Ophthalmic pterygium remains an 'enigma' despite numerous proposed pathogenetic theories, such as the molecular genetic alterations of stem cells located at the sclerocorneal limbus. The nasal conjunctiva may be particularly exposed to environmental irritants because of the presence of the 'lacrimal lake' formed in the area of the plica semilunaris. An oedematous enlargement of the latter may result from chronic ocular surface inflammation. A previous study has reported that nasal pterygia are associated with plica semilunaris ectopia, causing 'with-the-rule' corneal astigmatism because of the exertion of tractional forces and thus supported the idea that the ectopia of the plica semilunaris in pterygium is an epiphe-nomenon rather than a causative association.
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