1. Establish true presence of PU/PD

· Normal water intake typically 4-6% BWT/day.
PD is water consumption > 10% BWT/day (>50 litres per 500kg).
· Normal urine production typically 1-3% BWT/day.
PU is urine production > 5% BWT/day (>25 litres per 500kg).

2. Consider differential diagnoses

First rule out physiological causes such as lactation, hot weather, heavy work, diarrhoea (all resulting in PD but not PU) and excessive protein in diet – commonly alfalfa – leading to PU/PD due to increased urea excretion.

a. Common causes:
· Psychogenic polydipsia
· Equine Cushing’s Disease – may cause PU/PD in several ways:
i. Corticosteroid antagonism of insulin leading to secondary (Type II) Diabetes mellitus (DM)
ii. Reduced secretion of ADH caused by local presence of pituitary mass causing central Diabetes insipidus (DI).
iii. Diuretic effect of corticosteroids (?ADH antagonism).

b. Uncommon causes
· Chronic renal failure (CRF)– tubular failure to reabsorb water and electrolytes.
· Diabetes insipidus - central form caused by failure to secrete ADH.
- renal form caused by failure of nephron to respond to ADH.
· Primary (Type I) Diabetes mellitus – failure of pancreatic insulin secretion.

3. Initial laboratory work

a. Haematology
- anaemia common with CRF due to effect of uraemia and reduced erythropoietin.

b. Serum biochemistry
- azotaemia usually in CRF
- hypercalcaemia often in CRF
- hyperkalaemia sometimes in CRF
- hyperglycaemia sometimes in Equine Cushing’s Disease

c. Urinalysis

i. Specific gravity (SG). Normal urine specific gravity 1.020-1.060 (typically 1.030-1.040).
- hyposthenuria (<1.008) - often psychogenic polydipsia, DI
- isosthenuria (1.008-1.014) - often CRF
- hypersthenuria (>1.015) - often DM, Cushings disease or psychogenic polydipsia (in temporary remission)

ii. Glycosuria –indicates DM (Type I or Type II - Cushing’s Disease causing type II DM most likely).
(N.B. acute stress or ?2 agonist sedatives also cause hyperglycaemia and glycosuria).

iii. Urine creatinine : serum creatinine ratio Normally > 50. CRF < 40.

iv. Enzymuria – urinary GGT, AP and LDH may all be raised in CRF (more so in acute renal failure).

Enzyme levels calculated as follows after adjustment for urine creatinine concentration:

urinary enzyme (iu/l) x 1000 < 2.5 (GGT), 3.0 (AP), 1.0 (LDH) iu/mmol
urinary creatinine (mmol/l)

4. Further laboratory work

On the basis of the above results it should be possible to confirm/rule out CRF and DM. Cushing’s Disease (if not already suspected from DM) is usually suggested by typical clinical signs (old, hairy, laminitic etc..) or ‘overnight’ dexamethasone suppression test.

a. Water deprivation test
A water deprivation test will be required to differentiate psychogenic polydipsia from DI (both of which produce hyposthenuric urine). This test must not be performed on azotaemic horses. The object of the test is to establish whether or not the horse can produce concentrated urine (psychogenic polydipsia cases can; DI cases cannot).

Weigh horse accurately (if possible).
Check BUN and creatinine are normal (if not don’t proceed).
Take urine sample and measure SG (will usually be < 1.008 in cases requiring this test).
Remove water.
Check serum BUN and creatinine and urinary SG at least every 6 hours.

End of test is when one of the following occurs:
- 24 hours water deprivation
- 5% reduction in bodyweight
- clinical signs of dehydration
- azotaemia develops
- urinary SG > 1.020

Interpretation:
- if SG rises above 1.020 this confirms renal concentrating ability and therefore psychogenic polydipsia.
- if urine SG stays low and horse becomes dehydrated or loses 5% bodyweight this suggests DI (often happens by 12 hours with DI), although could be psychogenic polydipsia associated with ‘medullary washout’.
- if urine SG is still low after 24 hours but horse shows no clinical signs of marked dehydration this implies psychogenic polydipsia associated with ‘medullary washout’, although could be DI.

b. Modified water deprivation test
Strictly speaking this test is used to differentiate DI from psychogenic polydipsia. However, most DI cases will become rapidly dehydrated within 12 hours of water deprivation (see above) and this test is really used to confirm the suspected diagnosis of psychogenic polydipsia with medullary washout.

Start immediately following standard water deprivation test above if:
- urine SG < 1.020 after 24 hours water deprivation
- < 5% reduction in bodyweight
- no azotaemia
- no clinical signs of dehydration.

Test involves allowing restricted access to water for 2-3 days or until one of the criteria above is reached. Allow water consumption equivalent to 4% bodyweight per day (20 litres per 500kg) offered in several aliquots through the day.
Measure serum BUN and creatinine and urinary SG at least every 6 hours.

If urine SG rises above 1.020 this confirms psychogenc polydipsia. Continued inability to concentrate urine confirms DI.

c. ADH response test
Used to differentiate central and renal DI. Central DI cases successfully concentrate urine following ADH administration. Renal DI cases shown no response to ADH administration.

© The Liphook Equine Hospital 2005