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.
