21 Orbits


21 Orbits

21.1 Imaging Techniques

The orbits are osseous cavities in the anterior skull that contain the globe of the eye (“eyeball”) and serve as the source of vision. Imaging of the orbits plays a critical role in the diagnosis and management of various acute, chronic, and congenital conditions of the eye, orbit, and brain. In such imaging, computed tomography (CT) is superior for osseous detail and is rapidly available, whereas magnetic resonance imaging (MRI) is superior for revealing anatomic features and soft tissue characterization.

Computed tomography is typically the mainstay of acute orbital imaging for trauma and infection. In imaging for trauma, no intravenous contrast material is needed; however, postcontrast imaging is helpful for identifying abscesses in the setting of infection. Because of the dose of radiation needed, it is not appropriate to perform imaging both before and after the administration of contrast material, and if contrast is needed, the osseous structures of the orbit and surrounding areas can still be evaluated without dedicated unenhanced imaging.

Magnetic resonance imaging is very helpful in the examination of inflammatory conditions and both neoplastic and non-neoplastic masses affecting the orbit, and it is also helpful in characterizing congenital abnormalities of the eyes. Imaging both without and with contrast is typically indicated in MRI of the orbits, and for many indications it is important to also perform a dedicated MRI of the brain.

21.2 Anatomy

The globe of the eye has several substructures that can be identified through careful analysis of imaging. Along the anterior aspect of the globe is the lens, and anterior to this is the anterior chamber of the eye (Fig. 21.1 and Fig. 21.2). The movement of the globe is controlled by six extraocular muscles, five of which extend posteriorly to the orbital apex in a conelike configuration (Fig. 21.1 and Fig. 21.2), while the inferior rectus muscle attaches to the anterior floor of the orbit. The virtual cone created by the extraocular muscles is used to describe the location of structures in and around the orbit, with the optic nerve, superior ophthalmic vein, and intraconal retrobulbar fat in the intraconal space, and with the extraconal space predominantly containing fat as well as the lacrimal gland (Fig. 21.1 and Fig. 21.2).

Fig. 21.1 Anatomy of the globe and orbit. (a) Axial computed tomographic image of the right orbit shows the depth of the anterior chamber (green line) measured to the anterior margin of the lens (green arrowhead), and the axial length of the globe (red line). The medial rectus (red arrow) and lateral rectus (red arrowhead) muscles are seen, as is the optic nerve (green arrow). (b) Coronal computed tomographic image shows the optic nerve in the center of the orbit (red arrowhead), and above it the superior ophthalmic vein (green arrowhead). The image shows the medial rectus (double red arrowheads) and lateral rectus muscles (red arrow), inferior rectus muscle (double red arrows), and superior rectus/levator muscle complex (double green arrows). The superior oblique muscle is also seen (green arrow), but the inferior oblique muscle is seen only at the anterior aspect of the orbit because it does not extend posteriorly to the apex.
Fig. 21.2 Magnetic resonance image of orbital anatomy. Axial image made with fast imaging employing steady-state acquisition shows the iris (red arrow), which forms the margins of the pupil (red arrowhead) along the anterior aspect of the lens (green arrowhead). Along the posterior retina, the head of the optic nerve (green arrow) is seen at the insertion of the optic nerve (blue arrowhead). The optic nerve is surrounded by cerebrospinal fluid and bounded by a layer of dura known as the optic nerve sheath (blue arrow). The medial (white arrowhead) and lateral (white arrow) rectus muscles are seen.

21.3 Size and Position of the Globe

The size of the globe can be measured from the anterior aspect of the cornea to the position of the insertion of the optic nerve (Fig. 21.1a and Fig. 21.2), a measurement known as the axial length of the globe. The globe increases in size during infancy. In the setting of microcephaly, the appearance of the globes in relation to that of the head can suggest enlargement of the globes, and exact measurements can therefore help determine whether they are of appropriate size. The axial length of globe is approximately 18 mm in infancy, and reaches an adult size of approximately 24 mm over the first decade of life.

A measurement from the anterior aspect of the cornea to the lens provides the depth of the anterior chamber of the eye. This measurement can be inexact, especially because of slight obliquity of the lens, but typically ranges from 2 to 3.5 mm. A depth of the anterior chamber that exceeds 4 mm can be suggestive of glaucoma (Fig. 21.3). A shallow anterior chamber can be seen after a corneal laceration, and in some patients may be the only sign, on imaging, of an open globe (Fig. 21.4).

Fig. 21.3 Glaucoma. Axial T2 fat-saturated image of the anterior head of a 5-month-old female with congenital glaucoma shows a markedly increased depth of the anterior chamber (4.5 mm, red line), an increased axial length of the globe (23 mm, blue line), and a large width of the cornea (14 mm, green line).
Fig. 21.4 Vitreous hemorrhage. Axial computed tomographic image of the anterior head of a 16-year-old boy with orbital trauma shows vitreous hemorrhage and a shallow anterior chamber. The shallow anterior chamber is an indicator of an open globe injury.

The midpole of the globe should be at, or slightly posterior to, a line connecting the medial and lateral points on the orbital rim (Fig. 21.5). Projection of the globe anterior to this line represents proptosis, which can be seen with a retrobulbar space-occupying lesion, including a tumor, hematoma, or muscle hypertrophy in thyroid eye disease.

Fig. 21.5 Proptosis/retrobulbar hematoma. Axial computed tomographic image of the anterior head of a 13-year-old girl after trauma shows left-sided proptosis, with the midglobe extending beyond a line drawn from the medial to the lateral orbital rim (blue line).

The posterior contour of the globe should conform to that of a circle. Any outward deviation raises concern about a possible coloboma or morning glory disc anomaly (MGDA), which is discussed below (Fig. 21.6). Inward projection along the posterior globe is suggestive of papilledema, and warrants funduscopic examination and a workup for possible causes of increased intracranial pressures (Fig. 21.7).

Fig. 21.6 Morning glory disc anomaly (MGDA). Axial T2W image of the anterior head of an 8-year-old girl shows a focal protrusion (blue arrowhead) of the left globe at the location of the insertion of the head of the optic nerve, consistent with an MGDA.
Fig. 21.7 Papilledema. Axial T2W image of the anterior head of a 16-year-old girl with severe headaches and pseudotumor cerebri shows elevation of the optic nerve head (blue arrow), representing the correlate on magnetic resonance imaging of elevation/papilledema of the head of the optic nerve. There is also prominence of cerebrospinal fluid within the optic nerve sheaths (blue arrowhead), which with papilledema is suggestive of elevated intracranial pressure.

21.4 Orbital Region Infections

Infections in the region of the orbit typically originate in two locations, either the facial soft tissues or the ethmoid sinuses. Ethmoid sinus disease has the potential for extending through the lamina papyracea and into the medial extraconal fat. This is often accompanied by dehiscence/demineralization of the lamina papyracea, but it can be unclear whether the extension of the disease to the orbit is due to such demineralization or occurred through venous channels and then secondarily resulted in dehiscence. In either case, the infection can threaten vision after it reaches the orbit, and an abscess in the orbit represents a surgical emergency. Because this constitutes an infection within the orbit, it is referred to as an orbital infection, in the form of either orbital cellulitis or an orbital abscess. The anterior orbit has a fibrous septum that extends from the globe to the margins of the orbit, and any infection posterior to this septum is referred to as a postseptal process. Because of this, orbital cellulitis and postseptal cellulitis are synonymous terms for the same condition.

A preseptal cellulitis is one that involves the face and periorbital region (and is therefore also known as periorbital cellulitis), without extension posterior to the septum (Fig. 21.8).

Fig. 21.8 Preseptal cellulitis. (a) Axial post contrast computed tomographic image of the head of a 10-year-old boy with periorbital erythema and swelling shows subcutaneous stranding (red arrows) but no fluid collection. (b) Sagittal post contrast CT image shows facial subcutaneous soft tissue stranding (red arrow) extending to involve the lower (blue arrow) and upper (blue arrowhead) eyelids; however, there is no stranding in the postseptal fat (red arrowheads), which resembles subcutaneous fat in uninvolved areas of the face (green arrows). This represents preseptal (periorbital) cellulitis without postseptal extension and without abscess.

All infectious processes in the orbital region carry the risk of inducing a septic thrombophlebitis in the veins of the face and orbit (Fig. 21.9), and accordingly it is imperative to evaluate the caliber and patency of the superior ophthalmic vein and cavernous sinus in all studies of the orbit in which there is concern about infection. Following is a checklist for infectious processes affecting the orbit:

Fig. 21.9 Cavernous sinus thrombosis. Axial computed tomographic plus contrast image in an 11-year-old with eye pain and fever shows a normal right superior ophthalmic vein (blue arrowhead). There is an enlarged left superior ophthalmic vein (red arrowhead) that shows no central enhancement, and there is relative non-enhancement of the left cavernous sinus (red arrow). This represents left cavernous sinus and superior ophthalmic vein thrombosis.

  1. Preseptal or postseptal?

  2. Cellulitis or abscess?

  3. Any osseous demineralization?

  4. Involvement of the ethmoid sinuses?

  5. Patency of the superior ophthalmic vein and/or cavernous sinus?

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May 28, 2020 | Posted by in NEUROLOGICAL IMAGING | Comments Off on 21 Orbits

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