Renal capsule and flank pain in acute pyelonephritis
A swollen kidney in acute pyelonephritis (infection of the kidney) stretches its tough renal capsule, and that stretching causes flank pain, which is pain in the side.
Remember
The renal capsule is tough and non-distensible, meaning it hardly stretches, so a swollen kidney in acute pyelonephritis is held under tension and causes flank pain.
Memory aid
a tight glove on a swelling finger, where the glove does not give so the finger hurts
Picture the renal capsule as the glove and the swollen kidney as the finger: the capsule does not give, so the swelling hurts.
In acute pyelonephritis (infection of the kidney), the renal parenchyma, which is the working tissue of the kidney, becomes inflamed and swollen. The swollen tissue is held in by the renal capsule, a tough fibrous coat (a sheet of strong connective tissue) applied directly to the kidney. The capsule is non-distensible, meaning it hardly stretches, so the swollen kidney is held under tension and flank pain results.
Working outwards from the capsule, the layers are the perirenal fat (a cushion of fat around the capsule), the renal fascia (Gerota's fascia, a fibrous envelope round the perirenal fat and the adrenal gland) and the pararenal fat (extra fat outside the fascia). The capsule is the layer that touches the kidney directly. The fat and fascia further out are not what the swollen kidney itself stretches.
The renal fascia matters when infection tracks beyond the kidney. It is open inferiorly (at its lower end) along the line of the ureter, the tube that carries urine from the kidney to the bladder. A perinephric abscess, a collection of pus in the fat around the kidney, is therefore not sealed in and can track down into the pelvis, which lies below the kidney.
Key facts
- Capsule
- a tough, fibrous and non-distensible coat directly on the kidney, so swelling of the parenchyma in acute pyelonephritis produces flank pain.
- Order of the layers
- from deep to superficial, the renal capsule, perirenal fat, renal fascia (Gerota's fascia) and pararenal fat.
- Renal fascia
- encloses the perirenal fat and the adrenal gland, and is open inferiorly along the line of the ureter.
- Spread
- a perinephric collection (pus or fluid around the kidney) can track down into the pelvis, which lies below the kidney.
- Renal angle
- the angle between the 12th rib and the lateral border of the erector spinae, the muscle beside the spine; in acute pyelonephritis, tenderness (pain when pressed) there comes from the same stretching of the capsule.
Exam traps
- A renal fascia that is sealed at its lower end would tend to keep an abscess around the kidney, but the renal fascia is open inferiorly along the line of the ureter, so pus around the kidney is not trapped and can track down into the pelvis.
- A renal capsule that stretched easily would tend to relieve the pressure and the pain, but the renal capsule is non-distensible, and that is why a swollen, infected kidney is painful.
- Stretching of the renal pelvis, the funnel-shaped start of the collecting system where urine gathers before entering the ureter, causes pain when the collecting system is obstructed, for example by a stone, but when the swelling is in the parenchyma and not in the collecting system, as in the parenchymal swelling of acute pyelonephritis, the pain comes from the capsule being stretched.
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