrenal cortex.

Iatrogenic hypercortisolism. This situation occurs following prolonged and/or excessive administration of exogenous glucocorticoids. Besides the appearance of the classic signs and symptoms of Cushing’s Syndrome, this condition also causes inhibition of the hypothalamic-pituitary-adrenal axis with consequent atrophy of the adrenal cortex. The reason why some subjects are more sensitive than others to the “Cushingoid” action of glucocorticoids is not yet clear.

 

CLINICAL SIGNS AND SYMPTOMS

Chronic exposure to excessive levels of cortisol is manifested with a broad series of characteristic clinical signs which, nevertheless, may not always be present and may appear with varying degrees of severity.

Polyuria and polydipsia: these are extremely common signs during hypercortisolism (80-85% of subjects) and are often the main reason why the owner requests a veterinary consultation. A subject with hypercortisolism may consume 2 to 10 times more water than normal and thus more than 100 ml/Kg/day, which is the threshold that normally defines a state of polydipsia in the dog. Cortisol, produced in high quantities, interferes with the action of the antidiuretic hormone in the distal renal collecting tubules, causing a form of secondary nephrogenic diabetes insipidus. In these subjects a state of central diabetes insipidus - with a true antidiuretic hormone secretion deficiency - is also found.

Polyphagia: in the dog, the increase in appetite seems to be a direct effect of glucocorticoids. Polyphagia appears in over 90% of the cases of hypercortisolism and is probably due to an anti-insulin action of cortisol, which reduces glucose use by the tissues.

Barrel stomach (Figs. 1a and 1b): this symptom is detectable in 80% of dogs with hypercortisolism and is the result of an increase in abdominal content and a decrease in abdominal muscle tone due to the proteocatabolic effects of cortisol. The increase in abdominal content is attributable to the redistribution of body fats with accumulation in the omentum, to hepatomegaly (due to lipid infiltration and accumulation of glycogen) and to chronic bladder overdistension.

Muscular asthenia and lethargy: muscular asthenia manifests itself with exercise intolerance, difficulty in climbing stairs and carrying out intense physical efforts and it is the result of the protein catabolism mediated by glucocorticoids that affects the muscle tissue in 75-85% of subjects with Cushing’s Syndrome. Lethargy is probably the expression of the asthenia and of the muscle damage.

Skin manifestations: skin alterations during hypercortisolism are common and usually not associated with pruritus. Bilateral symmetric alopecia (Figs. 2 and 3) is one of the most common symptoms and is generally found on the trunk, sparing the head and limbs. It is the result of follicular and pilosebaceous atrophy, which causes hair loss and lack of regrowth.

 

The skin appears hypotonic and considerably thinned, with visible vascular markings (Fig. 4), tendency to develop haematomas, petechiae and suffusions.In addition, the skin is more subject to secondary infections that result from the immunosuppressive effect of glucocorticoids and are sustained by bacteria, fungi or mites (for example, demodicosis). Blackheads (Fig. 4) are often seen around the nipples and along the dorsal midline and they may sometimes be found along the entire trunk; areas of focal or widespread hyperpigmentation can be frequently observed. Calcinosis cutis is instead a less common finding (2-8%) [Figs. 5a and 5b), i.e. a dystrophic calcium deposit in the derma and in the subcutaneous tissue whose etiopathogenic mechanism is still not entirely known.

 

Dyspnoea: subjects with hypercortisolism have an increase in the deposition of adipose tissue in the chest and muscle weakness that also involves the respiratory muscles. These factors may lead to the onset of dyspnoea. The increase in pressure exerted on the diaphragm by the accumulation of abdominal adipose tissue, associated with hepatomegaly, may heighten the disturbances of the mechanism of ventilation. Other causes include pulmonary interstitial mineralisation and pulmonary thromboembolism.

Less common symptoms: dogs with hypercortisolism may occasionally show signs connected with the reproductive system, such as testicular atrophy or anoestrus. Myotonias are another quite rare phenomenon: occasionally, dogs with hypercortisolism develop a myopathy characterised by persistent muscle contractions and muscle stiffness (Video 1). In dogs, the cause of this alteration is still unknown.

Nervous system symptomatology: in subjects with pituitary macroadenoma, the compression on the surrounding structures may cause nervous system symptomatology characterised by alterations in the mental state (dull mentation, stupor), ataxia, tetraparesis, confusion and especially compulsive behaviour (Fig. 6) (Video 2).

PDH, pseudomyotonia

PDH, compulsion

 

 

 

 

 

 

 

LABORATORY TESTS

Complete blood count
The classic alteration present during hypercortisolism is the stress leukogram: 80% of subjects present lymphopenia and eosinopenia and 20-25% show a slight increase in total leukocytes. As a matter of fact, excessive cortisol production causes the demargination of the neutrophils and of the monocytes from the endothelium of the capillaries, causing neutrophilia and monocytosis. Another very common finding is thrombocytosis, while more rarely a mild erythrocytosis may be present.

Biochemical profile
The most common haematobiochemical finding is the increase in alkaline phosphatase (SAP). In fact, the excess in endogenous cortisol causes an increase in the corticosteroid-induced alkaline phosphatase. 85% of subjects with hypercortisolism has alkaline phosphatase values above 150 IU/L and these values often exceed 1000 IU/L. The liver enzymes, especially alanine aminotransferase (ALT), are generally increased due to the hepatopathy induced by steroids and by enzymatic induction. The discovery of an increase in ALT in the presence of a nearly normal AST is indicative of a hypercortisolaemic state. Glucocorticoids also stimulate lipolysis, thus causing an increase in lipaemia and in serum cholesterol in 90% of subjects. The insulin-resistance and the activation of haepatic gluconeogenesis caused by the endogenous cortisol may cause a moderate increase in glycaemia which in only 5-10% of subjects leads to a clinically overt diabetes mellitus. In 38% of subjects, the azotaemia and the creatininaemia may be lower than the reference range due to the heightened diuresis. Even serum electrolytes may show some alterations: in 33% of subjects a hypophosphataemia,caused by the increase in the renal excretion of phosphates, is detected; 50% of subjects show an increase in natremia and a slight drop in kalaemia. Haptoglobin is usually increased in dogs with hypercortisolism.

Chemico-physical examination of the urine
The most commonly found alteration in the urine of dogs with hypercortisolism is the decrease in specific weight. Indeed, in 85% of subjects the urine specific gravity is below 1.020. Around 40-50% of dogs suffer from urinary tract infections: the glucocorticoid-mediated immunosuppression, the chronic overdistension of the bladder and the presence of diluted urine facilitate the onset of lower urinary tract infections.

 

DIAGNOSIS

Specific endocrine tests
The specific endocrine tests for diagnosing hypercortisolism are based on the demonstration of the presence of excess blood glucocorticoid levels, as in the case of the urine cortisol-creatinine ratio (UCCR), on the demonstration of an excessive response following stimulation, on an abnormal activity of the pituitary-adrenal axis with the ACTH stimulation test, or by proving the loss of the negative feedback that circulating cortisol rates normally exercise on the pituitary gland in regard to the secretion of ACTH through the low dose (LDDST) or high dose (HDDST) dexamethasone suppression test in the blood or in the urine.

These tests are subdivided into screening tests and differentiation tests. Once the diagnosis of Cushing’s Syndrome has been reached, it is essential to be able to differentiate ACTH-dependent hypercortisolism, usually of pituitary origin, from ACTH-independent hypercortisolism, of adrenal origin, in order to set up a suitable treatment.

The screening tests used are the urine cortisol/creatinine ratio (UCCR), the ACTH stimulation test and the low dose dexamethasone suppression test (LDDST). In order to obtain diagnostic confirmation in a subject with suspected hypercortisolism, it is appropriate to have confirmation from at least two specific endocrine tests. The tests and the other differential type diagnostic methods include LDDST, HDDST on urine, HDDST on blood, endogenous ACTH measurement, abdominal ultrasound and advanced diagnostic imaging techniques like computed tomography and nuclear magnetic resonance.

Abdominal ultrasound
The abdominal ultrasound allows the evaluation of the shape and size of the adrenal glands. In addition, it makes it possible to highlight the presence of any alterations in the other abdominal organs, often present during hypercortisolism, such as hepatomegaly, the presence of urinary stones (uroliths) and liver tumour metastases.

During PDH, in approximately 97% of the cases, shape, contours, echogenicity and echotexture of the adrenal glands appear normal during ultrasound examination but with a symmetrical increase in dimensions. The best parameter for objectifying the increase in adrenal gland volume is the measurement of the diameter of the caudal pole of each adrenal gland, which should not exceed 7 mm regardless of the size of the animal.

Instead, during ADH, the echogenicity of the adrenal gland affected by the neoplasm appears variable and heterogeneous, with distortion of the contours and an irregular increase in dimensions; the contralateral adrenal gland, on the other hand, may appear atrophic or normal. It is important to recall that there are cases in which the tumour may involve both adrenal glands; in this case both adrenal glands appear larger and with altered echotexture and echogenicity.

Computed tomography
Pituitary gland dimensions differ among the various breeds and among dogs of the same breed. In one study on 35 healthy dogs, the tomographic measurements of the pituitary gland varied from 3.2 to 5.1 mm in height, from 4.2 to 6.9 mm in width and from 3.6 to 7.2 mm in length. The first sign of pituitary enlargement appears to be the increase in height (i.e. the dorsal extension into the suprasellar region); the gland is considered to be increased in volume when its dorsal contour protrudes above the suprasellar extension of the intercrural cisterns, easily detectable in the CT images.

As an alternative, we can consider the ratio between the height of the pituitary gland and the brain area measured in the image up to the centre of the pituitary gland (P:B ratio) to distinguish between an enlarged (P:B above 0.31) and a normal (P:B below or equal to 0.31) pituitary gland.

Large tumours are easily identified in CT images with contrast medium due to their size and altered shape (Fig. 6), while sometimes they can be difficult to detect without contrast medium. In 50-60% of dogs with pituitary hypercortisolism, a pituitary mass can be detected through CT; in the remaining 40-50% of the cases, the disease is caused by a microadenoma smaller than 3-4 mm, which is not even visible with contrast medium.

New CT techniques, which use a series of transverse scans passing through the centre of the pituitary gland during and after the rapid intravenous injection of a contrast medium (dynamic CT), are capable of detecting a difference in contrast between the neurohypophysis and the adenohypophysis. This is due to variations in blood flow; we notice a strong early contrast between the neurohypophysis in the centre (pituitary flush), as well as a weaker contrast, slightly delayed, of the adenohypophysis, more peripheral. A displacement or distortion of the neurohypophyseal flush may indicate the presence of small neoplasms.

 

TREATMENT

Hypercortisolism may be treated both medically and surgically.

PDH TREATMENT
Ideally, treatment of PDH should be aimed at eliminating the underlying cause of the endocrine disorder. In fact, in Human Medicine the surgical removal of the pituitary gland with a transsphenoidal approach is considered the treatment of choice for pituitary tumours. This type of operation has been performed successfully also in Veterinary Medicine, in the dog, and according to some authors it is the method of choice for treating canine PDH. Nevertheless, transsphenoidal hypophysectomy requires considerable surgical expertise and at this time only Dr. Meij of the University of Utrecht regularly performs this type of operation successfully. In the case of pituitary macroadenoma with concomitant neurological symptoms the only therapeutic possibilities are surgery and radiation therapy. Radiation therapy allows to considerably shrink the size of the pituitary mass.

The medical treatment of PDH involves the administration of active substances like trilostane, mitotane or ketoconazole.

Trilostane is a competitive inhibitor of 3β-hydroxysteroid dehydrogenase and it is currently the only drug approved in Italy for the treatment of canine hypercortisolism.This enzyme mediates the conversion of pregnenolone to progesterone and of 17-hydroxypregnenolone to 17-hydroxyprogesterone (Fig. 7). Cortisol, aldosterone and androstenedione are made from progesterone and from 17-hydroxypregnenolone (Fig. 7). Trilostane inhibits the production of progesterone and of 17-hydroxyprogesterone, with consequent reduction in the synthesis of the various steroids produced by the adrenal glands, by the gonads and by the placenta.

Several studies have been carried out in Veterinary Medicine that have highlighted a significant improvement in the clinical signs in dogs with hypercortisolism treated with trilostane (Fig. 8), even though variable responses were reported in the different subjects. Trilostane is an effective and safe drug for the treatment of PDH in the dog. Furthermore, this drug is the only one registered in Italy for the treatment of Cushing’s Syndrome in the dog.

The trilostane usage protocol was defined in a Consensus Meeting among European endocrinologists held on 19 April 2006.

Trilostane usage protocol
  1. The drug should be administered at a set time, in the morning, with food, at a starting dose between 2 and 5 mg/kg. In small dogs (<10 kg) start with the lowest possible dose. The first ACTH stimulation test should be performed 7-14 days after the beginning of treatment, 2-3 hours after the morning tablet (N.B. treatment should be made also on the day of the sampling). On this occasion, besides the clinical history and physical examination, it is recommended to perform some haemato-chemical-urine tests (control of kalaemia and of renal and haepatic parameters are especially important).
    •     If the clinical signs have regressed and the post-ACTH blood cortisol levels are between 1.5 and 5.4 mcg/dl (40-150 nmol/l), the treatment is defined optimal and maintained until the next controls.
    •      If the clinical signs have regressed, but the blood cortisol levels are below 1.5 mcg/dl in both samples, the recommendation is to discontinue treatment for 5-7 days and to then resume it at a lower dosage. In most cases these patients do not show signs of hypoadrenocorticism since blood cortisol levels tend to increase rapidly.
    •     If post-ACTH blood cortisol levels are above 5.4 mcg/dl and the dog does not show clinical improvement, the dosage is increased. No precise indications are available to date with regard to an optimal increase in the dosage for dogs that fall within this category; however, the following scheme proposed by Galac et al. (2008) may be used: from 10 to 20 mg, from 20 to 30 mg, from 30 to 40 mg, from 40 to 60 mg, from 60 to 90 mg, from 90 to 120 mg, etc.
    •     If the patient presents blood cortisol levels as in point a) but not clinical improvement is present, based on the severity of the symptoms decide on whether to increase the dose, change to administration every 12 hours or re-evaluate after 4 weeks continuing with the same dose.
    •     If the clinical signs have regressed but post-ACTH blood cortisol levels are between 5.4 and 9.0 mcg/dl (150-250 nmol/l), continue with the same dose and check again after 4 weeks.
    •     If the dog is in an obvious pathological state: discontinue treatment, identify the cause of the disease (perform ACTH stimulation test and haemato-chemical-urine tests) and treat symptomatically.
  2. Physical, haematological and biochemical signs are to be monitored and the ACTH stimulation test is to be repeated 4 and 12 weeks after the beginning of treatment, then every 3 months and 10 days after every dosage variation. The procedures listed in point 1 are also applicable for medium-long term monitoring.

Some authors maintain that the administration at low doses (0.5-2.5 mg/kg every 12 hours) gives better results, fewer side effects and makes it possible to normalise the blood-chemical profile. The monitoring protocol is similar to the one described above.

Mitotane (o,p’-DDD) is a molecule with adrenocorticolytic action that has been used for years as the drug of choice for treating PDH.Several mitotane usage protocols are available for canine hypercortisolism; some aim at the selective destruction of the reticular or fascicular zones, sparing the glomerular zone and the production of mineralcorticoids, while others aim at the complete destruction of the adrenal cortex, thus causing hypoadrenocorticism or Addison’s Disease, with consequent need for lifelong hormone supplementation.

The difference between the two types of approach is based on the amount of drug and the duration of the so-called loading phase. With the complete lysis of the adrenal cortex fewer relapses are reported; however, the owner should be well aware that the iatrogenic disease that is induced may be dangerous to the animal's life if a suitable glucocorticoid- and mineralcorticoid-based treatment is not carried out.

Mitotane usage protocol

Mitotane must be administered together with food as this increases its absorption. To obtain a selective action on the fascicular and reticular zones, treatment with o,p’-DDD involves a loading phase with a dose of 25-50 mg/kg subdivided into two administrations for 5-9 days. It is not necessary to administer glucocorticoids, but it is advisable to keep them on hand in case of emergency. It is appropriate to discontinue treatment whenever the patient shows a drop in appetite, when water consumption decreases below 60 ml/kg/day, in case of vomiting, diarrhoea or if the animal appears tired and lethargic. An ACTH stimulation test is performed after the first phase; if the response is normal or excessive, the treatment is continued with 3-7 day cycles. This is followed by the maintenance phase, in which the initial daily dose is administered within the space of one week (in 2-3 administrations). Treatment monitoring continues with the ACTH stimulation test. It is recommended to perform the ACTH stimulation test after the induction phase, one month after the beginning of the maintenance period, then every 3-6 months, unless problems occur, such as the reappearance of the signs and symptoms ascribable to Cushing’s Syndrome or symptoms of hypocortisolism or other side effects.

For complete lysis of the adrenal cortex the drug has to be administered for 25 continuous days at a dose of 50-75 mg/kg (100 mg/kg in small dogs) subdivided into three or four doses, always given with food, to reduce the gastrointestinal tract problems that may occur. The drug should be administered daily for the first 5 days and then every other day. The replacement therapy with cortisone acetate (2 mg/kg/day) (in alternative prednisolone) and fludrocortisone acetate (0.0125 mg/kg/day), both divided preferably into two daily administrations, begins on the third day. A follow-up visit is performed after the 25th day and the cortisone acetate/prednisolone dose may be reduced to 0.5-1.0 mg/kg per day. The subject must then be treated as an Addisonian for the rest of its life.

Undesirable effects may occur shortly after administration due to the alterations in the gastrointestinal tract and are characterised by nausea, vomiting, diarrhoea, or they may be due to the temporary glucocorticoid deficiency or to persistent hypoadrenocorticism, with signs and symptoms similar to the first group of side effects: vomiting, diarrhoea, weakness, ataxia and it is not always easy to distinguish between the two situations.

Today the use of mitotane has been widely reduced; it is used in the medical treatment of adrenal tumours (protocol with complete destruction of the gland) or when trilostane cannot be used.

Ketoconazole is a derivative of imidazole and is an antifungal agent. Nevertheless, at high doses, the drug also has an effect on steroid biosynthesis, suppressing cortisol secretion. The high cost of the drug, the inefficacy found in 20-25% of subjects and the need to administer it twice a day, have limited the use of ketoconazole in the treatment of hypercortisolism.

 

Ketoconazole usage protocol

The initial dose is 5 mg/kg twice per day for seven days; in the absence of changes in appetite and jaundice, the dose may be increased, reaching 10 mg/kg. Nevertheless, it has been reported that doses below 15 mg/kg may have an insufficient effect, while a high dosage (30 mg/kg) suppresses the hyperactivity of the adrenal cortex more adequately, but is often associated with the development of undesirable effects. Dosages that do not cause side effects are usually chosen, thus below 30 mg/kg.

 

ADH TREATMENT
ADH can be treated surgically, performing an adrenalectomy of the gland affected by the neoplasm. However, before the operation it is essential to evaluate whether or not the subject is a good candidate for the surgery. It is important to evaluate blood pressure; indeed, hypercortisolism causes hypertension in over 50% of subjects and the urinary protein/creatinine ratio is often increased. A marked increase in these parameters could indicate a high risk of thromboembolism. For the same reason it is advisable to make sure that the antithrombin III concentration in the serum is not below the norm.

During the operation and in the first six hours after surgery, it is important to administer 0.05-0.1 mg/kg i.v. of dexamethasone to the subject. The same dosage should be repeated 2 or more times during the day of the operation. On the following days the same dosage should be administered subcutaneously b.i.d. or t.i.d until the animal is able to take it orally without risk of vomiting.

Surgical treatment is not recommended when diagnostic imaging has detected a clearly inoperable neoplastic mass or metastasis to other organs. In these subjects it is possible to proceed with the medical treatment described beforehand and even though trilostane treatment is effective, it is preferable to choose the treatment with the adrenolytic agent mitotane (protocol of complete lysis of the adrenal cortex).

 

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