Pathogenesis and diagnosis of equine Cushing's disease (Proceedings)

Although the frequency of diagnosis and treatment of pituitary pars intermedia dysfunction (PPID) in horses has clearly increased over the past decade, there is no evidence that the prevalence of PPID is actually increasing. Increased recognition of the disease is likely a consequence of clients maintaining their horses to more advanced ages as well as improved health care (e.g., diet and dentistry) being provided to older horses. A recent survey of horse owners in Queensland, Australia revealed a prevalence of 15-20% of PPID in horses and ponies 15 years of age and older. There is no gender predilection and average age of affected horses is around 20 years. All breeds and types of equids can be affected with PPID but Morgan horses and ponies appear to be at greater risk (Figure 1).

Figure 1. Pituitary pars intermedia dysfunction (PPID) affects equids of all sizes and breeds although it may be more common in ponies (image courtesy of Dr. J. H. van der Kolk, Utrecht, Netherlands).

Pathophysiology

In humans and dogs, Cushing's disease is most commonly attributed to a corticotroph adenoma in the pars distalis of the pituitary gland. These adenomas are thought to arise spontaneously. In contrast, Cushing's disease in horses is almost exclusively attributed to hyperplasia or adenoma formation in the pars intermedia that appears to be due to loss of hypothalamic innervation. Abnormal pars intermedia tissue in horses contains markedly reduced amounts of dopamine, about 10% that of normal pars intermedia tissue, consistent with a specific loss of hypothalamic dopaminergic innervation. Recent evidence suggests that this loss of dopaminergic innervation is due to oxidant-induced injury to hypothalamic tissue. Thus, a risk factor for affected horses may be reduced anti-oxidant defense mechanisms in neural tissue. Further, insoluble aggregates of the neural protein α-synuclein have been found in dopaminergic nerve terminals of PPID-affected horses. These protein aggregates are also found in humans with Parkinson's disease suggesting that the two neurodegenerative disorders may share a similar pathogenesis. However, the population of neurons affected in horses, as compared to humans, appears to be somewhat different leading to the difference in clinical signs observed in each species.

Abnormal pars intermedia cells produce excessive amounts of pro-opiomelanocortin (POMC) and a number of POMC-derived peptides including adrenocoticotropin (ACTH). Also unlike Cushing's disease in humans and dogs, adrenocortical hyperplasia accompanying equine Cushing's disease is relatively uncommon, occurring in ~20% of affected horses. These differences in location and pathophysiology between human, canine, and equine pituitary adenomas have lead several authors to suggest that the disease in horses should not be called equine Cushing's disease; rather, pituitary pars intermedia dysfunction (PPID) has been advanced as a more appropriate descriptor.

Clinical signs

The classic clinical sign of PPID in horses is hirsutism, a long and curly hair coat that fails to shed. In some affected horses, coat color changes have also been observed (Figure 2, left). The pathogenesis of hirsutism, which is characterized by arrest of hair follicles in telogen, remains unclear. Hyperhidrosis is also observed in up to two-thirds of horses with PIPD, most commonly over the neck and shoulder areas, and has been attributed to a thermoregulatory response to the long hair coat. Weight loss and lethargy, or poor performance, are also commonly observed in horses with PPID. In addition to true weight loss, protein catabolism due to increased cortisol activity leads to loss of muscle mass. This is most notable in advanced cases as a loss of epaxial and rump musculature. Despite weight loss, appetite in affected horses is normal or even increased (polyphagia). However, dental abnormalities, leading to painful mastication and quidding, may compromise feed intake and contribute to weight loss in some horses. Combined with, or often preceding, loss of muscle mass is deposition of fat along the crest of the neck, over the tail head, and in the sheath of male horses. Another area where abnormal fat deposition may occur is above and behind the eyes (supraorbital area, Figure 2, center). Horses with PPID have also been described as overly docile and more tolerant of pain than normal horses. The latter signs have been attributed to increased plasma and cerebrospinal fluid concentrations of β-endorphin that are 60- and more than 100-fold greater, respectively, in horses with PPID than in normal horses.

Figure 2. Hirsutism with coat color change (left), supraorbital fat deposition (middle), and unilateral nasal discharge due to sinusitis (right) are some of the clinical findings in horses with PPID.

Chronic, insidious-onset laminitis is perhaps the major clinical complication of PPID with more than 50% of horses affected in most reports. Although the condition is more amenable to management in ponies due to their lower body weight, chronic or recurrent pain with exacerbation of laminitis or associated foot abscesses is often the reason for euthanasia. Polydipsia and polyuria (PU/PD) develops in about one-third of horses with PPID. Equids with PPID tend to have delayed wound healing and are frequently affected with secondary infections. Commonly recognized infections include skin infections (e.g., refractory "scratches" and fistulous tracts), recurrent subsolar abscesses, conjunctivitis, sinusitis (Figure 2, right), gingivitis, alveolar periostitis, and bronchopneumonia.

Other signs that have been reported in horses with PPID include persistent mammary secretions and infertility. Central nervous system (CNS) dysfunction, including ataxia, blindness, and seizure-like activity, are occasionally observed in equids with PPID. A major complication of hypercortisolism in affected human patients is osteoporosis. Although occurrence of this complication has not been investigated in horses, it is interesting to note that euthanasia of horses with PPID has been reported due to development of pelvic, pedal bone, mandibular, and multiple rib fractures.

Clinicopathologic findings

Abnormal laboratory data in horses with PPID may include mild anemia, an absolute or relative neutrophilia, and an absolute or relative lymphopenia. Although one or more of these abnormalities is usually found in a third or more of equids afflicted with PPID, the true prevalence is not well documented. As well as being increased in number, neutrophils in affected animals may appear hypersegmented. This finding reflects maturity of neutrophils and can be attributed to a longer half-life of circulating neutrophils because cortisol excess limits diapedesis from the vasculature. Eosinopenia is also recognized in human and canine patients with hyperadrenocorticism but is difficult to document in horses because equids typically have a low numbers of circulating eosinophils. The most common abnormality detected on serum biochemical evaluation is mild to moderate hyperglycemia, reported in 25-75% of cases, depending on the upper end of the reference range used. Additional abnormal biochemical findings may include elevations in liver enzyme activities, hypercholesterolemia, and hypertriglyceridemia.

Diagnosis

Practically, the diagnosis of PPID is most commonly made by observation of hirsutism and other clinical signs in older equids. However, establishing a diagnosis of PPID in less severely affected horses can be challenging. As a result, a number of endocrinologic tests have been used to evaluate horses with suspected PPID.

Plasma cortisol concentration and loss of diurnal cortisol rhythm

Resting cortisol concentration does not routinely exceed the upper end of the reference range in horses with PPID. Thus, measurement of plasma cortisol concentration alone is not a valid diagnostic test. Because plasma cortisol concentration has a diurnal rhythm of secretion, with an increase in the morning hours and a nadir around midnight (that should be at least 30% lower than morning values), loss of the diurnal rhythm has been advanced as a screening tool for evaluation of horses with suspected PPID. However, the effects of external stressors and disease on plasma cortisol concentration, not to mention collection of the evening sample at a late hour, makes loss of cortisol rhythmicity a poor screening tool for PPID.

Dexamethasone suppression test

The overnight dexamethasone suppression test (DST) is considered by many equine clinicians to be the "gold standard" endocrinologic test to support of a diagnosis of PPID. However, this statement is not without controversy and there is concern, although poorly documented, that administration of dexamethasone may induce or exacerbate laminitis in PPID-affected equids. In its most simple form, the overnight DST consists of measuring cortisol in the late afternoon (typically 5 pm) followed by administration of dexamethasone (40 µg/kg, IM = 20 mg to a 500 kg horse) and subsequently measuring plasma cortisol concentration 17 to 19 hours later (between 10 am and noon the following day). The major limitation of the overnight DST for ambulatory practitioners is that it requires two visits to the horse. However, considering the fact that the most important value is the cortisol concentration following dexamethasone administration, the overnight DST can be simplified by dispensing dexamethasone to the client for administration and limiting the test to one visit the following morning. When using this test, it is probably wise to consider dexamethasone as a "sledgehammer" in terms of feedback to the hypothalamic-pituitary axis. In other words, failure of dexamethasone to induce suppression of circulating endogenous cortisol concentration is strongly supportive of PPID. However, the overnight DST may be less effective in diagnosis of PPID in the earlier stages of the disease process. In this clinician's opinion, this is not an important limitation of the test because in the earlier stages of PPID, when DST results may be normal (not supportive of PPID), it may be difficult to justify treatments other than body clipping to limit hirsutism (unless laminitis is the primary clinical problem for which treatment with pergolide can be use on a "trial and error" basis).

Another limitation of using the DST is that seasonal variation can affect results. In a recent study of horses and ponies without clinical signs of PPID, abnormal DST results were found in 10 of 39 equids in September. To further examine the effect of season on DST results, the author performed the overnight DST monthly for a year in a group of 18 aged horses (>19 years) without clinical signs of PPID. Seven of 18 horses had normal overnight DST results throughout the year while 11 horses had overnight DST results supportive of PPID from 1 to 9 months of the year. Test results from late July through late October were most commonly affected by seasonal variation. Thus, the overnight DST is best performed from December through June and overnight DST results from July through November, if abnormal, should be interpreted with caution. Although the author prefers not to perform the test during these months, it warrants emphasis that normal overnight DST results during late summer to fall are valid and can be useful in case assessment. A further observation in the author's study that warrants mention is that no signs of laminitis were induced in this group of older horses during performance of 216 overnight DSTs.

Plasma ACTH concentration

Horses with PPID have excessive amounts of ACTH in abnormal pars intermedia tissue and increased amounts are released into plasma. Thus, plasma ACTH concentration would seem a likely choice for a single sample test to support a diagnosis of PPID. In fact, increased plasma ACTH concentrations, with a maximum reported value exceeding 12,000 pg/ml, have been documented in several reports of PPID in equids. Further, ACTH concentrations exceeding 27 or 50 pg/ml (~6 and ~11 pmol/l) in ponies and horses, respectively, have been reported to have a high sensitivity for diagnosis of PIPD. Limitations of using plasma ACTH concentration as the only endocrinologic test to support a diagnosis of PPID are that sample handling can be problematic and that different laboratories may use different assays for measuring ACTH. Because ACTH can be adsorbed onto glass and can be degraded by proteolytic enzymes in both whole blood and plasma, collection of blood into plastic tubes, rapid separation from red cells, and freezing of plasma prior to shipment for analysis has been recommended. Practitioners interested in using ACTH concentration as a diagnostic aid should contact the testing laboratory prior to sample collection for sample handling recommendations and should only send samples to a laboratory using an assay that has been validated as specific for ACTH in equine plasma. Another limitation of using plasma ACTH concentration is seasonal variation in test results. In normal ponies and horses without signs of PPID, plasma ACTH concentrations measured in September were above the threshold for diagnosis of PPID. Finally, ACTH is released in a pulsatile fashion from the pituitary gland. Consequently, plasma ACTH concentration can vary considerably over the day such that the absolute elevation in ACTH may not be all that useful for monitoring disease improvement with treatment. These limitations complicate use of plasma ACTH concentration as the sole endocrinologic test for both diagnosis and monitoring response to treatment of PPID.

Thyrotropin stimulation test and combined dexamethasone suppression/thyroptropin stimulation test.

Thyrotropin (TRH) is a releasing hormone for several pituitary hormones that has been shown to increase plasma cortisol concentration when administered to horses and ponies with PPID. Although the TRH stimulation test has not been as well validated as the overnight DST, it has been advocated for use in horses with laminitis because of concerns about exacerbating foot pain following dexamethasone administration. When used, a 50% increase in cortisol concentration between 15 and 90 minutes after administration of TRH can be supportive of a diagnosis of PPID. However, interpretation of the response is complicated by considerable variability of the initial cortisol concentration as well as the problem that up to 50% of normal horses may have a false-positive result with this test.

In an attempt to overcome these problems with this test, a combined DST/TRH stimulation test has been developed. Three hours prior to TRH administration, dexamethasone (40 µg/kg) is administered to suppress cortisol concentration to similar values in both PPID-affected and normal horses. Cortisol concentration is subsequently measured before and 30 minutes after TRH administration and equids with PPID show a greater increase in comparison to normal animals. After 24 hours, plasma cortisol concentration remains suppressed in normal horses while it returns to the basal (pre-dexamethasone) concentration in PPID affected horses. Although this combined test improves the accuracy of the TRH stimulation test, it is both more expensive for the client as well as less practical for the ambulatory clinician than the overnight DST. As a consequence, this combined test has not been widely used.

Domperidone stimulation test

The most recent endocrinologic test developed for diagnosis of PPID is a provocative test utilizing administration of domperidone, a dopamine receptor antagonist. In theory, this drug should exacerbate the loss of dopaminergic inhibition in horses with PPID and thereby increase release of endogenous ACTH by pars intermedia melanotropes. To test this hypothesis, plasma ACTH concentration was determined in 33 horses with or without clinical signs of PPID prior to and 4 and 8 hours after oral administration of domperidone (3.3 mg/kg). After testing horses were euthanized for histopathological examination of the pituitary glands. In this study, plasma ACTH concentration increased modestly (by about 50%) in horses without clinical signs of PPID or significant pars intermedia pathology while plasma ACTH concentration more than doubled in horses with clinical signs of PPID and more advanced pars intermedia histolopathologic abnormalities. Unfortunately, the domperidone challenge test was performed in the late summer and fall in some horses leading to seasonal variation as a possible confounding factor. Nevertheless, this novel test may offer promise of detection of PPID in the earlier stages of the disease and further investigation is warranted.

Serum insulin concentration

Some equids with PPID, especially ponies, may have insulin resistance. As a consequence, an elevated fasting serum insulin concentration could support a diagnosis of PPID. However, hyperinsulinemia can also accompany the "equine metabolic syndrome". Thus, use of serum insulin concentration alone for diagnosis of PIPD can be misleading because hyperinsulinemia is not specific to PPID. However, measurement of fasting insulin concentration may be of benefit in the initial evaluation of equids with suspected PPID because one case series found poorer survival in PPID-affected equids with hyperinsulinemia as compared to PPID equids with a normal insulin concentration.

Supplemental readings available on request to the author