PPID in horses

Question 1

What is PPID?

Answer 1

Pituitary pars intermedia dysfunction

Question 2

What are the cells and secretions of the pars intermedia?

Answer 2

Melanotrophes - POMC (proopiomelanocortin)

Question 3

What are the products of POMC and their functions?

Answer 3

ACTH - increases corticosteroid release via adrenal gland stimulation
MSH - regulation of appetite, sexual behaviour and melanin production
CLIP - modulation of pancreatic exocrine function
beta-endorphin - behaviour
beta-lipotropin - melanin production, steroidogenesis, lipolysis

Question 4

Why does POMC only produce ACTH in the pars distalis?

Answer 4

enzymes are not present to continue breakdown
ACTH production predominantly in pars distalis (only 2% in intermedia)

Question 5

Describe the pathophysiology of PPID

Answer 5

Pars intermedia adenoma => excessive production of POMC derived peptides

Question 6

Describe the control of POMC cleavage

Answer 6

CRH (corticotrophin releasing hormone) and ADH (arginine vasopressin) stimulate POMC cleavage
Dopamine inhibits POMC cleavage

Question 7

Why do horses with PPID not have high levels of cortisol?

Answer 7

Pars intermedia does not produce biologically active form of ACTH to act on adrenal glands

Question 8

What can occur secondary to PPID?

Answer 8

Growth of pars intermedia => compression of pars distalis and nervosa => clinical signs relating to other areas of pituitary damage

Question 9

Describe the pathogenesis of PPID

Answer 9

Lack of inhibitory control of pars intermedia cells via dopamine permits development of adenomas

Question 10

what is the action of administering D2-agonists to a horse with PPID

Answer 10

Increases inhibition of POMC cleavage (D2 = dopamine receptor):
- reduced clinical signs
- decrease ACTH conc

Question 11

What are the clinical signs of PPID?

Answer 11

Hypertrichosis (excessive hair growth)
Weight loss/wastage
PU/PD
Laminitis
Lethargy
Blindness
Impaired response to infection
Infertility/abnormal oestrus cycle

Question 12

Why does PPID cause hypertrichiosis?

Answer 12

Chronic elevation of MSH
Pituitary compression on hypothalamic thermoregulatory centre (prevent shedding in warm seasons)
Increase production androgens

Question 13

Why does PPID cause laminitis?

Answer 13

persistent hyperinsulinaemia (triggered by CLIP)

Question 14

Why does PPID cause PU/PD?

Answer 14

pituitary compression induces decreases ADH secretion

Question 15

Why does PPID cause weight loss?

Answer 15

Makes body use protein as energy source

Question 16

Why does PPID cause lethargy?

Answer 16

Beta-endorphin

Question 17

Why does PPID cause blindness?

Answer 17

compression of optic chiasm

Question 18

Why does PPID cause an impaired response to infection?

Answer 18

increased conc on immunosuppressive hormones

Question 19

Why does PPID cause abnormal oestrus cycle/infertility?

Answer 19

Compression of pars distalis or hypothalamus => abnormal release from gonadotrophs

Question 20

Describe the epidemiology of PPID (pars pituitary intermedia dysfunction)

Answer 20

common in older horses
Ponies are predisposed

Question 21

What history suggests PPID?

Answer 21

History:
- age
- persistent laminitis episodes
- recurrent infections

Question 22

What things on a physical exam can you look at to diagnose PPID?

Answer 22

muscle waste
Coat
fat deposits

Question 23

What tests can be used to diagnose PPID(pituitsry pars intermedia dysfunction) ?

Answer 23

Resting ACTH in EDTA tube (not used in horses)
TRH stimulation test
POMC - disproportionally higher than ACTH if positive
Diagnostic imaging (CT, MRI to identify pituitary enlargement
Comorbidities - evaluate insulin resistance in horses with laminitis