Murat V. Kalayoglu, M.D., Ph.D.
Contributing Editor
Penetrating ocular trauma with a foreign body is a rare but serious ophthalmic emergency. Intraocular foreign bodies (IOFB) can be inert, but often cause serious damage inside the eye and must be removed promptly. Unless the IOFB is removed and the wound repaired within 24 hours, the patient’s risk of a severe complication – such as endophthalmitis or vision loss – quadruples. Early removal of the IOFB facilitates the procedure, reduces formation of encapsulating material, and dramatically decreases tissue toxicity caused by the foreign body. The majority of IOFB are magnetic, and as such amenable to extraction with the aid of a magnet.
Two types of magnets are available for use in intraocular surgery. External magnets are powerful enough to exert a magnetic field to the IOFB when applied on the outside of the eye, and can be used to extract a limited number of magnetic foreign bodies depending on location, size and the local environment of the foreign body. Internal, rare earth magnets exert relatively less magnetism when compared with external magnets, but are small enough to be introduced inside the eye for precise delivery of the IOFB.
The Ophthalmologist must choose a mode of extraction depending on a variety of factors, including the nature of the penetrating trauma, type of IOFB, existing tissue damage, extent of vitreous organization around the foreign body, location of the IOFB, and size of the foreign body to be extracted. Several principles guide the Ophthalmologist’s decision in choosing the appropriate mode of extraction. For one, an estimate of the iron content of the IOFB is crucial, since the magnetism of the foreign body is proportional to its iron levels. This information is often gathered by a careful history and inquiry into the exact nature of the penetrating injury. For example, up to 80% of all work related penetrating intraocular traumas result from particles released by metal striking metal.
Second, the Ophthalmologist must consider that a magnetic foreign body tends to orient longitudinally in relation to the magnet. This positional change can be important if the foreign body has to move significantly in order to be oriented longitudinally towards the magnet, especially if the IOFB is lodged in delicate tissue, near a vessel, or has significant adjacent vitreous organization. Finally, the exerted magnetic force lessens with increasing distance, with the force inversely proportional to the cube of the distance between the magnet and the metallic foreign body.
In general, metallic IOFBs within the anterior segment are relatively easy to extract, and surgical outcomes tend to yield good results. Many such IOFBs can be removed by creating a limbal incision, then retrieving the foreign body with either a forceps or a hand held magnet. If the IOFB is inert and the entry site is closed, some surgeons may choose to leave the foreign body inside the eye and follow the patient. Posterior segment IOFBs are usually much more difficult to remove given the limited access and visibility that result in many traumatic injuries. A key principle in removing any IOFB from the posterior segment is obtaining excellent visibility. Using an external magnet with poor visibility can cause a myriad of complications. External magnets are usually reserved for cases where the view is excellent and the IOFB is not encapsulated by organized vitreous.
In these instances, the surgeon may choose to perform a pars plana sclerostomy and direct the external magnet towards the IOFB to retrieve it. The sclerostomy is created at the location yielding the shortest extraction route. Several studies suggest that in these patients, vitrectomy is not necessarily required before using the external magnet. However, since the majority of posterior segment IOFBs are obscured by opacification of media, embedded within tissues or encapsulated, many surgeons prefer using internal rare earth magnets and vitrectomy techniques to remove IOFBs. Vitrectomy is also useful to have immediate, unimpeded access to damaged tissues such as the lens, retina and choroid. During vitrectomy, rare earth internal magnets can be exchanged with non-magnetic grasping forceps or other vitreoretinal instruments to deliver the IOFB in a controlled fashion. Ultimately, forceps are needed for transscleral delivery of the IOFB, since the rare earth internal magnets are usually not powerful enough to maintain a hold on the IOFB when delivering it through the sclera.