Disorders of the lens
Cataract
The major disorder affecting the lens is cataract, which is a defect in the clarity of the lens. Cataracts occur in about 1 in 250 infants, account for about 15% of blindness in children and about two thirds are bilateral. In most cases cataracts are present at birth and are identified by screening using a direct ophthalmoscope. Screening for the ‘red reflex’ (a luminous red appearance seen upon the retina during retinoscopy examination) is mandatory for all newborns and subsequently at checks in primary care.
Cataracts may be congenital (present at birth) or of childhood onset. Congenital cataracts may be unilateral or bilateral. Most unilateral congenital cataracts occur as an isolated abnormality in an otherwise normal child. Unilateral cataracts may be associated with a smaller eye and abnormalities of the vitreous. There are many possible causes of bilateral congenital cataracts that include inherited (autosomal dominant), chromosomal abnormalities (the commonest is Downs syndrome), intrauterine infections (Rubella, chickenpox, toxoplasmosis), metabolic disorders and genetically determined systemic syndromes, although in about 40% of cases no underlying cause can be found.
Childhood onset cataracts may not give rise to visual problems until later childhood. They may be either congenital cataracts that are mild in infancy but progress later or those that occur for the first time in later childhood, for example those associated with trauma, inflammatory conditions or resulting from treatments such as radiotherapy or oral steroid therapy. Childhood onset cataracts are usually easier to diagnose and treat than congenital cataracts and the prognosis for vision is better.
Treatment depends upon whether the cataract is bilateral or unilateral.
All infants with bilateral congenital cataract are referred to an ophthalmologist. The ophthalmologist initiates a referral to the paediatrician and geneticist, if necessary. Complete bilateral cataracts require early surgery within the first months of life. Some partial cataracts have a good visual prognosis and surgery can be postponed until later in childhood if vision deteriorates during the course of monitoring. If the degree of opacity (clouding) differs between the two eyes, occlusion therapy (patching of the better eye) may be required to prevent amblyopia (lazy eye).
Surgery involves removal of the catarcatous lens. Visual rehabilitation for the loss of focussing mechanism of the lens (aphakia) usually involves correction by inserting a lens implant at the time of the operation. However, in children under the age of 2, correction is usually by use of contact lenses but may be by use of spectacles, but these are difficult to make and fit and may not be well tolerated in infants.
Amblyopia is a significant complication of cataract and is important as a cause of poor vision in these children. Vision can be significantly improved by occlusion of the better eye and correction of any refractive error.
If unilateral cataract is detected before eight weeks of age, good visual acuity can be achieved with prompt surgery. Amblyopia should be prevented by occlusion of the phakic eye (the eye which has not had the lens removed) and correction of any refractive error. Squint is common even with early treatment and glaucoma may occur as a late complication in up to 50% of children. If a dense unilateral cataract is identified after 4 months of age, amblyopia cannot be reversed and surgery is not usually undertaken.
Prognosis: The outlook for congenital cataracts is improving though it is still common to have mild to moderate visual impairment. Unilateral cataract has a worse prognosis than bilateral cataract.
Lens subluxation (ectopia lentis)
Lens subluxation (displacement of the lens within the pupillary space) is usually due to weakness of the suspensory ligaments that hold the lens in place. In most children it is bilateral and due to an underlying genetic defect, for example Marfan syndrome (a connective tissue multisystemic disorder) or homocystinuria (a metabolic disorder), but may be secondary to trauma. Subluxation (incomplete or partial dislocation) may progress to dislocation of the lens into the anterior (front) or posterior (rear) chamber of the eye.
Lens subluxation may be asymptomatic (without symptoms) and only detected at screening examinations. Symptoms result from the lens becoming more spherical and unstable within the eye and include fluctuating myopia (short sightedness) and astigmatism. Squint and secondary amblyopia are common. An early sign is iridodonesis (tremulous iris) which may look like the eye is wobbling. Rarely the child may present with an acute painful red eye with reduced vision due to secondary glaucoma when the lens displaces and blocks the pupil.
All children with lens subluxation should be referred to an ophthalmologist and a paediatrician for investigation to exclude any underlying cause.
Most children with lens subluxation can be treated conservatively with correction of any refractive defect, treatment of amblyopia and regular monitoring of intraocular pressure for glaucoma.
Surgery may be indicated if it is not possible to maintain good visual acuity despite use of spectacles or contact lenses or if the lens dislocates regularly into the anterior chamber. Surgery involves removal of the lens and correction of the refractive error with spectacles or contact lenses.
The visual prognosis for children with lens subluxation is fair with most children having impaired vision (mild to severe) in one eye. Amblyopia related to high refractive errors is the major cause of visual impairment.
