The normal production of tears can be subdivided into two components: basal tear production; and induced tear production. Induced tear production occurs following stimulation of the ophthalmic branch of the trigeminal nerve and in response to high light intensities. Loss of induced tear production most commonly occurs due to lesions of the ophthalmic branch of the trigeminal nerve, but the decreased tear production is less importantthan the loss of corneal sensation with consequent decreased blink frequency and development of neuroparalytic keratitis.

Lesions resulting in decreased tear production
The lacrimal gland is innervated by the parasympathetic portion of the facial nerve, a branch of which also innervates the lateral nasal gland (a serous-secreting gland that functions to keep the nose moist). Lesions affecting the parasympathetic portion of the facial nerve therefore result in neurogenic keratoconjunctivitis sicca and an ipsilateral xeromycteria. The presence of an ipsilateral dry nostril allows differentiation from immune-mediated keratoconjunctivitis sicca, as nasal mucosa hydration is dependent on the function of the lateral nasal gland and not on tear production. The majority of cases resolve spontaneously if the underlying disorder is addressed, but supportive management with supplementation of eye lubrication is essential. Because there is no underlying immune-mediated process, ciclosporin A is unlikely to have any effect in neurogenic keratoconjunctivitis sicca. The presence of denervation hyper-sensitivity following parasympathetic denervation of the lacrimal gland allows the opportunity to re-establish lacrimation in cases that do not spontaneously resolve with a direct-acting parasympathomimetic. The drug most commonly used is 1% pilocarpine, given in the food twice daily, but the dose should be carefully titrated to achieve an effect that stimulates tear production but avoids the development of deleterious side effects, such as vomiting and diarrhoea.

Lesions resulting in increased tear production
Paradoxical tearing (gustrolacrimal reflex or ‘crocodile tears’) describes the syndrome of excessive tear production while eating or during anticipation of a meal. The syndrome has been recognized for a considerable time in humans and was subsequently described in the cat. The underlying cause in human medicine is thought to be aberrant regeneration of facial nerve fibres following trauma, with fibres that usually innervate the salivary glands being misrouted to the lacrimal gland. The name ‘crocodile tears’ was derived from the popular myth that crocodiles cry while eating their prey.