The basis of tear ferning

Fern‐like crystallisation occurs in many body fluids when sampled and dried, including the tears. Tear ferning has been proposed as a diagnostic test of keratoconjunctivitis sicca (KCS), but the mechanism of reduced ferning in the dry eye is poorly understood. By microscopic examination of tear ferns and review of gynaecological and crystallographic literature, we seek an explanation for the reduced tear ferning in KCS. The physico‐chemical variables controlling the crystal ‘habit’ (viscosity, osmolality, protein content, and mucous quality or quantity) are examined to ascertain which might be responsible for the decreased ferning of dry eye tear samples. We hypothesise that a shift in the salt‐to‐macromolecule ratio, increased lipid contamination of mucus and altered tear rheology are the features of dry eye which best explain the altered ferning patterns in this condition.