Henoch-Schönlein purpura (HSP) is the most common systemic vasculitis in children. It belongs to the group of small-vessel vasculitis associated with immune complexes, and the proper term is IgA vasculitis (IgAV). In HSP in the wall of small vessels, deposits of immune complexes composed of IgA1 and C3 complement component are found. The incidence of HSP is estimated at 3-26.7/100,000 children per year. The changes most often affect the skin, lower limbs, digestive tract, and kidneys and are accompanied by arthralgia and arthritis. In this review article we pay attention to the pathogenesis, possible clinical manifestations, irregularities in laboratory tests, and proper management of patients with HSP. The treatment should depend on the clinical condition of the patient and is primarily symptomatic. Good knowledge of the differentiation of clinical symptoms allows for quick diagnosis and the qualification of patients for appropriate supervision.

We present the history of a nine-month-old male infant born prematurely with extremely low birth weight, who was admitted to the paediatric nephrology department with dehydration, acute kidney injury, hyponatraemia, hyperkalaemia, and metabolic acidosis. While the crucial first step in the diagnosis of hyponatraemia includes the assessment of the patient's fluid status, we focus in the discussion on the causes, diagnosis, and treatment of hypovolemic hyponatraemia. With the notable exception of congenital adrenal hyperplasia (CAH) and other primary adrenal diseases, in which there is a deficiency in aldosterone synthesis, many other salt-losing disorders share the common feature of inducing secondary hyperaldosteronism. In the presented case hyponatraemia was caused by NEC-related ileostomy with, typically, hyperkalaemia despite secondary hyperaldosteronism. The clinical picture can be very similar to pseudohypoaldosteronism type 1 (PH 1), with the renal handling of sodium being the key differentiating feature.