What distinguishing characteristic can help differentiate between conjunctivitis and iritis?

The distinguishing characteristic that helps differentiate between conjunctivitis and iritis is a ciliary flush. Ciliary flush refers to the redness that occurs around the cornea and is typically a sign of inflammation in the anterior segment of the eye. This phenomenon is more pronounced in iritis, where there is inflammation of the iris itself, indicating involvement of deeper ocular structures.

In contrast, conjunctivitis generally presents with a more diffuse redness that is more uniform across the conjunctiva and does not typically involve the ciliary vessels, which are primarily affected in iritis. The presence of a ciliary flush in a patient thus strongly suggests iritis rather than conjunctivitis.

Other factors, such as eye discomfort, slow progression, and changes in vision, may overlap in both conditions but do not serve as reliable distinguishing factors. For instance, eye discomfort can be present in both cases, and while iritis may progress slowly, conjunctivitis can also have variable progression based on its cause. Changes in vision can occur in both conditions as well, but significant vision changes are more common in iritis due to the deeper involvement of the eye's structures. Hence, the presence of a ciliary flush is the most definitive characteristic for differentiating iritis from conjunctivitis.