8/9/2019 Pterygia May Be Classified as Inactive Or

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Pterygia may be classified as inactive oractive. An inactive pterygium shows little

or no evidence of progression over a long

period. Conversely, an active pterygiumbehaves in a far more aggressive fashion,

with an advancing margin of greyish opacification

and hyperaemia within the tissue.

Beneath the body of the lesion there maybe destruction of Bowman's layer andthe superficial corneal lamellae. The

pterygium may invade the superficial

peripheral cornea (with the ape of thelesion towards the cornea! and move towards

the pupillary area, eventually causing

corneal distortion and visualThe development of instruments to

analyse corneal topography has created

the opportunity to study corneal surfacechanges not apparent by other methods."

#ideo$eratoscopy is very important in theevaluation of pterygium because it can

provide an assessment of the amount of

corneal toricity and i r r eg%l a r i ty.T%h evideo$eratoscopic image can also be used

to measure pterygium encroachment onto

the cornea as the distortion of the otherwiseregular pattern helps define the limits

of the pterygium."

The development of a pterygium canlead to significant corneal distortion and

astigmatism. A pterygium generally causes

localised flattening central to the ape ofthe pterygium.& As this flattening is along

the horiontal meridian, it usually causeswiththerule corneal astigmatismg )istinct

from the symmetry of most forms of

corneal astigmatism, that induced by apterygium is usually hemimeridional on

the side of the pterygium."

*t has been postulated that the cause ofthe astigmatism associated with pterygium

is tear film pooling at the ape of the pterygium.

l+ The proposed mechanism in thiscase involves a tear meniscus developing

between the corneal ape and elevatedpterygium, causing an apparent flattening

of the normal corneal curvature in that

area, as the head of the pterygium approachesthe ape of the cornea.&' Another

possible eplanation for the astigmatism

is traction on the cornea by thepterygium (due to the ingrowth of

fibrovascular tissue!, as this is sometimes

obvious and may restrict ocular ductions.There is a significant correlation between

the etension of the pterygium onto

the cornea and the amount of induced

astigmatism. Pterygia appear to have aminimal effect on the central cornea until

they eceed -per cent of the corneal

radius (or reach within 3.2 mm of thevisual ais!. nce this critical sie isreached, increasing degrees of astigmatism

are induced./ The patient described

in this case report had a pterygium encroachingonto the cornea by about four

millimetres. This figure is greater than -

per cent of the corneal radius and so the

high degree (approimately 0.++ )! ofinduced withtherule astigmatism was not

unepected. (The term 'corneal radius' isused here to denote the semidiameter of

the cornea and not the cornea's centralaial curvature.!There is a poor correlation between

pterygiuminduced astigmatism measured

topographically and that measured bymanifest refraction, with much higher

degrees of astigmatism revealed by theformer.' Again, this is supported by the

findings of this case report, in which the

topographic cylinder was about 0.++ ),while the refractive cylinder was less than

half this amount. This wea$ correlation between

topographic and refractive astigmatismis probably due to the asymmetric

nature of pterygiuminduced astigmatism,with the more normal (usually temporal!side of the cornea reducing the refractive

effect of the changes in the nasal cornea./

Pterygium ecision usually induces a reversal ofpterygiumrelated corneal flat pterygium, the refractive

cylinder was retening.&

Conse1uently, successful ptery duced to 2.++ ) and visualacuity improved

gium surgery should reduce pterygium to 3 "4". Thischange in the againsttheinduced

refractive astigmatism and rule direction is most evident in

the early