Both are primarily caused by the polymerization of sickle cells in the kidney microvasculature due to the low O2 tension, high osmolarity, and low acidity. This polymerization fills and occludes blood vessels such as the vasa recta in the kidneys leading to microinfarctions, leakage into surrounding tissues and potentially papillary necrosis and renal infarcts. Renal papillae are especially susceptible to damage eventually causing papillary necrosis since these vessels are only supplied with blood by the vasa recta. The sickling of the cells also contribute to two other mechanisms which are chronic hypoxia and chronic hemolysis. Hypoxia is caused from both the insufficient ability for the red blood cells to transport oxygen alongside blood vessel occlusions promoting the activation of Hypoxia Inducible Factor-1𝛂. The hypoxia also causes the over expression of endothelin-1 and functional nitric oxide deficiency and due to the chronic hemolysis, reactive oxygen species are produced leading to vasoconstriction and further medullary hypoxia. This nitric oxide deficiency alongside endothelin-1 overproduction leads to the inability to properly respond to stress and hemodynamic changes which increases the likelihood of experiencing acute kidney injury.The hyperfiltration has multiple contributing factors such as the increased cardiac output caused by the normal physiological response to anemia leading to greater renal blood flow and an increase in glomerular filtration rate (GFR). This is not the only factor because having multiple blood transfusions does not reverse this effect.
Functional tubule abnormalities such as nephrogenic diabetes insipidus result from marked reduction in vasa recta blood flow, combined with ischemic tubule injury and sickled erythrocytes in the vasa recta of the inner medulla impairing free water absorption, all causing the production of dilute urine. The concentrating defect also occurs in individuals with sickle trait. This can lead to symptoms such as polyuria and dehydration due to the low water reabsorption.Increase risk of urinary tract infection from encapsulated bacteria due to hyposplenism from spontaneous infarctions in the spleen (autosplenectomy). Additionally, papillary necrosis can increase the risk of a UTI and all infections should be dealt with promptly to prevent sickle cell crisis.Initially, hyperfiltration occurs in pediatrics and then after 30 years old, the GFR slowly declines which is proportional to the development of proteinuria. Worsening proteinuria is gradual, but a sudden onset could have a secondary cause such as nephritic syndrome from FSGS or minimal change disease, membranoproliferative glomerulonephritis, or hepatitis C.Tubular dysfunction, specifically incomplete distal renal tubular acidosis, is caused by the impaired potassium and hydrogen excretion as well as the impaired bicarbonate reabsorption. These contribute to the development of metabolic acidosis, high blood potassium and defects in uric acid excretion which, combined with increased purine synthesis in the bone marrow, results in high blood uric acid levels.Renal infarcts from total occlusion which can present with pain, vomiting, fever, and high blood pressure. Complications
Kidney complications of sickle cell disease include cortical infarcts leading to loss of function, persistent bloody urine, and perinephric hematomas.