I recently consulted on a patient with AKI who had undergone a bone marrow transplant a couple of months previously. While reviewing his labs, I noticed that he had ongoing BK viremia and I wondered whether or not he could have BK nephropathy.
BK viruria occurs in 50% of BMT recipients, usually within 2 months of transplant although it can also be detected pre-transplant. The virus usually clears spontaneously as time progresses. Although in the renal transplant world, BK virus is most often associated with nephropathy, for BMT recipients, hemorrhagic cystitis is the commonest pathology seen with this infection and was first described more than 20 years ago. Hemorrhagic cystitis occurs in 10-25% of BMT patients. Early post-transplant cystitis was thought to be related to chemotherapy and its incidence has fallen with the routine use of bladder protection. BK-related cystitis tends to occur later after transplant and can be associated with significant morbidity. The main risk factors for cystitis are BK viremia, especially pre-transplant, GVHD and increased doses of immunosuppression. Treatment is generally supportive as the cystitis usually resolves spontaneously in 2-3 weeks although cidofivir and leflunamide have been used with limited success.
Despite the high incidence if BK viremia and viruria, BK nephropathy is extremely rare in these patients, with a very small number of cases reported in the literature. In the majority of these cases, the nephropathy was diagnosed >1 year after transplant and occurred in patients who required prolonged high-dose immunosuppression for GVHD. Interestingly, cystitis did not necessarily precede the development of nephropathy in these patients.
So why do more BMT recipients not develop nephropathy? Perhaps, in most patients, as the immune system reconstitutes, the BK virus is cleared spontaneously while in patients with GVHD, the viremia persists allowing the damage to occur.
Although rare, BK virus nephropathy has also been described in recipients of heart, pancreas and lung transplants.
So, given that my patient not long after transplant, it is unlikely that he had BK nephropathy. However, it is something to consider in patients with sustained increases in serum creatinine in the medium term after BMT.