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BLOG: Why does orbital fat prolapse?
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Say you’re doing binocular indirect ophthalmoscopy on a patient and you want to examine the inferior retina. You ask him to look down, you pull up on his upper eyelid and suddenly – under his superior-temporal conjunctiva – a blob of pinkish-yellowish tissue sags into view.
Has this happened to you? The first time it did, you might have been fairly nervous about it. But there’s no need to send this patient for head imaging. This month we’re going to talk about orbital fat prolapse.
I distinctly remember going to an attending during my residency year and trying to calmly inform her that my patient has a mass in his eye. Thankfully, she wasn’t as flustered as I was. She told me that this condition is prolapsed orbital fat and is a benign consequence of aging.
She was right, but I never really grasped what was actually happening in the eye. How did the fat get under the conjunctiva? The answer has to do with Tenon’s capsule. This is – in my opinion – a tricky visualization for many new clinicians.
We learn about the conjunctiva, episclera and sclera. We learn how to differentiate inflammation in each layer. And then we learn that the Tenon’s capsule also exists, but mostly only hear about it in connection with surgeries or injections. Sub-Tenon’s injections, for example.
Tenon’s capsule is another layer to protect the globe; its anterior face touches the back of the conjunctiva, and its posterior face touches the front of the sclera/episclera. Remember that the conjunctiva is only in the anterior portion of the orbit and only layers onto the anterior portion of the globe.
Tenon’s functions to completely envelope the whole globe, running from the limbus back to the optic nerve, just anterior to the sclera. Its fascia is perforated by ciliary nerves and vessel, and also by extraocular muscles. This is where orbital fat prolapse comes in.
Although the globe is surrounded by orbital fat, it doesn’t penetrate under the layers of the eye because Tenon’s capsule keeps it out. Orbital fat sneaks under Tenon’s when there is a discontinuity of the capsule. One way for this to happen is after surgery.
Strabismus and scleral buckle surgeries are just two operations in which Tenon’s is compromised. After the surgery, orbital fat can make its way under the capsule and then migrate anteriorly, since that’s the path of least resistance. But most of the time, prolapsed orbital fat isn’t a complication of surgery, it simply happens because Tenon’s becomes thin and fragile with age, and small breaks start to occur. There are also large gaps in Tenon’s where the extraocular muscles insert, and it’s thought that age-related weakening around these gaps is where most of the fat comes under Tenon’s. Once a little bit of fat tissue penetrates the capsule, a lot of it often follows. This herniation of fat tissue can happen all at once, sometimes causing a subconjunctival hemorrhage from the force of it.
An astute clinician will notice that the fat rarely makes it all the way to the limbus, usually stopping at least 3 mm to 4 mm posterior. This phenomenon is likely due to the fact that Tenon’s is strongly fused to the underlying sclera until approximately 5 mm posterior from the limbus. Thus, the fat has a hard time going any further. This is also why when clinicians make an injection into the sub-Tenon’s space, they use a forceps and pull up the conjunctiva/Tenon’s and make a “tent” 5 mm to 10 mm posterior to the limbus and insert the needle there.
The surgical correction for prolapsed orbital fat (performed mostly for cosmetic reasons) is relatively straightforward. Make an incision in the overlying conjunctiva, then make an incision in the overlying Tenon’s capsule, remove the prolapsed fat, cauterize and suture the wounds closed. A good closing suture job would also function to make a barrier to prevent future re-herniation, although recurrence is possible.
One last point: I think a fantastic way to understand this (and a lot of ocular conditions) is to watch a surgery for it. Seeing the ocular anatomy as it’s being dissected or manipulated can really change your perspective of it and illuminate things that a slit lamp simply can’t. I would encourage you to view YouTube videos of orbital fat prolapse surgery, as it lets you see these layers in a way that a text or a figure doesn’t.
References:
Coats DK, et al. Strabismus Surgery and its Complications. Springer Science: New York. 2007.
Jordan DR, et al. Can J Ophthalmol. 1987;22:173-179.
Kim E, et al. Am J Neuroradiol. 2011;doi.org/10.3174/ajnr.A2313.
Kim YD, et al. Korean J Ophthalmol. 1994;doi:10.3341/kjo.1994.8.1.42.
Nakamura N, et al. Clin Ophthalmol. 2015;doi:10.2147/OPTH.S91598.