Orbital Fat Prolapse

Summary
Orbital fat prolapse is a soft, harmless bulge of normal body fat that has slipped forward out of the bony eye socket and come to rest just beneath the thin, clear membrane that lines the inside of the eyelids and covers the white of the eye. The fat is normal tissue in an abnormal place. It is not a tumor, not an infection, and not a cyst, and it does not spread or invade anything around it.

Behind and around your horse's eyeball sits a generous cushion of fat that supports the globe and lets it move smoothly. A sheet of tough fibrous tissue normally holds that fat pad back where it belongs. If a weak spot or small tear develops in that sheet, a lobe of fat can push through it and slide forward. The commonest place for this to happen is the inner corner of the eye, near or behind the third eyelid, because that is where the fibrous layer is naturally thinnest.

What you are most likely to observe is a smooth, rounded, movable swelling at the inner corner of the eye, often about the size of a marble, sometimes larger and oblong. It feels soft and squishy through the eyelid. It is usually pale pink or slightly yellow and is covered by conjunctiva that looks completely normal. It does not appear overnight in most horses; owners often report that it has been there a while and is slowly becoming more obvious. Most affected horses show no discomfort at all, do not squint, and have normal vision. If the mass is large enough to interfere with the third eyelid sweeping across the eye, you may notice tearing, a little discharge, or intermittent squinting.

This condition is uncommon, and this is the important part: you cannot tell it apart from several far more serious problems just by looking at it. Squamous cell carcinoma, other tumors, summer sores caused by stomach worm larvae, cysts, and abscesses can all produce a swelling in exactly the same place. A benign lump of fat and an early cancer can look very similar to an owner. Any new mass on or around the eye needs to be examined by your veterinarian rather than watched from a distance.

DIAGNOSIS: Your veterinarian confirms what the mass is before deciding anything else. A complete eye examination, sometimes a needle sample, and often removal of the tissue for laboratory analysis are how the answer is reached. One useful clue is that fat will not draw up into a needle, so a soft swelling that yields nothing when sampled points toward fat rather than fluid.

TREATMENT: If the prolapse is small and causing no trouble, it can reasonably be left alone and monitored. If it is large, is interfering with the eyelids, is being rubbed and irritated, or the diagnosis is not certain, it is removed surgically. This is generally a straightforward standing procedure done with sedation and local anesthetic blocks; general anesthesia is usually not required.
The excised tissue should be sent for laboratory confirmation. Horses recover quickly and the eye almost always looks normal again once the post-operative swelling settles.

Relevant_Factors



Prevention
There is no reliable way to prevent orbital fat prolapse, because the underlying weakness in the fibrous tissue behind the eye is not something you can see coming or strengthen.

Two things are still worth doing. Keep your horse at a sensible body condition, which is good practice for many reasons and reduces the total amount of fat available to push forward. Protect the eye from blunt injury and from flies with a well-fitted fly mask, since trauma is one of the recognized triggers and flies aggravate any exposed tissue around the eye.

After surgical removal, the most useful preventive measure is simply keeping the eye protected and clean while it heals, and having it re-checked so any early return is caught while it is still small.

Anatomy
The eyeball sits in a bony socket called the orbit. It does not sit directly against bone. It rests in a cushion of fat, the orbital fat pad, which fills the space around and behind the globe, absorbs shock, and allows the eye to rotate smoothly as your horse looks around.

That fat is held in place by sheets of tough fibrous connective tissue that wrap the eyeball and separate the fat from the surface tissues at the front of the eye. Over the front of the globe and lining the eyelids lies the conjunctiva, a thin, moist, semi-transparent membrane.

The third eyelid is a triangular fold of tissue at the inner corner of the eye, stiffened by a T-shaped plate of cartilage, that sweeps across the eye surface to clean and protect it. A small tear-producing gland sits at its base.

In orbital fat prolapse, a lobe of the orbital fat pad pushes through a defect or a thinned area in the fibrous sheet and comes forward to lie underneath the conjunctiva. The inner, lower corner of the eye near the third eyelid is the usual site because that is where the fibrous barrier is thinnest. The overlying conjunctiva stays intact and normal-looking, which is why the mass appears as a smooth lump under a clear membrane rather than as an open or angry-looking lesion.

Breeds_Affected
No breed predisposition has been established. It has been reported across a range of light horse breeds and body types. Reports exist in animals from foals through aged horses, so this is not a breed-specific problem.

Affected_Groups
Most reported cases are adult horses, and middle-aged to older horses appear over-represented, which fits with age-related weakening of the fibrous tissue behind the eye. It has, however, been described in a young foal, so age is not a requirement.

Horses in heavy body condition are a group worth flagging. More total body fat means more orbital fat, and more forward pressure on the tissue holding it back. This is a reasonable mechanical explanation rather than a proven risk factor.

Horses that have had blunt trauma to the head or eye region, or previous surgery around the eye or third eyelid, are also a recognized group, since either can create the defect the fat escapes through.