Componentes principales de TRIXBOX

Classification o f human inherited cataract is difficult because o f the wide variation in morphologies observed though a number o f observers have proposed classification systems'

The lens develops by the formation o f an embryonic nucleus during morphogenesis, around which lens fibres are deposited throughout life, initially forming the fetal nuclear region and thereafter the cortex (figures 3 and 4). Animal models suggest that the genes so far implicated in cataractogenesis are expressed in a time-ordered, sequential fa sh io n " C a teg o risa tio n therefore, more weighted towards the location o f opacification rather than appearance w ill accommodate these developmental considerations and best reflect the underlying genotype. Such a system is also clinically convenient.

1.7.1 Nuclear cataract

Cataract affecting the nucleus is common and suggests an abnormality o f gene expression in early development. Opacities are inherited in an autosomal dominant manner and may be confluent or discrete. Affected individuals show bilateral symmetrical involvement with variable expressivity. An exception is the pulverulent cataract where the type and distribution o f the opacities can vary not only between family members but also between eyes o f the same patient'^’ .

Pulverulent cataract derives its name from the dust-like ‘"pulverised” appearance o f the opacities, which can be found in any part o f the lens. Largely historical attempts have been made to sub- classify this form o f cataract to reflect possible aetiologic differences. The first detailed description of an affected family was published by Nettleship in 1906^. In this, the Coppock family, the cataract was confined to the embryonic nucleus and has been termed central

pulverulent^^®; in all probability, the phenotype described as Doyne’s discoid cataract^^’ The Coppock family has not been the subject o f a published linkage study, unlike the genealogically unrelated pedigree whose cataract has been described as Coppock-like^'^ which has been linked to the crystallin gene cluster region on 2q. It is o f note that the Coppock phenotype and the cataract investigated by Renwick and Lawler in the “Ev family from Southern England”'® have

become synonymous in the literature. However, the latter involves the larger fetal nucleus with opacification increasing in density towards the periphery and is therefore identical to the family with zonular pulverulent cataract described by Poos^^^

Many other families with pulverulent cataract have now been described^ 2 2 7 -2 2 9 - g ^lear is that

significant intra- and inter- familial variation, both in the distribution o f the cataract and the degree o f opacification, distinguish this phenotype fi'om all others.

1.7.2 Lamellar cataract

The concentric deposition o f secondary lens fibres that occurs during growth o f the normal lens results in the formation of lamellae. Opacities confined to a specific lamella therefore reflect a short period o f developmental disturbance (usually during the foetal period) resulting in usually symmetrical bilateral lens opacification. Lamellar cataracts have also been called zonular, perinuclear, polymorphic^^® or Mamer's cataract'^'*). Commonly, cataract occurs at the anterior and posterior Y sutures and may be associated with cortical riders. The degree o f (R eification is variable and visual acuity may be well preserved or reduced enough to require surgiœl

intervention^^ ^.

1.7.3 Cortical cataract

Cataract limited to the cortex is rare and differs firom lamellar cataract since opacification is limited to a sector o f outer cortical, often inferior, lens fibres, adjacent to the lens capsule. The nucleus is unaffected. The pathogenesis is unknown but its distribution, early onset and subsequent progression suggest an abnormality o f the later stages o f lens development.

1.7.4 Polar cataract

The presence o f families with cataract limited to either the anterior or posterior pole is less amenable to explanation in terms o f lens development. Anterior polar cataracts are bilateral.

usually symmetrical, well-circumscribed lens ( ^ c itie s that are rarely progressive and can be inherited as dominant, recessive or X-linked traits®^® Larger opacities often have a pyramidal shape, the apex o f which may extend into the anterior chamber’’^ Associations with micrqihthalmia^^^ and astigmatism^^ implicate a gene involved in anterior segment development. Visual ftmction is usually well preserved^^.

Families with posterior polar cataracts are less common. Affected individuals have bilateral, symmetrical lens opacities, which are usually inherited as a dominant trait. Since opacification is close to the optically crucial, nodal point o f the eye, vision is commonly reduced^. In some families, progressive accumulation o f further posterior cortical opacities can lead to total cataract formation’

1.7.5 Blue dot cataract

The blue-dot cataract, first described by Vogt^^ is not truly congenital, but develops in childhood and progresses through early life^^. The discrete pinhead-shaped blue-white cqiacities are distributed throughout the lens becoming more numerous in the cortex where they may form large cuneiform (wedge-like) shapes in the mid-periphery. Within a pedigree, this phenotype is consistent in its distribution but variable in its severity. Acuity is usually w ell preserved; cataract extraction, if necessary, only being required in adult life and associated with a good outcome^^^^.

1.7.6 CoraUiform cataract

A peculiar and rare form o f cataract, “coralliform”, originally described by Nettleship^^®, is characterised by finger-like protuberances extending from the nucleus that resemble sea coral^^^

The visual impact is variable but cataract extraction is usually required in the early years o f life.

1.7.7 “Total” cataract

“Total” cataract, that is lens opacity apparently affecting both nuclear and cortical regions, has been reported in families both with autosomal dominant^^ as well as X-linked recessive congenital c a t a r a c t ' I t has also been reported as the end result o f the progression o f the phenotypes outlined above.

Other phenotypes have been described in isolated cases, but not documented in families.