1.6 The Abnormal Pupil

Question 1

What is iris coloboma?

What is typical and atypical presentation?

Answer 1

the abnormal development of the iris leaving a defect (a hole) in the collarette

• typical presentation is ventral (6 o’clock position)
• atypical is anywhere else

Question 2

When should the pupillary membrane regress normally?

Answer 2

by day 14 PP, when the eyes open

• if not, this is a PPM

Question 3

What is a uveal cyst?

Answer 3

spherical cysts formed from the uveal epithelium

• usually free-floating in the anterior chamber
• they can burst and leave pigment on the corneal endothelium

Question 4

What is iris atrophy?

Answer 4

a potential effect of ageing in which the iris thins and wears away, leaving a ragged pupil margin

• this may be mistaken for mydriasis if the margin is not examined closely

Question 5

What are differentials for darkening of the iris?

Answer 5

• benign melanosis
• iris melanoma (or other neoplasia)

uveal melanomas are the most common occular tumor in dogs and cats; they can change the shape or size of the pupil, reduce PLR, and cause glaucoma; they can also metastisize to other organs

Question 6

What is posterior synechia?

Answer 6

when the iris adheres to the anterior lense capsule

• complication of uveitis (the iris becomes “sticky”)
• can cause decreased or absent PLR, and glaucoma (if the adhesion is 360 degrees)

Question 7

How does lense luxation occur?

Answer 7

the lens is anchored in place by numerous fibers, called lens zonules (“zonules of zinn” attach lens to ciliary body)

• if these fibers break down, the lens begins to drift from its central position
• lens luxation occurs when all of these fibres are broken and the lens becomes loose within the eye, where it can move forwards or (less commonly) backwards in the eye
• anterior lense luxation can obstruct the pupil from constricting

Question 8

What can cause miosis?

Answer 8

• uveitis: miosis alongside aqueous flare, conjunctival hyperemia, and hypopion/hyphema if severe
• Horner’s syndrom: miosis alongside enopthalmos, third eyelid protrusion, and ptosis (upper eyelid drooping)

Horner’s syndrome will NOT have aqueous flare (greatest differentiator from uveitis)

Question 9

What can cause mydriasis?

Answer 9

• glaucoma: non-functioning optic nerve causes fixed, mid-dilated pupil
• dysautonomia: disregulation of the autonomic nervous system, which causes many other systemic signs (megaesophagous, atonic blader, etc.
• fear: heightened sympathotic tone leading to reduced or absent PLR
• central blindness: many causes

Question 10

Understand the PLR pathways.

Answer 10

check notion for a refresher

• know the neural pathway of the eye (wall image)