equine pituitary dysfunction

Question 1

the pars intermediate produces:

Answer 1

melanotrophes:
- POMC = precursor for the below
- alpha MSH
- CLIP
- beta endorphin
- beta MSH
- beta LPH
- ACTH

Question 2

what hormones upregulate POMC activity and which downregulate

Answer 2

upregulate:
- coticotrophin RH
- arginine vasopressin

downregulate:
- dopamine

Question 3

ACTH is a minor post translational product of POMC. why? whch one is most active under adenoma influence

Answer 3

ACTH is further cleaved into other products
- beta endorphin is the more active form

Question 4

what is the difference between PPID and HAC in dogs

Answer 4

• adenoma is in the pars intermedia rather than adenohypophysis
• leading to excessive productio of POMCs derived from peptides

Question 5

what is the mechanism behind PPID

Answer 5

less dopamine from hypothalamus to inhibit POMC cleavage

Question 6

how is PPID treated

Answer 6

• aim is to inhibit expression of POMC mRNA expression and POMC hormone release
• give D2 agonist (decreases ACTH concentration in most cases)

Question 7

PPID is a ____ disease that results from the ________ of the pituitary _______

Answer 7

PPID is a central disease that results from the hypertrophy of the pituiatary pars intermedia

Question 8

what are the clinical signs of PPID

Answer 8

• generalised hypertrichosis
• weight loss/wastage
• PU/PD
• laminitis
• recurring infections
• poor performance
• regional adiposity/potbelly
• docility/lethargy
• neurologic signs
• infertiltiy

Question 9

Why do horses with PPID experience hypertrichosis

Answer 9

• do not shed, even in warm seasons
• chronic elevation of MSH?
• pituitary conpression on the hypothalamic thermoregulation centre
• increased production androgens

Question 10

why do horses with PPID experience laminitis

Answer 10

• persistent hyperinsulinaemia triggered by CLIP release
• possibly high glucocorticoid concentration

Question 11

why do horse with PPID experience lethargy/docility

Answer 11

beta endorphins have sedative effect (increase in circ)

Question 12

Why do we see neurologic impairment in horses with PPID

Answer 12

• blindness due to compression of optic chiasm

Question 13

what factors and tests help make a diagnosis of PPID

Answer 13

• hx: age (over 15 years, but at least 7), pony?, persistent laminitis episodes, recurrent infections etc.
• physical exam: muscle wasting, coat chanes, fat depsoits
• biochemistry: variable (sometimes hyperglycemia/hyperinsulinaemia, rarely neutrophilia/lymphopenia/hypertriglyceridemia
• testing: resting ACTH or TRH stim

diagnosis only confirmed post mortem

Question 14

describe the process of a resting ACTH test for diagnosis of PPID

Answer 14

• collect blood in EDTA tubes and separate plasma within 4 hours of collection (careful of high temps in car)
• submit chilled
• positive horses should hace high ACTH levels

Question 15

describe the process of TRH stim testing to diagnose PPID

Answer 15

• take baseline blood sample
• administer 1 mg TRH iv
• blood sample 10 minutes later
• measure ACTH concentration
• positiv if ACTH is >110pg/ml at 10 mins

inaccruate in autumn months

Question 16

what tests do we NOT do for PPID

Answer 16

• basal cortisol
• dexamethasone suppression test
• TRH stim test measuring cortisol levels
• combined dex suppression and TRH

Question 17

how can we theoretically measure POMC to diagnose PPID

not avaible commercially

Answer 17

• beta endorphin, alpha MSH and CLIP will be disproportionally higher than ACTH in horses with PPID
• not availible commercially
• not validated in horses

Question 18

how can we use imaging to diagnose PPID and what are the pros/cons

Answer 18

• CT, MRI might identify pituitary enlargement
• CT possible but requires contrast
• MRI might be useful but hardly availible
• both require GA or heavy sedation
• costly
• poor sensitivity

Question 19

why dont we perform ACTH stim tests on horses

Answer 19

useless in horses because unlike other species, equine cushings is central and not peripheral in origin