Introduction:  Non-alcoholic  fatty  liver  disease  (NAFLD) is a socially significant health problem and it affects around a billion people on a global scale. NAFLD is the most common reason for liver damage worldwide and is considered the hepatic manifestation of the metabolic syndrome. Non-alcoholic fatty liver disease is generally associated with obesity and the related comorbidities, but it can also develop in subjects with a normal body mass index (BMI). This sub-phenotype of NAFLD is called lean NAFLD. Lean subjects with NAFLD have milder features of the metabolic syndrome when compared with obese patients. Nonetheless, they have a higher prevalence of metabolic alterations (e.g., dyslipidemia, arterial hypertension, insulin resistance, and diabetes) compared with healthy controls. The diagnosis of NAFLD requires the exclusion of both secondary causes and of a daily alcohol consumption ≥ 30 g for men and ≥ 20 g for women.

Clinical Case: We present the clinical case of a 29-year-old man, who was admitted to the hospital because of diagnostic evaluation of ultrasound findings of severe liver steatosis accompanied with mild hepatosplenomegaly. There was no evidence of diabetes mellitus, hypertensive disease or obesity. The patient was overweight but without obesity: height – 187 cm., weight – 108 kg, BMI – 30, waist circumference – 111 cm. Laboratory examination revealed: normal blood count, normal level of CRP, AST, ALT, AF, total bilirubin, mild elevation of GGT and normal synthetic liver function tests. The lipid profile showed elevated level for LDL cholesterol and low HDL cholesterol. Underlying chronic viral hepatitis, autoimmune liver diseases, Wilson's disease and hemochromatosis were ruled out. The calculated non-invasive scores for steatosis were high. The NAFLD fibrosis score: -3.35 and FIB-4 index: 0.52, ruled out presence of significant fibrosis. Abdominal ultrasound examination showed liver steatosis grade 3, upper limit of normal size, mild splenomegaly. Liver transient elastography (TE) with fibroscan revealed normal liver stiffness – 4.8 kPa and CAP – 304 db/m that proved the presence of severe steatosis. Based on physical examination data, definite results of non-invasive scores for steatosis, ultrasound examination and fibroscan elastography, and after excluding alcohol consumption and other common causes for steatosis, we established the diagnosis lean non-alcoholic fatty liver disease. We performed an examination for lysosomal enzymes from dried blood that ruled out classic Gaucher disease. Low activity of acid sphingomyelinase was found, which was indicative of Niemann-Pick disease A/B – rare inherited autosomal recessive condition involving lipid metabolism.

Conclusion: The clinical case presents a patient without obesity, with definite laboratory, ultrasound and fibroscan data for non-alcoholic fatty liver disease. As a differential diagnosis we found a rare inherited condition – Niemann-Pick disease, characterized by impaired lipid metabolism. Our clinical case proves the broad spectrum of differential diagnoses of patients with NAFLD.

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