Clinical and morphological features of kidney involvement in primary Sjögren's syndrome

Background. Primary Sjogren's syndrome is a connective tissue disorder affecting primarily the lacrimal and salivary glands, resulting in xerophtalmia and xerostomia. Extraglandular manifestations are frequent and may include renal involvement. Methods. We studied the prevalence and nature of kidney involvement in 60 Italian patients with primary Sjogren's syndrome, diagnosed according to the European classification criteria. The following renal laboratory tests were performed in all patients: electrolytes in serum and in 24-h urine, creatinine in serum and in 24-h urine, venous pH and HCO3 , urinalysis, urine culture, urinary osmolality and urine pH. A water deprivation test was performed in patients with morning urine osmolalities below the reference values adjusted for age. An oral ammonium chloride loading test was performed in patients with urine pH above 5.5 from morning samples. Renal biopsy was performed in patients with renal involvement. Results. Sixteen patients (27%) had laboratory evid- ence of tubular anduor glomerular dysfunction. A variable degree of creatinine clearance reduction was found in eight patients (13%); frank distal tubular acidosis in three (5%); hypokalaemia in four (7%); and pathological proteinuria in 12 (20%). Urine concentrating capacity was defective in 10 out of 48 (21%) tested patients. Only four patients presented with overt clinical manifestations, including hypo- kalaemic tetraparesis (1), nephrotic syndrome (2), recurrent renal stones with flank pain and haematuria (1). In two patients, signs of renal involvement pre- ceded the onset of sicca syndrome. Renal biopsies from nine patients showed tubulo-interstitial nephritis in six and glomerular disease in three. Patients with renal involvement had a significantly shorter disease duration compared with patients without renal abnormalities. Conclusions. Kidney involvement is a frequent extraglandular manifestation of primary Sjogren's syndrome. It is rarely overt and may precede the onset of subjective sicca syndrome.