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240 Chapter 6: Genitourinary system

Prognosis Investigations
2−
Treatment ameliorates some of the symptoms and bio- Serum Ca 2+ may be normal or low, PO 4 is high and
chemical disturbances; however, not all patients respond often alkaline phosphatase is raised.
to treatment. Renal transplantation offers the best treat- Raised parathyroid hormone.
ment, but is of limited availability. 25(OH)D 2 (Vitamin D 2 )isnormal but 1,25(OH) 2 D 3
(Vitamin D 3 )islow.
Renal osteodystrophy
Management
Definition Serum PO 4 2− must be lowered by restricting dietary
This term refers to the various bone diseases that develop intake and taking phosphate binders such as calcium
due to chronic renal failure. carbonate with meals. Vitamin D supplements (active
analogues, e.g. alfacalcidol which do not require renal
Pathophysiology activation) are used in addition to ensuring an ade-
The main mechanism is that of secondary hyperparathy- quate calcium intake. The aim of treatment is to suppress
roidism: thePTHtoslightlyhigherthannormallevels,controlthe
1 Chronic renal failure causes reduced hydroxylation parameters, avoid ectopic calcification and bone disease.
of 25(OH)D 2 to the active vitamin D metabolite,
1,25(OH) 2 D 3 .This leads to reduced absorption of cal- Glomerular disease
cium from the diet and therefore lowers serum cal-
cium levels. In addition, phosphate levels rise, due to The glomerulus is an intricate structure, the function of
reduced renal excretion. This binds calcium, further which depends on all its constituent parts being intact
lowering serum calcium levels and also causes calcium (see Fig. 6.7).
deposits in tissues. 1 Blood reaches the glomerular capillary system via the
2 In response to the low serum calcium, the parathyroid afferent arteriole. On the vascular side of the bar-
glands in the neck are stimulated to produce increased rier between the blood and the filtrate is endothe-
amounts of parathyroid hormone (i.e. secondary hy- lium, fused to the glomerular basement membrane
perparathyroidism). (GBM). On the filtrate side is an epithelium composed
3 Hyperparathyroidism leads to increased osteoclastic of podocytes, attached to the GBM by foot processes.
activity (to mobilise calcium from bone, and partially The connective tissue, which supports the capillary
restoreserumcalciumlevelstonormal.Hyperparathy- network, is called ‘mesangium’.
roidism also releases PO 4 ,which cannot be excreted 2 The pressure in the glomerular capillaries is higher
and so contributes further to hyperphosphataemia. than that in the urinary lumen, so that constituents
Other mechanisms include: of the blood are filtered into the urinary lumen. This
Metabolic acidosis also promotes demineralisation of ‘ultrafiltrate’ is almost an exact mirror of plasma ex-

bone. cept for proteins because the GBM is relatively im-
On long-term renal dialysis aluminium is retained, permeable to high molecular weight proteins such as

which deposits in bone, blocking calcification of the albumin. It is also less permeable to negatively charged
osteoid. molecules.
There are three main types of glomerular disease:
Clinical features Glomerulonephritis describes a variety of conditions
See Osteomalacia, Osteoporosis, Secondary and Tertiary characterised by inflammation of glomeruli in both
Hyperparathyroidism for the clinical features and X-ray kidneys, which have an immunological basis.
findings. Vasculitis which can mimic glomerulonephritis, by
Osteosclerosis may also occur, where there are alter- damage to the glomerular vessels.
nate stripes of sclerotic and osteoporotic bone. This Glomerular damage may also occur due to infiltration
affects the trabecular bone of the spine, to produce a by abnormal material, such as by amyloid (see page
‘rugger-jersey spine’ appearance on X-ray. 513).