PPID in horses Flashcards
What is PPID?
Pituitary pars intermedia dysfunction
What are the cells and secretions of the pars intermedia?
Melanotrophes - POMC (proopiomelanocortin)
What are the products of POMC and their functions?
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
Why does POMC only produce ACTH in the pars distalis?
enzymes are not present to continue breakdown
ACTH production predominantly in pars distalis (only 2% in intermedia)
Describe the pathophysiology of PPID
Pars intermedia adenoma => excessive production of POMC derived peptides
Describe the control of POMC cleavage
CRH (corticotrophin releasing hormone) and ADH (arginine vasopressin) stimulate POMC cleavage
Dopamine inhibits POMC cleavage
Why do horses with PPID not have high levels of cortisol?
Pars intermedia does not produce biologically active form of ACTH to act on adrenal glands
What can occur secondary to PPID?
Growth of pars intermedia => compression of pars distalis and nervosa => clinical signs relating to other areas of pituitary damage
Describe the pathogenesis of PPID
Lack of inhibitory control of pars intermedia cells via dopamine permits development of adenomas
what is the action of administering D2-agonists to a horse with PPID
Increases inhibition of POMC cleavage (D2 = dopamine receptor):
- reduced clinical signs
- decrease ACTH conc
What are the clinical signs of PPID?
Hypertrichosis (excessive hair growth)
Weight loss/wastage
PU/PD
Laminitis
Lethargy
Blindness
Impaired response to infection
Infertility/abnormal oestrus cycle
Why does PPID cause hypertrichiosis?
Chronic elevation of MSH
Pituitary compression on hypothalamic thermoregulatory centre (prevent shedding in warm seasons)
Increase production androgens
Why does PPID cause laminitis?
persistent hyperinsulinaemia (triggered by CLIP)
Why does PPID cause PU/PD?
pituitary compression induces decreases ADH secretion
Why does PPID cause weight loss?
Makes body use protein as energy source
Why does PPID cause lethargy?
Beta-endorphin
Why does PPID cause blindness?
compression of optic chiasm
Why does PPID cause an impaired response to infection?
increased conc on immunosuppressive hormones
Why does PPID cause abnormal oestrus cycle/infertility?
Compression of pars distalis or hypothalamus => abnormal release from gonadotrophs
Describe the epidemiology of PPID (pars pituitary intermedia dysfunction)
common in older horses
Ponies are predisposed
What history suggests PPID?
History:
- age
- persistent laminitis episodes
- recurrent infections
What things on a physical exam can you look at to diagnose PPID?
muscle waste
Coat
fat deposits
What tests can be used to diagnose PPID(pituitsry pars intermedia dysfunction) ?
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
