Lizard Eye

Introduction

Common mydriatics have no effect on the reptilian eye because both the iris sphincter and ciliary muscle contain striated (or skeletal) rather than smooth muscle. This means that they are under conscious control. Therefore, unlike mammals, the consensual pupillary response is absent, the cornea does not contain a Descemets membrane and scleral ossicles are often present. This renders the ocular examination more challenging but more importantly, the parasympatholytic topical drugs (that reduce the activity of the parasympathetic nervous system) used to dilate the pupil (mydriasis) in mammals are totally ineffective.

Another characteristic of the lizard eye is its ability to occupy both scotopic (light limited) and photopic (light rich) environments. Indeed, the morphology of the eye adapts and varies according to activity in these different light environments.

Parietal Eye

A well developed parietal "eye" (with a rudimentary lens and retina) is found on the dorsal midline in some lizards, including the green iguana, blue-tongue skink and water dragon. This sensory organ, connected to the central nervous system and the pineal gland by the small parietal nerve, functions in hormone production (including reproduction) and thermoregulation. Although sensitive to changes in light, it cannot form images. Sometimes referred to as "pineal eye" or "third eye", it it visible as an opalescent gray spot on the top of some lizard's heads.

Eyelids

They are usually present. The lower lid is movable, moving upwards to close the eye; in some lizards, this lid might be transparent, allowing vision even when the lids are closed. Lizard also have a well developed, semi-transparent, highly mobile third eyelid which is called the 'nictitating membrane'; this membrane can close across the eye even while the eyelids remain open.

In some chameleon species as well as some geckos and the oscellated skink (Ablepharus sp.) the eyelids have fused to produce a circular, immobile and transparent dermal aperture called a spectacle. It create an impervious barrier to topically applied medications.

The pupil

The pupil is usually round and relatively immobile in diurnal species and is usually slit-like in nocturnal species. Geckos, such as the Tokay gecko, have a serrated pupillary opening resembling a series of small holes when the pupil is completely closed. This specificity allows for acute vision even in dim light.

Ophthalmologic Examination

The eyes are usually considered as a barometer of general health and environmental conditions; therefore a full ophthalmologic examination should be performed in all cases.

Vision can and should be evaluated by observing certain behaviours such as a menace response.
Following a general assessment of the eye and associated structures (eyelids, tear film, globe), a more detailed examination should be performed using a form of focal illumination and a magnification system.
Examination of the ocular fundus to assess the retina (fundoscopy) can be carried out using a direct or indirect ophthalmoscope.
Mydriasis can be attempted either through general anaesthesia or intracameral injection of neuromuscular blocking agents such as curare or d-tubocurarine. These agents can also be applied topically; however their effectiveness may be affected by the variable corneal penetration of the drugs.
Other diagnostic tools, such as tonometry (measurement of the intraocular pressure), stains, cytology, bacteriology, histopathology and electron microscopy, in addition to routine diagnostic tools (haemotology, biochemistry, radiology and ultrasound) can also be used to detect an ocular disease or underlying problem in lizards.

Behaviour

When threatened or molested by other animals, such as dogs, coyots or foxes, certain horned lizards (Phrynosoma cornutum, P. coronatum and P. solare) will respond by squirting an spectacular amount of blood from their eyes. They accomplish this by constricting venous outflow from the head which increases blood pressure and causes leakage from the ocular venous sinuses.

References

Engbretson, G.A. & Lent, C.M. (1976). Parietal eye of the lizard: neuronal photoresponses and feedback from the pineal gland. Proceedings of the National Academy of Sciences 73, 654-657
Girling, S.J. (2004). BSAVA Manual of Reptiles. British Small Animal Veterinary Association. pp. 350. ISBN 0905214757
Hall, M.I. (2008). Comparative analysis of the size and shape of the lizard eye. Zoology 111, 62-75
Mader, D.R. (2005). Reptile Medicine and Surgery. Saunders. pp. 1264. ISBN 072169327X