A ‘swollen eye’ typically means one of two scenarios:

1. Buphthalmos – enlargement/stretching of the globe due prolonged elevated intraocular
pressure from chronic glaucoma.
2. Exophthalmos – anterior globe protrusion due to expansion of orbital tissues posterior to
the globe.

So, how can you differentiate between the two?

 Buphthalmic globes will have corneal and globe diameters greater than the unaffected/normal eye (unless both eyes are affected!), whereas exophthalmic globes will have corneal and globe diameters equal to the other eye.

Buphthalmic globe with no nictitans elevation

 Buphthalmic globes are not associated with nictitans elevation, whereas exophthalmic globes usually have nictitans elevation (the exception is ‘intraconal orbital disease’ – see next blog!).                                                                                                                                                                                                       Buphthalmic globes are usually associated with significantly elevated intraocular pressures due to glaucoma (although pressure will eventually normalize is some chronic glaucoma cases), whereas exophthalmic globes are associated with normal or mild elevation in intraocular pressures (the latter being due to expanded orbital contents placing pressure on the globe)

 Buphthalmic globes often have significant ocular changes such as corneal oedema, dilated pupil, lens subluxation (due to globe stretching), whereas exophthalmic globes often feature a relatively normal eye unless there is significant exposure keratitis from an inability to blink over the protruding globe.                                                                                                                                     

Exophthalmic globe with nictitans elevation

 Buphthalmic globes are usually blind due to chronic uncontrolled glaucoma with no pupillary light reflex, dazzle reflex or menace response, whereas exophthalmic globes are usually visual unless the orbital disease process has affected the optic nerve.                              

 Pain on palpation is not a good differentiator – some buphthalmic and some exophthalmic globes can be very painful, moderately painful or painless on globe palpation, depending on the chronicity (buphthalmos) and cause (exophthalmos –see next blog) of the underlying condition.

https://www.voreyevet.com.au/ocular-disease/swollen-eye-part-2-exophthalmos-and-its-causes/