Peters anomaly is a congenital defect in the embyonic development of the anterior segment of the eyeball, causing patients to be born with an opaque area in the centre of the cornea. 

The cornea is located in the front most part of the eyeball. In normal conditions, it is a transparent tissue due to the absence of blood vessels and the distribution of the fibres comprising it. It has 5 layers: the epithelium, the Bowman's membrane, the stroma, the Descemet membrane and the endothelium. 

Peters anomaly is an uncommon pathology that affects around 1 in every one million people and is responsible for 40% of congenital corneal opacities. It may affect one or both eyes, although it is bilateral and asymmetrical in 80% of cases. Its exact cause is unknown, but ethiopathogenic factors are involved in the genes affecting the embryonic development of the eye. 

These genetic mutations may occur in at least 4 genes: PAX6, PITX2, FOXC1 and CYP1B1. The PAX6 is the gene that regulates the embryonic development of the eyeball and the mutations in this gene may cause other pathologies like congenital aniridia. The majority of cases are sporadic, but recessive and dominant autosomal inheritance patterns have been described, in which there is more than one family member with the same condition. 

Peters anomaly is usually detected by the paediatrician straight after birth, as newborns usually have a white spot in the centre of the cornea. The Descemet membrane, posterior stroma and endothelium of the centre of the cornea do not develop properly, leading to opacity in its centre (leukoma, from the Greek 'leucus' which means white). 

The corneal periphery is generally respected and a characteristic sign is the presence of adherence (synechiae) between the corneal defect and the iris. The loss of transparency of the central part translates as decreased vision,  which will become more significant, the larger the size and density of the leukoma.  

Despite the fact that opacity is the most characteristic sign, Peters anomaly is usually associated with other defects like glaucoma (an increase in eye pressure which can damage the optic nerve and is the most severe complication of this anomaly), nystagmus (involuntary movement of the eyes due to the absence of cooridnation in the central retina), cataracts (a loss of transparency in the crystalline lens) and retinal detachment. 

In the most severe cases, the eyeball fails to develop properly, which leads to a rudimentary eye which we call microphthalmos. On occasions, the genetic defect (mutation) can be multiple and affect other organs and systems. This condition is called Peters Plus Syndrome and also causes cardiopathy, deafness, small stature, a cleft lip and cleft palate.

Patients require treatment with a multidisciplinary approach, including a cornea specialist and a paediatric ophthalmologist. In bilateral cases, surgical treatment via a penetrating keratoplasty (full thickness corneal transplant) is indicated. The surgery involves substituting the opaque cornea for a healthy cornea from a donor. 

If the patient also has cataracts, they can also be removed in the same operation, the prognosis being somewhat worse. Children under the age of one do not receive an intraocular lens instead of their crystalline lens, which is why early visual rehabilitation using glasses or soft contact lenses is required if the corneal transplant so permits.

Associated glaucoma should be treated to regulate intraocular pressure as the damage to the optical nerve is irreversible. On occasions, the use of topical hypotensive eyedrops is sufficient, while in others surgical treatment will be required to regulate intraocular pressure.

Furthermore, it is important to rule out possible associated deformities, get a full family history and have an eye examination performed on the parents and siblings, in order to provide genetic advice. Lastly, we must not forget that treatment for these children whose visual system is still developing must be provided as early as possible to get the best possible visual prognosis and minimise the risk of amblyopia (lazy eye).