Urolithiasis Flashcards
What are struvite uroliths made of? Why are they formed?
- Magnesium ammonium phosphate (MAP)
- Supersaturation of urine by MAP leads to struvite urolith
What are risk factors for struvite uroliths in dogs and cats?
Dogs
* Usually associated with UTIs caused by urease producing bacteria - these produce uraese which increase in pH of urine allows for urolith formation
* Account for 40% of stones affecting lower urinary tract of dogs
Risk Factors
* Abnormal retention of urine; any condition predisposing to UTI – eg diabetes mellitus, hyperadrenocorticism
* Susceptible breeds – Miniature schnauzer, Shih Tzu, Bichon
Cats
* 90% are sterile – in contrast to dogs (infection-induced seen predominantly in cats > 10 yrs)
* Account for 50% of stones affecting lower urinary tract of cats
Risk Factors
* Abnormal retention of urine
* Formation of concentrated urine – moisture content of food, water intake
* Urine-alkanising metabolites in diets
Why are calcium oxalate uroliths formed? What are specific risk factors?
Formation:
* Hypercalciuria, hyperoxaluria, hypocitraturia
* Hypercalciuria – increased intestinal reabsorption of calcium or reduced renal tubular reabsorption - Hypercalcaemia – renal tubular reabsorption mechanisms overwhelmed
* Recurrence common
* Account for approx. 40% of stones in dogs and cats
Risk factors:
* Acidifying diets
* Oral calcium supplements given outside of mealtimes
* Excessive dietary protein
* Formation of concentrated urine
How are urate uroliths formed? What are risk factors?
Formation:
* Composed of uric acid, sodium urate or ammonium urate
* Impaired conversion of uric acid to allantoin leads to increased concentration of uric acid in serum and urine.
* 5-8% of uroliths in dogs and cats
* Dalmations and Black Russian terriers have breed predisposition
* Often associated with PSS – due to impaired hepatic metabolism of uric acid and ammonia - so urate uroliths also associated with breeds predisposed to PSS eg Yorkshire Terriers
Risk factors:
* High Purine intake (glandular meat)
* Persistent aciduria in a predisposed animal
Why are cysteine urotliths formed? What are risk factors?
Formation:
* Cystinuria – inborn error of metabolism caused by defective tubular resorption of cysteine and other amino acids
* Breeds with genetic mutations include Newfoundlands, Labradors, Australian cattle dogs, mastiffs and bulldogs
* Not all cystinuric dogs develop cysteine uroliths – cystinuria is a predisposing factor
* Predominantly intact male dogs affected
Risk factors:
* Genetic predisposition to cystinuria
* Acidic, concentrated urine
* Urine retention
Why are calcium phosphate uroliths formed? What are risk factors?
Formation:
* Pure Calcium Phosphate uroliths - less than 1% of all uroliths in dogs and cats
* Often a minor component of Struvite and Calcium Oxalate uroliths
Risk factors:
* Excessive dietary calcium, primary hyperparathyroidism
* UTI – with urease producing bacteria
Why are xanthine uroliths formed? What are risk factors?
Formation:
* Impaired Xanthine oxidase activity leads to hyperxanthinaemia and xanthinuria
* Familial or congenital defect e.g. CKCS
* Allopurinol therapy can lead to acquired xanthinuria
Risk factors:
* Genetic predisposition
* Acid urine, highly concentrated urine
* Urine retention
* Allopurinol treatment
What clinical signs are associated with urolithiasis?
- Lower urinary tract signs – dysuria, haematuria, pollakiuria
- +/- signs of urinary obstruction
- Urate – signs of PSS
How are uroliths diagnosed? Why is it important to undertake diagnosis?
Urinalysis
* Not as helpful as you might think!
* pH is useful – partly clue to urolith type, but also for monitoring treatment/dietary management
* Need to rule out UTI – and if present, must treat especially if struvite uroliths
Imaging
- radiography
- ultrasound
A diagnosis of urolithiasis without knowing the composition makes it impossible to select effective management.
Which uroliths are radiopaque and radiolucent?
- Radiopaque uroliths – Calcium Oxalate, Struvite, Calcium phosphate - make up 80-90% of uroliths - good because can see on Xray
- Radiolucent uroliths – Xanthine - cant see on Xray
- Variable – Urate, Cysteine
What urolith is this an example of?
struvite - usually round/faceted, appear smooth
What urolith is this an example of?
Calcium oxalate - irregular appearance
What medical treatment can be used to treat uroliths?
- Analgesia – usually NSAID
- Antibiotic if concurrent UTI (e.g. with Struvite) – culture and sensitivity as will usually need prolonged course if uroliths present
- Lithotripsy – only one place in the UK currently, owners may ask about it.
Specific urolith treatment - Urate
* Allopurinol – Xanthine Oxidase inhibitor – for dissolution – takes at least 4-6 weeks, sometimes required long term
* Not effective if PSS – need to manage the shunt
How is diet used to treat uroliths?
- Can dissolve struvite if treating the UTI with antibiotics
- Calcium oxalate - cannot dissolve but will prevent
- diets are mainly used for prevention won’t stones are removed
What surgical treatment can be used for uroliths?
Which uroliths?
* Calcium Oxalate, Calcium phosphate, and larger Struvite uroliths (as will take a while to dissolve)
* Cystotomy, urethrotomy
How?
* If blocked – try and push urolith(s) back into bladder by retrograde urohydropulsion
* Cystotomy – rarely an emergency – remember lower ASA risk if planned not emergency, cam ensure adequate personnel available, do in morning so time to recover and usually home same day
* Post-op – warn owner of likely haematuria, need to monitor urine output, watch for dysuria
* Peri and post op analgesia – Opiod, NSAID
* If Struvite suspected, take urine sample for C & S (unless done recently)
How can you prevent uroliths forming?
Really important – high rate of recurrence especially Calcium Oxalate – 50% of dogs reform uroliths in 2-3 years; 30% of cats
