Intrinsic acute kidney injury: when the damage is inside the kidney

The NHS describes acute kidney injury (AKI) as the situation in which something suddenly causes the kidneys to stop working as well as they should. Doctors usually sort the possible causes into three groups depending on where the problem lies. Pre-renal causes reduce the blood supply reaching the kidneys. Post-renal causes block the outflow of urine. Intrinsic, or intra-renal, causes are those in which the kidney tissue itself has been damaged.

NICE Clinical Knowledge Summaries explain that intra-renal AKI is a consequence of structural damage to the kidney, affecting the tubules, the glomeruli (the filters), the interstitium (the supporting tissue between them) or the small blood vessels within the kidney. It may also follow a pre-renal or post-renal problem that has gone on long enough to damage the kidney cells, and more than one cause is often present at the same time.

Knowing which group applies matters because the treatments differ. Restoring fluid helps a dry, under-perfused kidney, and draining a blockage helps an obstructed one, but damage inside the kidney may need medicines that calm inflammation or removal of the substance that caused the harm.

NICE CKS give examples of each pattern of intra-renal injury. Which part of the kidney is affected shapes both the cause and the treatment.

NICE CKS list toxins and nephrotoxic drugs among the renal causes of AKI, giving as examples certain antibiotics, non-steroidal anti-inflammatory drugs (NSAIDs), proton pump inhibitors, allopurinol, iodine-based contrast and chemotherapy. Exposure to some herbal remedies and poisonous plants and animals is also listed as a risk.

On the immune side, the NHS explains that glomerulonephritis, which is damage to the tiny filters inside the kidneys, is often caused by the immune system attacking healthy tissue. It can be caused by inflammatory conditions such as systemic lupus erythematosus and vasculitis, by infections including HIV, hepatitis B and C and endocarditis, by some cancers and by certain medicines. Sometimes the cause is not clear.

Who is more likely to be affected? NICE CKS list people aged 65 or over, those with a history of AKI, chronic kidney disease, heart failure, liver disease or diabetes, those with sepsis or dehydration and those who have recently taken medicines such as NSAIDs, ACE inhibitors, angiotensin receptor blockers, diuretics or aminoglycoside antibiotics. The NHS adds recent surgery and use of illegal drugs.

The NHS says many people do not get symptoms of AKI, especially if it is mild, and it is often found because blood tests are being done for another reason. When symptoms occur, the NHS lists the following.

• Peeing a lot less than usual.

• Feeling very thirsty.

• Feeling sick or being sick.

• Swollen feet or legs.

• Feeling tired, dizzy, confused or sleepy.

• Feeling breathless, especially when lying down.

The NHS says to ask for an urgent GP appointment or get help from NHS 111 if you think you may have symptoms of AKI. If you also have the features of a kidney infection, such as being confused, drowsy or having difficulty speaking, the NHS says to call 999 or go to A&E.

With glomerulonephritis, the NHS says to see a GP if you notice blood in your urine, or have other symptoms such as joint pain, tiredness and shortness of breath, because the cause of blood in the urine should always be investigated.

The first step is the blood test for creatinine. NICE CKS state that AKI may be diagnosed when serum creatinine rises by 26 micromol/L or more within 48 hours, when it rises by 50% or more within a week, or when urine output falls below 0.5 mL/kg/hour for more than 6 hours.

To work out where the damage lies, assessment includes asking about symptoms, fluid intake, urine output and medicines, assessing fluid status and signs of obstruction, checking kidney function and potassium and doing a urine dipstick test for an underlying cause and kidney disease. The NHS adds that you may have a urine sample, an ECG and scans such as an ultrasound, and may be referred to a kidney specialist (nephrologist) if the AKI is severe or the cause is unclear. If glomerulonephritis is suspected, the NHS says a kidney biopsy, taken under local anaesthetic with ultrasound guidance, may be done.

There is no single treatment, because the aim is to remove or treat the cause and support the kidneys while they recover. The NHS lists medicine to treat an infection, raise blood pressure, reduce swelling or calm the immune system, stopping or changing medicines that may be harming the kidneys, fluids through a drip, and a catheter or surgery if there is also a blockage. Where the immune system is the problem, the NHS says immunosuppressant medicine may be used for glomerulonephritis.

If the AKI is severe, the NHS says a machine may do the job of the kidneys through dialysis, usually only for a few days or weeks until they recover. NICE CKS add that complications of AKI include high potassium, acid build-up in the blood, fluid overload and chronic kidney disease, which is why potassium and fluid balance are closely monitored.

Follow-up matters. According to NICE CKS, the AKI episode should be documented, kidney function monitored to look for the development or progression of chronic kidney disease, and any medicines that were paused reviewed. Kidney Care UK says the GP will monitor kidney function and blood pressure for at least three years after an episode of AKI.

The NHS says AKI is serious and can be life-threatening if not treated, but that many people recover fully within a few weeks. Kidney Care UK similarly says most people make a full recovery and need no long-term treatment. The outlook is worse in people who are older, have other long-term illnesses or have more severe injury; NICE CKS report that people with pre-existing chronic kidney disease have a four- to five-fold increased risk of end-stage kidney disease after AKI.

Some people are left with permanent kidney damage. The NHS says you are more likely to develop chronic kidney disease if your AKI is severe or you already have other long-term health problems. To lower the chance of a repeat, the NHS suggests staying hydrated, speaking to a doctor before taking new medicines or supplements and knowing the warning symptoms.