CKD
Kidney failure is one of the most common causes of death in cats. Renal disease occurs in approximately 2% of the cat population, but in 10% of cats 10 years and older and about 30% of cats older than 151.
The terms renal or kidney insufficiency and failure have been used interchangeably. It is now preferred to use the term Chronic Kidney Disease (CKD) along with the International Renal Interest Society (IRIS) stage to indicate severity.
- What happens in CKD?
- Regulatory mechanisms in CKD
- Disease progression in CKD
- What’s the difference between systemic and glomerular hypertension?
- What is proteinuria and why is it important?
Reference 1: D. Polzin (2006) NAVC, 684-687.
In CKD, many glomeruli in the kidney have been destroyed. Therefore, each remaining glomerulus has to work harder, filtering more blood, in order to maintain overall glomerular filtration rate (GFR). This is done by increasing the pressure within the glomerulus (glomerular hypertension), which is mediated by the renin-angiotensin-aldosterone system (RAAS).
The GFR varies according to the volume and pressure of blood passing through the glomeruli. In response to reduced renal blood flow, the juxtaglomerular apparatus releases renin. Just as in heart disease, this triggers the RAAS and angiotensin II is produced. Angiotensin II causes vasoconstriction and in the kidney, angiotensin II also specifically acts on the efferent arteriole. Constriction of the efferent arteriole reduces outflow from the glomerulus, causing increased pressure within the glomerulus (glomerular hypertension). This increased pressure causes an increase in GFR.
In the short term, maintaining GFR by raising glomerular pressure is good. However, continual (chronic) high pressure within the glomerulus eventually leads to damage of the glomerular membrane. The damaged membrane allows larger molecules such as protein to pass through. It is thought that this protein further damages the renal tubules, which causes further progression of the disease.
Eventually the glomerulus becomes so damaged that it is no longer able to function. The loss of this nephron forces the remaining nephrons to work even harder. This, in turn, leads to a vicious cycle of disease progression.
What’s the difference between systemic and glomerular hypertension?
Systemic hypertension means there is high blood pressure in the circulation throughout the entire body. Glomerular hypertension is when there is high pressure just within the glomerulus.
If systemic hypertension is present, the pressure in the glomerulus is also elevated. However glomerular hypertension may be present with a normal systemic blood pressure. This means all patients with systemic hypertension, regardless of the underlying cause, are at risk of kidney damage as they will have glomerular hypertension.
Conversely, a patient with kidney disease, but normal systemic blood pressure, will still have glomerular hypertension and is, therefore, also at risk of further kidney damage.
What is proteinuria and why is it important?
Proteinuria means that protein is present in the urine. Proteins are large molecules and cannot normally pass through the glomerular membrane into the urine. Proteinuria is caused for one of three reasons:
- There are abnormal (small) protein molecules in the blood: ‘prerenal’ (very rare)
- The glomerular membrane is damaged (as in CKD) and has become more ‘leaky’, allowing bigger molecules, such as proteins, to pass through: ‘renal’ (the most common cause)
- There is inflammation of the lower urinary tract (for example from cystitis) which is releasing protein into the urine: ‘postrenal’
Proteinuria is important because it has been clearly shown in people, dogs and cats that the worse the proteinuria, the shorter the survival.
Proteinuria can be measured and this is useful in the diagnosis of CKD – renal proteinuria shows that there is damage to the glomerulus. However, proteinuria also causes further damage to the kidneys, and increasing proteinuria suggests progression of CKD.