South African Equine Veterinary Association Congress 2015 Protea Hotel Stellenbosch
Figure 1. Plasma ACTH concentrations measured
monthly in groups of normal horses (filled
squares) and PPID-affected equids (open circles)
demonstrating greater seasonal increases in
ACTH in fall months in PPID-affected horses; the
hatched line is the seasonally-adjusted upper limit
of the reference interval (from Copas VEN,
Durham AE. Equine Vet J 2012;44:440).
Dexamethasone suppression test (Tier 1): The overnight dexamethasone suppression test (ODST)
is still considered by some 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 ODST consists of
measuring cortisol in the late afternoon (typically 5 pm) followed by administration of
dexamethasone (40 g/kg, IM = 20 mg to a 500 kg horse) and subsequently measuring plasma
cortisol concentration between 17 and 19 h hours later (between 10 am and noon the following
day) (Figure 2). The major limitation of the ODST 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 ODST 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. Unfortunately, as with plasma ACTH concentration, the ODST may
be less effective in diagnosis of PPID in the earlier stages of the disease when test results remain
normal.
Figure 2. Overnight dexamethasone suppression test
(ODST results in 43 horses with pituitary pars
intermedia dysfunction (PPID) confirmed at
necropsy and 18 non-PPID horses. Endogenous
cortisol was measured prior to dexamethasone
administration (40 g/kg, IM) and again 15 and 19
hours later. Only 2 of 43 PPID-affected horses had
an endogenous cortisol concentration <1.0 µg/dL
(≈30 pmol/L, dashed line) at 15 hours and all 43
horses had an endogenous cortisol concentration
>1.0 µg/dL at 19 hours. In contrast, all 18 non-PPID
horses had suppression of endogenous cortisol
concentration to <1.0 µg/dL at both 15 and 19 hours.
(adapted from Dybdal NO, Hargreaves KM,
Seasonal
alsoMed
affect
ODST results but to a lesser extent than ACTH concentration.
Madiganvariation
JE, et al. Jcan
Am Vet
Assoc
To1994;204:627).
examine the effect of season on ODST results, the author performed the test 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, results of tests performed
from July through November, if abnormal, should be interpreted with caution. However, it
warrants emphasis that normal ODST 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
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