Imaging Findings

Acute rhinosinusitis does not require a radiologic study of the paranasal sinuses because the symptoms reported by the patient in association with the endoscopic examination are the only diagnostic steps required for making a correct diagnosis (Phillips 1997).

When an orbital complication is suspected, generally secondary to acute ethmoiditis, CT permits differentiation between edema, phlegmon, and abscess, and precise identification of the site of the lesion, which is necessary for proper treatment planning (HAhNEi et al. 1999). CT may discriminate between preseptal cel-lulitis, subperiosteal inflammation, and intraorbital (extra- or intraconal) spread. Involvement of orbital muscles and posterior extension of the inflammatory collection towards orbital fissures are additional critical issues, the latter entailing an obvious risk of intracranial spread.

Preseptal cellulitis (Group 1 according to ChANDiER et al. 1970) is confined to the anterior compartment of the orbit (eyelid, periorbital soft tissues). CT shows thickening of the orbital septum, increased density of orbital septum and periorbital soft tissues without involvement of the orbital cavity or exophthalmos (OiiVERiO et al. 1995).

Increased density of intraorbital fat tissue is the hallmark of orbital cellulitis (Group 2 according to ChANDiER et al. 1970). It is often observed at the level of the retrobulbar space amid muscles and optic nerve.

Subperiosteal abscess is located between the inner surface of the orbital walls and the periorbita (Group 3 according to ChANDiER et al. 1970) (Fig. 6.2). Both CT and MR may demonstrate a fluid collection with

Orbital Abscess

Fig. 6.2 a,b. Complicated acute rhinosinusitis: subperiosteal abscess. Axial CT after contrast administration. a Fluid inflammatory material occupies the left ethmoid labyrinth (asterisk). Thickening and increased density of periorbital soft tissues at the level of medial orbital angle, eyelid, and nasal pyramid: preseptal edema (arrows). An inflammatory collection is detected between the lamina papyracea and the medial rectus muscle (arrowheads). b The small gas bubbles in the upper section contained in the collection is bordered by a thin hypodense line, probably the periorbita (arrowheads): subperiosteal abscess. Thickening of the prenasal tissue (black arrows)

Fig. 6.2 a,b. Complicated acute rhinosinusitis: subperiosteal abscess. Axial CT after contrast administration. a Fluid inflammatory material occupies the left ethmoid labyrinth (asterisk). Thickening and increased density of periorbital soft tissues at the level of medial orbital angle, eyelid, and nasal pyramid: preseptal edema (arrows). An inflammatory collection is detected between the lamina papyracea and the medial rectus muscle (arrowheads). b The small gas bubbles in the upper section contained in the collection is bordered by a thin hypodense line, probably the periorbita (arrowheads): subperiosteal abscess. Thickening of the prenasal tissue (black arrows)

a peripheral enhancing rim. CT better depicts subtle defects of the bony walls adjacent to the abscess (Yousem 1993). Gas bubbles within the collection herald the presence of anaerobic agents or indicate fistulization from contiguous paranasal cavities.

Abscesses (Group 4 according to ChANDiER et al. 1970) secondary to ethmoid sinusitis are generally observed along the lamina papyracea, displacing the orbit anteriorly and laterally, whereas fluid collections complicating frontal sinusitis are located along the superior orbital wall and dislocate the ocular bulb anteriorly and inferiorly. A key point to be ruled out at CT is intraconal extension of the abscess through a breach of the periorbita. In this case, a precise assessment of the relationships between the lesion, extrinsic muscles, ocular bulb, and optic nerve is necessary (Fig. 6.3).

MR better demonstrates further vascular complications, such as superior ophthalmic vein or cavernous sinus thrombosis (Group 5 according to Chandler et al. 1970) (Yousem 1993; Oliverio et al. 1995).

The entity of bone changes - perfectly depicted at CT - is widely variable: intraorbital spread of inflammation in the absence of detectable defects of the lamina papyracea can often be observed in pediatric patients. More aggressive inflammatory processes may induce osteitis or osteomyelitis. Both CT and MR may demonstrate irregular areas of sclerosis - indicating chronic osteitis - as well as bone destruction with sequestration, typical of active osteomyelitis (Fig. 6.4).

Overall, CT may provide a correct diagnosis of orbital complication in up to 91% of cases, being significantly more accurate than clinical examination alone (81%) (Younis et al. 2002c).

Intracranial complications are generally secondary to frontal sinusitis. They are observed even in the absence of sinus wall defects, as they may be secondary to thrombophlebitis of valveless diploic veins (Lerner et al. 1995). Imaging is mandatory, in order to correctly assess the degree of involvement of in-

tracranial structures. In this setting, MR should be considered the technique of choice, its accuracy being superior to CT, in particular in differentiating du-ral reaction from epidural/subdural or intracerebral abscess, and in demonstrating thrombosis of sagittal or cavernous sinus (Hahnel et al. 1999; Rao et al. 2001; Younis et al. 2002c).

CT findings of meningitis may be unremarkable. Early signs are represented by mild enlargement of ventricles and subarachnoid spaces. Large amounts of inflammatory exudate may efface subarachnoid spaces, inducing marked enhancement of the meninges. This is probably related to extravasation of contrast agent from small vessels or to the presence of granulation tissue. Dural enhancement is also demonstrated at MR, on Gd-enhanced SE T1 images, especially at the level of the falx, tentorium, and convexity (Younis et al. 2002c).

At CT, subdural/epidural abscess is detected as an extracerebral hypodense fluid collection with a convex shape, separated from the parenchyma by a thick and enhancing rim (Fig. 6.5).

The CT appearance of brain abscesses is widely variable in the different phases of evolution. During the cerebritic phase, a focal hypodense area may be observed, characterized by a superficially gyri-form and deeply granular pattern of enhancement.

Ethmoid Sinus Abscess Scan

Fig. 6.3a-c. Complicated acute rhinosinusitis: from subperiosteal to intraconal abscess. Plain CT scan on the axial plane (a,b); contrast enhanced CT scan obtained 24 h after surgery (c). Acute rhinosinusitis: maxillary sinus and ethmoid labyrinth are occupied by inflammatory secretions. Preseptal edema (PE) and a subperiosteal abscess - bordered by medial rectus muscle - (arrows) are demonstrated in (b). CT scan performed 24 h after surgery (c) shows a large residual cavity after partial eth-moidotomy and a breach in the medial orbital wall. Though subperiosteal abscess has been drained, an intraconal inflammatory collection has developed behind the eyeball (arrowheads)

Fig. 6.3a-c. Complicated acute rhinosinusitis: from subperiosteal to intraconal abscess. Plain CT scan on the axial plane (a,b); contrast enhanced CT scan obtained 24 h after surgery (c). Acute rhinosinusitis: maxillary sinus and ethmoid labyrinth are occupied by inflammatory secretions. Preseptal edema (PE) and a subperiosteal abscess - bordered by medial rectus muscle - (arrows) are demonstrated in (b). CT scan performed 24 h after surgery (c) shows a large residual cavity after partial eth-moidotomy and a breach in the medial orbital wall. Though subperiosteal abscess has been drained, an intraconal inflammatory collection has developed behind the eyeball (arrowheads)

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