Interstitial nephritis (or tubulo-interstitial nephritis) is a form of nephritis affecting the interstitium of the kidneys surrounding the tubules, i.e., is inflammation of the spaces between renal tubules. This disease can be either acute, meaning it occurs suddenly, or chronic, meaning it is ongoing and eventually ends in kidney failure.
This disease is also caused by other diseases and toxins that damage the kidney. Both acute and chronic tubulointerstitial nephritis can be caused by a bacterial infection in the kidneys known as pyelonephritis, but the most common cause is by an adverse reaction to a medication. The medications that are known to cause this sort of reaction are β-lactam antibiotics such as penicillin and cephalexin, and nonsteroidal anti-inflammatory drugs (aspirin less frequently than others), as well as proton-pump inhibitors, rifampicin, sulfa medications, fluoroquinolones, diuretics, allopurinol, mesalamine, and phenytoin. The time between exposure to the drug and the development of acute tubulointerstitial nephritis can be anywhere from 5 days to 5 months (fenoprofen-induced).
At times, there are no symptoms of this disease, but when they do occur they are widely varied and can occur rapidly or gradually. When caused by an allergic reaction, the symptoms of acute tubulointerstitial nephritis are fever (27% of patients), rash (15% of patients), and enlarged kidneys. Some people experience dysuria, and lower back pain. In chronic tubulointerstitial nephritis the patient can experience symptoms such as nausea, vomiting, fatigue, and weight loss. Other conditions that may develop include a high concentration of potassium in the blood, metabolic acidosis, and kidney failure.
Urinary findings include:
- Eosinophiluria: Original studies with Methicillin-induced AIN showed sensitivity of 67% and specificity of 83%. The sensitivity is higher in patients with interstitial nephritis induced by methicillin or when the Hansel's stain is used. However, a 2013 study showed that the sensitivity and specificity of urine eosinophil testing are 35.6% and 68% respectively.
- Blood in the urine and occasional RBC casts
- Sterile pyuria: white blood cells and no bacteria
- Nephrotic-range amount of protein in the urine may be seen with NSAID-associated AIN
Treatment consists of addressing the cause, such as by removing an offending drug. There is no clear evidence that corticosteroids help. Nutrition therapy consists of adequate fluid intake, which can require several liters of extra fluid.
The kidneys are the only body system that are directly affected by tubulointerstitial nephritis. Kidney function is usually reduced; the kidneys can be just slightly dysfunctional, or fail completely.
In chronic tubulointerstitial nephritis, the most serious long-term effect is kidney failure. When the proximal tubule is injured, sodium, potassium, bicarbonate, uric acid, and phosphate reabsorption may be reduced or changed, resulting in low bicarbonate, known as metabolic acidosis, low potassium, low uric acid known as hypouricemia, and low phosphate known as hypophosphatemia. Damage to the distal tubule may cause loss of urine-concentrating ability and polyuria.
In most cases of acute tubulointerstitial nephritis, the function of the kidneys will return after the harmful drug is not taken anymore, or when the underlying disease is cured by treatment. If the illness is caused by an allergic reaction, a corticosteroid may speed the recovery kidney function; however, this is often not the case.
Chronic tubulointerstitial nephritis has no cure. Some patients may require dialysis. Eventually, a kidney transplant may be needed.
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