Improved life expectancy and the need for robust tools to monitor renal safety of emerging new therapies have fueled the interest in renal function in Duchenne muscular dystrophy (DMD) patients. We aimed to establish a methodology to accurately assess their renal function. Twenty DMD patients (5-22 years) were included in this prospective study. After obtaining medical history, all patients underwent a clinical examination, 24-hour ambulatory blood pressure monitoring, ultrasound of the kidneys, direct GFR measurement ( 51 Cr-EDTA, mGFR), complete blood and urine analysis. Seventeen of 20 patients were treated with corticosteroids and 5/20 with angiotensin converting enzyme inhibitor (lisinopril). No patient suffered from urinary tract infections or other renal diseases. Hypertension (systolic or diastolic blood pressure >P95) was found in 9/20 patients (8/9 patients were on steroid treatment) and a non-dipping blood pressure profile in 13/20 subjects (10/13 patients were on steroid treatment). Urinary protein to creatinine ratio was elevated in 17/18 patients, whereas 24-hour urine protein excretion was normal in all subjects. Median interquartile range (IQR) mGFR was 130.4 (29.1) mL/min/1.73 m 2 . Hyperfiltration (mGFR >150 mL/min/1.73 m 2 ) was found in 5/20 patients. Inverse correlation between mGFR and age was observed (R 2 = 0.45, p = 0.001). Serum creatinine based estimated GFR (eGFR) equations overestimated mGFR up to 300%. eGFR based on cystatin C Filler equation was closest to the mGFR (median eGFR (IQR) of 129.5 (39.7) mL/min/1.73 m 2 ). Our study demonstrates a high prevalence of hyperfiltration and hypertension in children and adolescents with DMD. Because the majority of hypertensive patients were under corticosteroid treatment, the iatrogenic cause of hypertension cannot be excluded. Serum or urine creatinine measurements are of no value to evaluate renal function in DMD patients due to the reduced skeletal muscle mass.

The correct order of application in this DMD population should be BM correction first, followed by BSA indexing, and not vice versa. In general, the order of applying the BM correction and BSA normalization becomes more important with increasing slow GFR and extreme low BSA. The order of application is of less importance for people with normal BSA and/or normal GFR.

We observed a male newborn with bilateral nystagmus and central hypothyroidism without hypoprolactinemia due to a deletion of chromosome band Xq26.1q26.2, containing FRMD7 and IGSF1. These two loss-of function mutations are known to cause, respectively, congenital nystagmus and the ensemble of central hypothyroidism, hypoprolactinemia and testicular enlargement. These latter two features may not yet be present in early life.

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