LAB BOOK
INVESTIGATING PITUITARY PARS INTERMEDIA DYSFUNCTION (PPID)
“EQUINE CUSHING’S DISEASE”
Equine Cushing’s Disease is caused by gradual and progressive hyperfunction of the pars intermedia of the pituitary gland as a result of
loss of inhibitory dopaminergic innervation. It represents a “normal” ageing change and as the disease is progressive in nature defining
when the disease is present is perhaps theoretical. Of greater concern is recognition of when PPID may be contributing to or increasing the risk of other clinical disease, most notably laminitis. Although PPID advances with age it is not merely a disease of geriatrics and
should be considered as a possible factor in the development of laminits in all animals over 10 years old and perhaps even younger.
A number of tests have been advocated for the diagnosis of PPID, however recent expert consensus is that ACTH
provides the most reliable screening test with a diagnostic accuracy of around 90%.
Accuracy of ACTH measurement may be improved by measurement of paired samples to eliminate the effects of pulsatile ACTH
release. Unpublished data from our laboratory and other centres indicates that variation in ACTH concentration between samples only
occurs in a small minority (<5%) of cases to a level that it might affect clinical interpretation. It is our view that whilst measurement of
paired samples may be helpful it is not strictly necessary. If going to the trouble and expense of measuring ACTH twice then there is a
strong argument for doing a TRH stimulation test rather than measuring paired samples.
ACTH fluctuates on a seasonal basis and local seasonally-adjusted reference ranges need to be applied (Copas et al. 2012). Between
August and October pituitary activity increases far more in PPID than in normal horses and this represents the optimal time of year to
test for PPID (Figure 1).
plasma ACTH pg/mL
250
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N
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Figure 1. Illustration of seasonal variability of plasma ACTH in normal (n=156, white boxes) and PPID horses (n=962, grey boxes). Boxes
represent median values and whiskers represent interquartile range.
Copyright © 2012 The Liphook Equine Hospital.
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LAB BOOK
The effects of pain and stress on ACTH concentration are minimal (by contrast to the marked effects on insulin concentration) and it
can therefore be measured in cases of acute laminitis. ACTH is temperature sensitive and has to be handled appropriately between
collection and analysis. ACTH concentration typically decreases by 10-20% per day if samples are held at room temperature so whilst
it is preferable that samples are kept chilled it is possible to estimate the changes that are likely to have occurred if sample handling was
sub-optimal.
Measurement of ACTH concentration
1.
Collect a single blood sample into an EDTA (purple) tube
2.
Chill the sample within 3 hours of collection.
3.
Separate plasma from the cell fraction prior to posting, either by centrifugation or by gravity
4.
Chill the sample en route to the laboratory using specialised chiller packs – supplied free on request
t
Timing of separation is unimportant as long as the sample is chilled within 3 hours of collection.
t
Freezing is unnecessary although might help if the delivery is delayed.
t
Never freeze samples that have not been separated by centrifuge.
TRH Stimulation Test:
Measurement of ACTH following stimulation with thyrotropin stimulating hormone (TRH) has slightly higher diagnostic accuracy than
measurement of resting ACTH and is a very useful test when the results of ACTH screening are equivocal. An alternative to the TRH
stimulation test is the dexamethasone suppression test, however this test is affected by season and is generally regarded to be less sensitive than other methods. The effects of season on the TRH stimulation test have not been published though research is underway. For
the time being this test should probably be avoided between mid July and the end of October. When looking back at older literature
it is important to realise that the TRH stimulation test was originally performed using paired cortisol measurements which (just like the
measurement of resting cortisol) was not reliable in diagnosing PPID.TRH is supplied by The LEH laboratory on request.
PPID is indicated by either a baseline plasma ACTH value greater than the seasonally adjusted reference range (typically >29 pg/mL)
and/or a post stimulation plasma ACTH value greater than 100 pg/mL.
Performing a TRH Stimulation Test
5.
Collect a baseline plasma sample for ACTH
6.
Inject 1mg TRH IV (please contact us for details on obtaining TRH)
7.
Collect a further plasma sample 10 and/or 30 minutes later for assessment of post stimulation ACTH
8.
Process and dispatch samples as per previous section on ACTH measurement reporting the time post collection to facilitate interpretation
(Figure 2).
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Copyright © 2012 The Liphook Equine Hospital.
LAB BOOK
1750
1500
ACTH pg/ml
1250
1000
750
500
250
0
0
10
20
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60
Time post-TRH(mins)
Figure 2. ACTH responses following administration of TRH in horses with PPID (squares) and healthy horses (diamonds).
Further Reading:
Copas, V. and Durham, A.E. (2012) Circannual variation in plasma adrenocorticotropic hormone concentrations in the UK in normal
horses and ponies, and those with pituitary pars intermedia dysfunction. Equine Vet J44, 440-443.
Beech, J. et al.(2007) Adrenocorticotropin concentration following administration of thyrotropin-releasing hormone in healthy horses
and those with pituitary pars intermedia dysfunction and pituitary gland hyperplasia. J Am Vet Med Assoc231, 417-426.
Copyright © 2012 The Liphook Equine Hospital.
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