The orbit is the space in the skull in which the eye and extraocular muscles are located. It is cone-shaped, widest anteriorly and tapering to the orbital apex, through which the optic nerve passes. The orbits are demarcated by the bones of the skull (Figure 26–1). The roof consists of a portion of the frontal bone and the lesser wing of the sphenoid bone. The frontal lobes of the brain lie above this area. The medial orbital wall is composed of the lacrimal bone and portions of the ethmoid, maxilla, and sphenoid bones. The ethmoid sinuses are adjacent to this wall. The floor of the orbit is composed of the maxilla and a portion of the zygomatic bone. It lies above the maxillary sinus. The lateral wall is formed by the zygomatic bone and the greater wing of the sphenoid.

Openings in the orbit allow for passage of other structures (Figure 26–2). The optic nerve, ophthalmic artery, and sympathetic fibers pass through the optic foramen posteriorly at the apex of the orbit. Cranial nerves III, IV, and VI, and portions of cranial nerve V travel through the superior orbital fissure. The infraorbital nerve (a branch of cranial nerve V) is transmitted through the infraorbital fissure.

All of the extraocular muscles except the inferior oblique muscle arise at the annulus of Zinn at the apex of the orbit and extend anteriorly to insert on the globe. A layer of connective tissue joins these muscles to form a cone-shaped space. Orbital disorders may be classified as intraconal (within this group of muscles) or extraconal (outside of the muscle cone but still within the orbit). The orbital septum lies beneath the orbicularis muscle, and separates the orbit into the preseptal and postseptal spaces. The lacrimal gland is located in the lacrimal fossa in the superolateral portion of the orbit.

Embryology

Most of the orbital structures arise from the frontonasal and maxillary processes of neural crest cells that surround the optic cups. The various bones that comprise the orbital walls usually fuse during the sixth month of gestation.

Measurements between various eyelid and ocular structures may be used to describe some orbital disorders. Hypotelorism is a smaller-than-normal distance between the medial orbital walls. Hypertelorism is a greater-than-normal distance between the orbits. Telecanthus is a greater-than-normal distance between the medial canthi. In telecanthus, the orbits themselves are often normal (Figure 26–3).

Orbital disorders are uncommon in children. The most frequent abnormalities are discussed in this chapter. There are a large number of rare entities that can affect the orbit, which are beyond the scope of this text.

Orbital disorders may arise from a variety of conditions, including congenital anomalies, trauma, infections, tumors, vascular and lymphatic malformations, and inflammatory conditions. Because the orbit is demarcated by bony structures, space-occupying lesions can create problems either by causing forward displacement of the eyeball (proptosis) or by compression of vital structures, most importantly the optic nerve.

Orbital problems are most commonly identified by proptosis, in which the globe is displaced anteriorly and appears to bulge from the socket. As the eyeball moves forward, the eyelids are displaced, which can give the clinical appearance of eyelid retraction, in which the sclera is visible between the upper and lower eyelid and the cornea. In some patients the asymmetric appearance of the eyes may be mistakenly interpreted as ptosis of the normal eye. In some conditions, particularly traumatic fractures, the eye may be displaced posteriorly (enophthalmos) and appear sunken-in. In inflammatory and infectious processes, the periocular area may become erythematous and edematous. The presence of pain is variable, and this symptom may be helpful in establishing a diagnosis.

If proptosis progresses, patients develop signs and symptoms of corneal exposure due to incomplete coverage of the eye by the stretched eyelids. These symptoms include ocular redness, light sensitivity, and foreign body sensation. If the extraocular muscles are involved, patients develop strabismus and diplopia. If the optic nerve is compressed, vision may decrease.

Craniofacial Malformations

Many craniofacial malformations have associated orbital findings. In most, the orbit is shallow and the eye appears protuberant. Disorders secondary to craniosynostosis include Crouzon, Apert, Pfeiffer, and Saethre-Chotzen syndromes. The most common ocular problems associated with craniosynostosis syndromes are corneal exposure and strabismus (Figure 26–4A and B). Compression of the optic nerve may also occur.

Anophthalmos and Microphthalmos

In these disorders the eye is underdeveloped (microphthalmos) or absent (anophthalmos). True anophthalmos is very rare. In most cases at least some vestige of ocular tissue is present. Microphthalmos may be associated with an orbital cyst. These cysts may be the same size or larger than a normal eye. Because normal orbital growth depends on the presence of an eye, the orbital bones are often underdeveloped in patients with microphthalmos and anophthalmos. Treatment with ocular prostheses or tissue expanders may be used to enlarge the hypoplastic orbit (Figure 26–5).1

Encephalocele

Encephaloceles may present in infancy as a bulge medial to the eye (Figure 26–6). These result from bony defects within the skull that allow protrusion of intracranial material. They may superficially appear similar to infantile mucoceles. Encephaloceles can be distinguished from mucoceles by their location above the medial canthus, noninflamed appearance, and frequent presence of visible pulsations within the lesion (due to transmission of intracranial pressure).

Blunt Orbital Trauma

Blunt trauma to the eye may cause fractures of the orbital bones. Major trauma, such as a motor vehicle accident, can cause multiple fractures. The most common injuries result from direct trauma to the ocular area. The bony rims of the orbit extend beyond the eyeball itself, and therefore the globe is relatively protected from blunt injuries with large objects. Smaller objects, such as fists or small balls, may impact the globe directly. Transmission of the force from such an impact, or by direct trauma to the inferior orbital rim, may cause a blowout fracture, in which the floor of the orbit is fractured. This may cause herniation of the extraocular muscles or the globe through the fracture.

Presentation of Blunt Orbital Trauma

Patients with orbital trauma usually present with periocular edema and ecchymosis. Specific clinical signs of blowout fracture include enophthalmos (the eye appears sunken in the orbit) and strabismus. Strabismus most commonly results from restriction of the inferior rectus muscle, which limits elevation of the eye (Figure 26–7A and B).

Extraocular muscles that have limited movement can be assessed with forced ductions, in which the eye is grasped with forceps (after instillation of a topical anesthetic) and moved by the examiner. An inability to move the eye suggests entrapment. In the early posttraumatic period, however, edema of the muscle may also cause some limitation of forced ductions. If the eye moves freely with forced ductions, but does not function normally, then the movement abnormality is likely due to damage to the nerve that supplies the muscle. A diagnosis of entrapment can be confirmed by imaging studies, most commonly computed tomography (Figure 26–8). Children with orbital trauma should be examined for associated ocular injuries.

Treatment of Blunt Orbital Trauma

Treatment of small, nondisplaced fractures is usually not necessary. Surgical repair is indicated if there is radiographic evidence of entrapment and significant enophthalmos or strabismus. In most cases surgery is delayed 1 to 2 weeks to allow edema to decrease and to determine whether the strabismus improves (which can occur due to reduction of muscle edema or hemorrhage).

An important exception requiring early treatment in children is the white-eyed blowout fracture, in which the inferior rectus muscle becomes entrapped in the orbital floor fracture. These patients have only mild periocular signs of trauma. They are unable to elevate their eyes. They often experience nausea or bradycardia when they attempt to do so, due to the oculocardiac reflex. Urgent repair is indicated in patients with white-eyed blowout fractures due to the risk of muscle necrosis.2