When confronted by a flying missile, the normal reaction is to attempt to close the eyelids and to rotate the eyes upward. This is the reason why the commonest point of impact is the lower
temporal part of the eye and it is in this region of the iris that one is most likely to see peripheral iris tears ("iridodialysis").
When the eye is compressed the iris periphery is torn at its root, leaving a crescentic gap, which looks black, but through which the fundus and red reflex can be observed. Such an injury also provides an excellent view of the peripheral part of the lens and the zonular ligament (Figure 16.2).
Contusion can result not in a tear of the iris root, but in a tangential splitting of the iris and ciliary body from the sclera producing recession of the angle of the anterior chamber; the appearance is often associated with secondary glaucoma, sometimes many years after the injury and is identified using the special contact lens known as the gonioscope.
A sudden impact on the eye can also produce microscopic radial tears in the pupillary
sphincter of the iris. This could be a subtle microscopic sign of previous injury when no other signs are present, or the damage might be more severe, resulting in persistent dilatation of the pupil (traumatic mydriasis). Unless the eye is examined, this widening of the pupil after injury can be mistaken for a third cranial nerve palsy.
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