Page 939 - Equine Clinical Medicine, Surgery and Reproduction, 2nd Edition

914 CHAPTER 6

VetBooks.ir Diagnosis insulin dysregulated. Measurement of endogenous
ACTH concentration is a popular option to diag-
In addition to the clinical signs, certain changes may
be seen in a complete blood count and on a serum
higher physiological values observed in the autumn,
chemistry panel that support a presumptive diag- nose PPID. Circannual variability, with normal
nosis. These include a mature neutrophilia, lym- has led to seasonal reference intervals being devel-
phopenia and mild hyperglycaemia due to increased oped by laboratories to help properly interpret the
glucocorticoid secretion. results, especially in early stage cases.
The pituitary gland can be visualised by advanced The dexamethasone suppression test (Table 6.7)
imaging techniques (computed tomography [CT] is another common test to diagnose PPID that
or magnetic resonance imaging [MRI]), but these relies on the ability of an exogenous glucocorticoid
techniques are not used routinely because of poor to induce a negative feedback and reduce cortisol
reference intervals and the possible need for gen- secretion. There is some fear that administration of
eral anaesthesia or heavy sedation. They may also dexamethasone to a horse with PPID and laminitis
lack sensitivity for early detection of the disease as might exacerbate the latter. While the risk of this
dysfunction of the pars intermedia of the pituitary appears to be low and not demonstrated, many other
gland could be observed before significant enlarge- tests carry no risk at all. Additionally, response to
ment occurs. the dexamethasone suppression test is not highly
Several tests have been described for the diagno- repeatable and may produce false-positive and false-
sis of PPID, but many of them have low sensitivity negative results.
and/or specificity. Measurement of serum cortisol The TRH stimulation test (Table 6.8) has
concentration is not useful as it is usually within been developed to increase testing sensitivity in
the reference interval. It is thought that horses with milder cases in the months outside the autumn.
PPID lose the normal circadian variation of serum Stimulation of TRH receptors on the cortico-
cortisol and it has been proposed that PPID could tropes of the pituitary pars distalis and on the
be diagnosed by finding a <30% decrease in serum melanotrophs of the pars intermedia results in
cortisol measured in the late afternoon relative to an increase in ACTH. This increase, however, is
measurement in the early morning. However, loss of greater in horses with PPID. Domperidone is a
a circadian pattern of cortisol secretion is not spe- dopamine receptor antagonist that can stimulate
cific to PPID, thus using it to establish a PPID diag- the melanotrophs of the pars distalis resulting in
nosis would result in a high number of false-positive an increased secretion of ACTH. In horses with
results. PPID, the oral administration of 3.3 mg/kg of
Before EMS was identified as a separate entity, domperidone results in a larger ACTH secretion
increased fasting serum insulin was thought to be than in control horses.
suggestive of PPID. Subsequently it has been shown There is evidence that the results of diagnos-
that PPID has no significant effect on insulin levels tic tests, when performed on multiple occasions,
and that less than 30% of horses with PPID were may vary within an individual horse to the point

Table 6.7 Protocol for the dexamethasone suppression test

• Collect baseline plasma sample for cortisol concentration around 17.00 hours
• Administer dexamethasone (40 μg/kg i/m)
• Collect a plasma sample for cortisol concentration 17–19 hours later (around 10.00 hours)
• Collection of plasma samples at 15, 19 and 23 hours may be more useful, particularly in young or mildly affected horses. Some
horses that are just starting to manifest clinical signs of pituitary pars intermedia dysfunction may be suppressed at 8.00 hours
or 12.00 hours noon, but are no longer suppressed by 12.00 hours noon or 16.00 hours.