The most common cause of feline lower urinary tract disease is idiopathic cystitis. Feline idiopathic cystitis, formerly
called idiopathic feline lower urinary tract disease, is defined as a disease of undetermined etiology characterized by hematuria,
dysuria, pollakiuria and possible urethral plug formation. This condition overlaps with 2 clinical syndromes of dysuria/pollakiuria
syndrome and urinary obstruction. The syndrome of dysuria/pollakiuria in cats is often associated with hematuria and was initially
referred to as the "feline urologic syndrome" or "FUS". Idiopathic cystitis is one of several differential diagnoses for dysuria/pollakiuria
in cats. Unfortunately many veterinarians assume all cats with dysuria/pollakiuria or urinary obstruction should all receive
the same stereotyped treatment without first obtaining a diagnosis. This leads to frustration when cases don't resolve with
treatment.
Diagnosis
Differential diagnoses for lower urinary tract diseases in young adult cats from most common to least common include idiopathic
cystitis (with or without urethral plug formation), urolithiasis, iatrogenic disorders (urethral tears, urethral stricture),
bacterial UTI, neoplasia (lymphoma, transitional cell carcinoma), fungal UTI, prostate disease, and idiopathic detrusor instability.
The role of urachal diverticula in lower urinary tract disease in cats is controversial. While cats with diverticula may
have persistent clinical signs that have been suggested to be caused by the diverticula, diverticula may spontaneously regress
with resolution of lower urinary tract disease suggesting the diverticula are a result of the disease rather than a cause
of the disease.6 The causes of lower urinary tract disease in geriatric cats are different than young adult cats. In one study of geriatric
cats, the causes from most common to least common were UTI (46%), urolithiasis with UTI (17%), urolithiasis without UTI (10%),
urethral plugs (7%), traumatic injury (7%), idiopathic cystitis (5%), and neoplasia (3%). The higher incidence of UTI in
geriatric cats versus only 1-2% incidence of UTI in young adult cats with lower urinary tract disease is due to underlying
diseases that predispose geriatric cats to UTI such as chronic kidney disease, diabetes mellitus, and hyperthyroidism.
Diagnosis of idiopathic cystitis is based on ruling out other known causes of lower urinary tract disease; it is an exclusion
diagnosis. The minimum work-up should consist of a urinalysis, urine culture, and abdominal radiographs, although these do
not rule-out all possible causes of lower urinary tract disease. Cases that persist beyond 5 to 7 days may require additional
work-up, such as ultrasound or contrast radiographs to rule out radiolucent uroliths and neoplasia, and urine cultures for
unusual organisms (mycoplasma and ureaplasma). Cystoscopy may be used to confirm the diagnosis of idiopathic cystitis. 
