Primary lymphoedema in a 45-year-old woman

Case scenario

Miriam, a 45-year-old shop assistant, presented with mild symmetrical pitting oedema of both legs of relatively abrupt onset. She said that the swelling would subside overnight but redevelop during the day, causing her legs to ache and feel heavy.

She was extensively investigated but no pathology was found. In particular, she was neither anaemic nor hypertensive, she was euthyroid, she had no deep vein thromboses or venous valvular incompetence, and she had no abdominal or pelvic lesion on CT scan. She was taking no medication and she otherwise felt well. She was overweight (BMI, 30 kg/m²) but there had been little change in her weight over the preceding five years.

It was finally assumed that she had developed impaired lymphatic drainage. What is likely to have caused this?