The Lens

11 The Lens





DEVELOPMENT AND GROWTH OF THE LENS


The lens is an unique organ in that its epithelium is inverted so that the cell apices face internally with their base lying on the basement membrane that encapsulates the lens (the capsule). It is not innervated and lacks a blood supply after regression of the tunica vasculosa lentis so that nourishment must come through the aqueous and vitreous. The lens grows throughout life by continuous mitosis in the equatorial epithelium, these cells mature into lens fibres. There is no means to shed fibres or catabolize protein yet the lens must remain transparent. New fibres are constantly produced and move centrally with each generation, and as they do so their cell nucleus is lost and the protein is compacted. The lens contains a very high concentration of protein, about 30% of its weight. Most of the protein is soluble and comprises the α and βγ crystallins. α Crystallin has a chaperon function whereby it binds denatured proteins and prevents the formation of large aggregates which would scatter light. The insoluble proportion of these proteins increases with age and their concentration increases from the cortex to the nucleus, accounting for the increased refractive index in the nucleus. Glucose metabolism is conducted by anaerobic pathways.









image

Fig. 11.4 The radius of curvature of the anterior surface of the lens becomes progressively shorter with age (i.e. more convex), resulting in increased postive spherical aberration (see Ch. 1). The cornea has a positive spherical aberration too so that with age there is increasing aberration which leads to image degradation and loss of contrast. This is of some interest as intraocular lenses are now being manufactured with a negative spherical aberration to neutralize the corneal aberration leading to better contrast post- operatively.











ACCOMMODATION


By contraction of the ciliary muscle (under parasympathetic 3 rd nerve supply) the lens changes shape and increases its diopteric power to focus near objects on the retina. The physiological basis of accommodation was developed by Helmholtz and his theory has been confirmed by many observations since then. The increase in diopteric strength of the lens is largely accounted for by a shortening in radius of curvature (increased convexity) of the central part of the anterior surface which also moves forward slightly. The curvature of the lens, which at rest is close to spherical, becomes more conoid on accommodation. This aspherical change appears to be brought about by a difference in behaviour between the nucleus and the cortex; the nucleus undergoes the greater change and distends the anterior axial capsule which is comparatively weaker centrally. The force required to change the shape of the lens comes from the capsular elasticity which moulds the lens by its elastic force as the tension from the suspensory zonules on the capsule changes.


Accommodation is measured in dioptres, thus 1D of accommodation is needed to focus from infinity to 1 m or 3D to focus at 33 cm. A child has as much as 14D of accommodation but by 60 years of age this has virtually disappeared.







ANOMALIES OF SHAPE AND POSITION


Anomalies of lens shape and position are rare disorders either resulting from primary lens pathology or due to secondary zonular changes. They are either genetic (in which case there may be other systemic abnormalities) or a result of trauma or pseudo lens exfoliation, which is associated with zonular weakness. Pupil block glaucoma is a common feature of subluxating or dislocating lenses (see Ch. 8). Intact lenses that dislocate posteriorly can lie in the vitreous or on the retinal surface for many years without causing ocular damage.

















CATARACT


A cataract is any opacity within the lens. Cataracts are classified according to their morphology and position within the lens and graded by the degree of opacity or ‘maturity’ produced. If lens damage is insufficient to progress to maturity a localized opacity is produced in the injured region that becomes surrounded by new lens fibres as they are laid down beneath the capsule (see glaucomflecken Ch. 7). The three major types of age-related cataract are nuclear, cortical and posterior subcapsular opacity; many patients have combinations of these. It has been suggested that these represent different disease processes: nuclear changes being caused by protein denaturation, cortical by damage to lens fibres and posterior subcapsular cataract by migration of lens epithelial cells posteriorly. This remains to be proven. Occasionally the morphology of a cataract may give an indication of its aetiology (e.g. posterior subcapsular cataract with trauma or steroids) and this may have important medicolegal implications. The morphology does, however, influence the patient’s symptomatology.


Genetic factors have shown to be important risk factors for age related nuclear and cortical opacity; other recognized cataractogenic environmental risks are sunlight, smoking, dehydration and chronic diarrhoea (Table 11.1).


Table 11.1 Causes of cataract
























Congenital Acquired
Maternal infection (e.g. rubella) Age related
Genetic Metabolic (diabetes, hypothyroidism, atopy)
Metabolic (e.g. galactosaemia) Drugs (steroids)
Chromosomal (e.g. Down’s syndrome) Intraocular disease (uveitis, retinitis pigmentosa)
Ocular developmental (e.g. Peters’ anomaly) Trauma (blunt injury, radiotherapy, intraocular surgery)
Trauma Genetic (age related nuclear and cortical, Dystrophia myotonica)


Jun 4, 2016 | Posted by in OPHTHALMOLOGY | Comments Off on The Lens

Full access? Get Clinical Tree

Get Clinical Tree app for offline access