One Eye Squinting Suddenly: the 2-Hour Line and 3 Causes

A cat that starts squinting one eye is either in pain or avoiding light, and the two look almost the same at 1 m. What separates them is not the squint itself but what the eye does in the next 2 hours. Surface injuries declare themselves fast, pressure inside the eye changes the pupil within 120 minutes, and nerve problems drift over 1 to 3 days.

So the clock is short on purpose. Two hours is long enough to see a trend and short enough that a deep corneal ulcer has not yet perforated. Measure both eyes, write 3 numbers down, and use the table to decide between a same-day visit and an emergency.

The 2-hour line, in 3 blocks

Minutes 0 to 30: note the exact time the squint started, and whether the eye was normal at the last meal. Check for a foreign body by looking at the surface from 2 angles, and count blinks for 60 seconds. Minutes 30 to 120: repeat the blink count and the pupil measurement at the 1-hour mark as well, because a pupil that changes 1 mm in an hour is a pressure or inflammation case, not a scratch. After 120 minutes: if the squint is unchanged and the cat is eating, book within 24 hours; if it is worse, go now.

Thirty seconds of comparison: the good eye is the ruler

Record 4 numbers. Pupil width in millimeters, in the same light, with a phone torch held 30 cm away and 2 seconds of light only. A difference of 1 mm or more between the eyes is real anisocoria; in bright light a normal pupil is 1 to 2 mm and in a dark room up to 8 or 9 mm. Lid opening in millimeters, comparing the gap above the pupil. Blinks per 60 seconds; a difference of 5 or more in a minute is a genuine asymmetry. And wetness: a wet streak under the eye with a squint usually means the tear film is running over, which is the pattern behind sudden tearing in one eye.

Group 1: the surface, which declares itself within 2 hours

Corneal scratch, ulcer, a plant awn under the third eyelid, a hair or eyelash curling in, or chemical contact. The eye weeps, the cat rubs it with a paw, and the squint is worse in bright light. A scratch that is 2 mm wide at 8 a.m. can be 4 to 5 mm and infected by evening, so this group owns the 2-hour rule. Do not put anything in the eye except sterile saline; a human drop with a steroid can turn a 1 mm ulcer into a melting one in 24 hours. Chemical exposure needs 5 to 10 minutes of gentle flushing and a call, and the toxin list covers the common household splashes.

Group 2: inside the eye, where the pupil moves first

Uveitis and raised pressure. Normal pressure in a cat is 15 to 25 mmHg, and a difference of 5 mmHg between eyes counts. In uveitis the pupil goes small and the eye is tender to light; in glaucoma it goes wide and fixed, the surface clouds within 24 to 48 hours, and pain is severe. A pupil that is 2 mm wider than its neighbour and does not react to a torch at 2 hours is the pattern that needs a vet the same day. Glaucoma can take vision in 24 to 48 hours, which is why this group sits between 2 and 6 hours, not 48.

Group 3: nerve, lid, and tear film, which move slowly

Horner syndrome gives a small pupil, a drooping upper lid, and the eye sitting 1 to 2 mm deeper in the socket, and the squint is mild. A nerve problem after a neck injury, a middle ear infection, or a bite can add a head tilt or a wobble within 48 hours, which is when the picture stops being an eye problem (sudden wobbly walk). Entropion and low tear production are slower still: a tear test under 15 mm per minute, or a lid margin rolling in by 1 to 2 mm, produces a squint that comes and goes over days and worsens in a dry room.

Table: 3 groups, 4 numbers each

Group	Pupil gap (mm)	Change in 2 h	See a vet within (h)	First check
Surface: ulcer, foreign body	0 to 1	worse by 1 to 2 mm of defect	2 to 6	Blinks per 60 s, light test
Inside: uveitis	1 to 2, smaller pupil	1 mm an hour, pupil shrinking	2 to 4	Pupil width, tenderness
Inside: glaucoma	2 to 4, wider pupil	fixed pupil, clouding by 24 h	0 to 2	Pupil reaction to torch
Nerve: Horner	1 to 3, smaller pupil	no change in 2 h	24 to 48	Lid droop, eye depth
Lid: entropion, dry eye	0 to 1	no change in 2 h	48 to 72	Tear test, lid margin

Three things to do while you wait, three to avoid

Do: dim the room to 30 percent brightness so photophobia is easier to read; keep a 2-hour log with time, pupil millimeters, and blinks per minute; and offer water and food, because a cat that stops eating with a squint moves up a tier. Avoid: touching the eye to test sensation, wiping with cotton wool, and any leftover drop from a previous prescription, since a 2-day-old bottle grows bacteria and a steroid drop on an ulcer makes the ulcer worse within 24 hours.

What the vet does in 15 minutes

A fluorescein stain read under blue light, a pressure reading in both eyes, a tear test, and a look under the third eyelid with a cotton tip. Those 4 checks separate the 3 groups in about 15 minutes, and each has a number attached: stain uptake area, pressure difference over 5 mmHg, tear rate under 15 mm per minute, and pupil size before and after a drop. Bring your 2-hour log; the trend saves a repeat visit.

Today: 5 steps

1. Write the minute the squint started, and check the eye in daylight at 30 and 120 minutes.
2. Measure both pupils in the same light and record the difference in millimeters.
3. Count blinks for 60 seconds on each eye and note which side weeps.
4. Do not instill any drop; flush only for a known chemical splash, 5 to 10 minutes.
5. Go now for a wide fixed pupil, clouding, or a wobble; book within 24 hours for a stable mild squint.

Terms mentioned in this article

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