Three conditions, one mechanism. Rhabdomyolysis, tumour lysis, and the crystal nephropathies all overwhelm the kidney with a filtered load — pigment, urate and phosphate, or a precipitating drug — that clogs and injures the tubule. That shared idea explains the shared treatment: in every case the first and most important move is early, generous fluid to dilute the load and keep it flowing. The disease-specific work — a urate oxidase here, an antidote there — sits on top of that foundation, and the urinary-pH question must be answered differently for each.
The unifying idea: an overwhelmed tubule
It helps to hold the three conditions together. In each, something is released or administered in such quantity that the kidney cannot handle it: myoglobin from dying muscle, urate and phosphate from dying tumour, or a poorly soluble drug given too fast to a dry patient. That load then does two things in the tubule — it precipitates into casts or crystals that obstruct flow, and it injures the epithelium directly. Volume depletion makes both worse by concentrating the load and slowing flow, which is why volume is the universal antidote. Urine pH modifies solubility, but in opposite directions for different substances, which is the trap that runs through the chapter. Keep the unifying idea in mind and the specifics fall into place.
Rhabdomyolysis: pigment, and the electrolytes that kill
Skeletal muscle breakdown — from crush, prolonged immobility, seizures, extreme exertion, ischaemia, statins, toxins, or inherited myopathies — spills the muscle's contents into the blood: myoglobin, creatine kinase, potassium, phosphate, and urate. The diagnosis rests on a markedly elevated CK, and the AKI risk climbs with it, becoming substantial above several thousand and marked at very high levels. The urine gives the now-familiar clue: the dipstick reads blood, but microscopy shows no red cells, because the pad is detecting myoglobin's heme. CK is the more dependable marker, since myoglobin clears quickly and the urine can normalise while injury continues.
Myoglobin injures the kidney three ways — heme casts obstruct the tubule, especially in acid urine; the heme molecule's iron drives oxidative tubular injury; and myoglobin scavenges nitric oxide, causing renal vasoconstriction — all compounded by the volume depletion of fluid sequestering into injured muscle. But the immediate threat is often not the kidney: it is potassium. Rhabdomyolysis releases large amounts of potassium early, and with AKI on top, hyperkalaemia can be severe and rapidly fatal. Calcium behaves in a characteristic sequence — it falls early as it deposits in damaged muscle, then rises late as that calcium is released during recovery. The practical consequence is to treat early hypocalcaemia only if it is symptomatic or causing arrhythmia, because aggressive calcium loading sets up a dangerous rebound hypercalcaemia.
Rhabdomyolysis: what to do, and what is unproven
The treatment is early and it is fluid. Begin aggressive isotonic resuscitation as soon as rhabdomyolysis is suspected — before the creatinine has even moved — aiming for a brisk urine output in the region of 200 to 300 mL/h, which dilutes the pigment, restores flow, and corrects the pre-renal component. This is the one intervention that clearly helps. The traditional additions are less certain. Urinary alkalinisation with bicarbonate, intended to reduce cast formation and heme toxicity, is theoretically attractive but has never been shown superior to isotonic saline alone, and it can worsen the hypocalcaemia and cause alkalosis; it is, at best, optional and not routine. Mannitol, proposed as an osmotic diuretic and scavenger, is likewise unproven and can itself harm. So the honest message mirrors the ATN chapters: fluid is the mainstay, bicarbonate and mannitol are not routine, hyperkalaemia is treated aggressively, a compartment syndrome is decompressed, and established AKI is supported with renal replacement for the usual indications — most often refractory hyperkalaemia.
Tumour lysis: the crystals and the alkalinisation trap
Tumour lysis syndrome is the malignant mirror of rhabdomyolysis: instead of muscle, it is tumour that breaks down — usually after the first cytotoxic therapy of a bulky, rapidly proliferating, chemo-sensitive cancer such as Burkitt lymphoma or acute leukaemia, occasionally spontaneously. The dying cells release potassium, phosphate, and nucleic acids that are metabolised to uric acid, producing the quartet of hyperuricaemia, hyperkalaemia, hyperphosphataemia, and a secondary hypocalcaemia from calcium-phosphate precipitation. The AKI comes from two crystals: uric acid precipitating in acid urine, and calcium-phosphate precipitating when the calcium-phosphate product is high. And here is the central trap. Alkalinising the urine dissolves uric acid — which sounds helpful — but simultaneously promotes calcium-phosphate precipitation, a worse problem, so urinary alkalinisation is no longer recommended in tumour lysis. The contrast with rhabdomyolysis and with methotrexate, where alkalinisation is at least neutral or helpful, is exactly why urine pH cannot be a blanket rule.
Tumour lysis: prevent, and choose the urate-lowering agent
Tumour lysis is managed by risk-stratifying before treatment and preventing. Every at-risk patient gets aggressive isotonic hydration to dilute and flush the load. The urate-lowering choice then depends on risk and turns on a mechanistic distinction. Allopurinol inhibits xanthine oxidase, so it prevents the formation of new uric acid but does nothing about urate already present; it suits lower-risk patients and is started before chemotherapy. Rasburicase is recombinant urate oxidase — it enzymatically converts existing uric acid into highly soluble allantoin, dropping the level rapidly — and so it is the agent for high-risk or established tumour lysis. Two cautions are non-negotiable: rasburicase is contraindicated in glucose-6-phosphate dehydrogenase deficiency, where it causes haemolysis and methaemoglobinaemia, so screen first; and because it continues to break down urate in the test tube, blood samples must be transported on ice or the reported urate will be falsely low. Beyond urate, manage the hyperkalaemia that can kill, bind phosphate, treat hypocalcaemia only if symptomatic, and start renal replacement at a lower threshold than usual given the relentless load.
The crystal nephropathies: volume, pH, and rate
Beyond urate, a range of substances crystallise in the tubule. Drug crystals are the commonest: aciclovir, especially given as a rapid intravenous bolus to a dehydrated patient; sulfadiazine; high-dose methotrexate; the protease inhibitors indinavir and atazanavir. Endogenous crystals include calcium oxalate — from enteric or primary hyperoxaluria and, classically, ethylene-glycol poisoning with its envelope-shaped crystals — and the calcium-phosphate of acute phosphate nephropathy after phosphate-based bowel preparations. The prevention is generic and specific at once: maintain volume to dilute, slow the infusion and adjust the dose of the offending drug, and set the urine pH to the drug's solubility — alkalinise for methotrexate and sulfadiazine, for instance. Specific antidotes matter where they exist: glucarpidase rapidly lowers toxic methotrexate levels, and fomepizole blocks the metabolism of ethylene glycol, with dialysis for severe poisoning. The thread back to the unifying idea is constant: dilute the load, and respect that pH is friend or foe depending on what is crystallising.
Where the evidence is firm, and where it argues
The firm parts are reassuringly simple: early, generous fluid helps in all three, rasburicase lowers urate faster and more reliably than allopurinol, and urinary alkalinisation is the wrong move in tumour lysis. The arguments cluster around the additions. Whether bicarbonate adds anything to saline in rhabdomyolysis has never been settled by a convincing trial, and most practice has drifted toward saline alone. Mannitol's place is similarly unproven. And the exact CK threshold, fluid rate, and target urine output are guided more by physiology and consensus than by randomised data. The honest position is the one this volume keeps returning to: the cornerstone intervention is well supported, the fashionable additions mostly are not, and the discipline is to do the proven simple thing aggressively and early.
