Keywords
computed tomography - orbital fractures - vitrectomy - X-rays
A 58-year-old lady underwent pars plana vitrectomy with intravitreal gas (14% C3F8) tamponade for retinal detachment repair. She complied with prone posture and surgical wounds were stable postoperatively. Unfortunately, she fell down the stairs at home the day after discharge from hospital, and readmitted for head injury. Investigation by skull X-ray showed radiolucency over the right orbit in bubble configuration ([Fig. 1]). Attending emergency physician further worked up with computed tomography scan to rule out orbital fractures which revealed negative density within the vitreous cavity (−990 to −400 Hounsfield’s unit) with the shape of the eyeball maintained ([Fig. 2]).
Fig. 1 The bubble eye sign. Orbital X-ray (Waters’ view) showing radiolucency over the right orbit. The gas was taking the configuration of a bubble, compatible with gas-filled intravitreal cavity confounded by the eyeball.
Fig. 2 Computed tomography (transverse cut) of orbit. Right eye intravitreal cavity was filled by radiolucent gas bounded by the scleral shell and the natural lens over this transverse cut.
Being a confined space filled by the eyeball and muscles and fat tissue, orbit is free of gas content normally. Without any natural openings, external gas could only enter the orbit via periorbital sinuses, opening of septum either traumatically or surgically; or rarely developed in situ by gas forming anaerobes’ infection. Intraocular space is a sterile environment and intraocular gas is sometimes introduced intraoperatively for the intention to retain postoperatively for tamponade purpose. Commonest usage of gas tamponade would be on retinal detachment or macular hole surgery, when long-lasting intraocular gas is utilized as tamponade effect to appose the detached neurosensory retina toward the retinal pigment epithelium, postoperatively.[1] In addition, gas tamponade is also utilized after posterior lamella keratoplasty for graft apposition to host’s corneal stroma.[2] Depending on the nature of gas, complete resolution varies from days, for air, up to few weeks to months for SF6 or C3F8, etc.[3]
Volume of intravitreal gas is larger than intracameral gas, and the approximated sphere shaped vitreous cavity allows the intravitreal gas to keep its bubble configuration throughout the whole resorption stages. As gas is radiolucent on radiography, intravitreal gas appears as a radiolucency bubble on X-ray and computed tomography, bounded by the scleral shell, outlining the shape of the eyeball. This “Bubble Eye sign” differentiates intravitreal gas from small orbital emphysema in orbital fracture, when gas is located outside the globe but confined by the orbit, giving a crescent or concave shape usually flowing upwards over the superior orbit ([Figs 3]
[4]–[5]).
Fig. 3 Orbital X-ray (Waters’ view) of another orbital fracture case for comparison. Left orbital emphysema classically seen after orbital fracture was evidenced by the rim of radiolucent gas over the superior orbit. It also outlined the eyeball position, but by its extraocular location.
Fig. 4 Orbital X-ray (Caldwell’s view) of the same patient in Fig. 3. Discontinuity of the left medial orbital wall suggested the site of orbital fracture.
Fig. 5 Computed tomography (transverse cut) of orbit of the same patient in Fig. 3. Left orbital emphysema was well illustrated by the hypodense gas-filled spaces in between the soft tissue planes.
Traumatic fall is not rare after intraocular surgeries for few reasons. First, there is loss of usual stereopsis. Adaptation to new visual sensation does require time,[4] and a gas-filled eye not only affects visual acuity, but also distorts images by its meniscus level. Second, with prolonged prone posturing after vitreoretinal surgery, patients would feel dizzy on standing and navigating around. Last, in elderly patients with poor vision over the contralateral eye, their comorbidities, like poor muscle strength, balancing, and gait problems, all contribute to accidental fall even in familiar places.[5]
In conclusion, by identifying the “Bubble Eye sign,” physicians can differentiate intravitreal gas from orbital emphysema caused by orbital fracture, when more in-depth ophthalmological history taking and examinations are more indicated than computed tomography scan with radiation exposure to patients.
