Glomerular hyperfiltration in the classic model of diabetic nephropathy
In the classic teaching model of diabetic nephropathy (kidney damage from long-standing diabetes), the first change is glomerular hyperfiltration, an abnormally high filtration rate, with raised capillary pressure.
Remember
In the classic model the kidney first overworks and later wears out: hyperfiltration with raised capillary pressure can occur before overt proteinuria (protein in the urine) and before the GFR (the volume of blood filtered per minute) falls.
Memory aid
pressure first, scars later
It reminds you that the pressure change comes before the scarring in the classic model.
Diabetic nephropathy (kidney damage from long-standing diabetes) is taught as a sequence that begins with a functional change rather than a structural one. The first step in the classic model is glomerular hyperfiltration: the glomeruli (the kidney's filtering units) filter more plasma than normal, because the pressure inside their capillaries is raised. This is a haemodynamic change (a change in blood flow and pressure), and it can occur before overt proteinuria (protein in the urine) or a falling GFR (the volume of blood filtered per minute).
Why does the pressure rise? In diabetes, extra glucose and sodium chloride are filtered, so more of both is reabsorbed. Less sodium chloride then reaches the macula densa (a sensor in the distal tubule wall that tells the glomerulus how much salt is arriving). That feedback helps to widen the afferent arteriole (the inflow vessel), and raised glucose helps to dilate it too. Meanwhile high local angiotensin II narrows the efferent arteriole (the outflow vessel). So more blood goes in than comes out, the capillary pressure climbs, and the GFR is initially increased.
The sustained pressure then drives structural damage. The glomerular basement membrane (the thin layer that forms the filtering barrier) thickens, the mesangium (the supporting tissue between the capillaries) expands, podocytes (the cells wrapped around the capillaries) are injured, and over time glomerulosclerosis (scarring) develops.
Remember that this is a teaching model. Functional and structural changes can overlap, and individual patients do not all follow the same order.
Key facts
- Earliest change
- in the classic model, glomerular hyperfiltration with raised capillary pressure is the first change, and it is functional rather than structural.
- Afferent arteriole
- it widens because less sodium chloride reaches the macula densa, and raised glucose adds to the widening.
- Efferent arteriole
- it is narrowed by high local angiotensin II.
- Early GFR
- the GFR (the volume of blood filtered per minute) is initially increased.
- Later damage
- basement membrane thickening, mesangial expansion and podocyte injury follow, then glomerulosclerosis (scarring).
- Late course
- nodular glomerulosclerosis and a falling GFR belong to established disease, not the start.
- Caveat
- functional and structural changes can overlap, and patients do not all follow the same sequence.
Exam traps
- In the classic model, thickening of the glomerular basement membrane is an early structural change, but it follows the haemodynamic change rather than starting the disease, because it is driven by the raised pressure.
- Nodular glomerulosclerosis (rounded scars, also called Kimmelstiel-Wilson lesions) is distinctive of diabetic nephropathy, but it is seen after years of injury, so it is not the earliest change.
- In the classic model, a progressive fall in glomerular filtration rate is a late functional consequence, seen once overt proteinuria has developed, because early in the disease the filtration rate is raised rather than falling.
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