Chronic Interstitial Nephritis Chronic Interstitial Nephritis (CIN) is recognized as the most prevalent microscopic diagnosis underlying Chronic Kidney Disease (CKD) in geriatric cats. It is characterized by chronic inflammation and subsequent fibrosis (scarring) within the interstitium, the structural tissue surrounding the kidney's filtration tubes. Rather than a disease of the filters themselves, CIN is defined by the destruction of the tubules and the space between them. This process leads to tubular atrophy (wasting away of the tubes) and the eventual replacement of functional tissue with non-functional scar tissue. Clinically, this manifests as the small, irregular, and firm kidneys often detected during a physical examination. While the specific initiating cause is frequently undetermined, the end result is the progressive loss of nephrons and the irreversible decline in kidney function known as CKD.
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