Orbital diseases are distinct from primary ocular disorders in that they require consideration of a much larger group of differential diagnoses. Thus, ptosis may be attributable to a "simple" problem in the anterior segment, but may also be the clinical presentation of a more general disorder, such as Horner's syndrome, oculomotor paralysis, or myasthenia gravis. One must also consider orbital involvement in primary disorders of the periorbital structures, including the paranasal sinuses and the intracranial space.
Structures surrounding the orbit and the proportional distribution of structures within the orbit are schematically diagrammed in ■ Fig. 9.1. The close relationship of the orbital walls with the paranasal sinuses is particularly important (■ Fig. 9.2).
In company with the optic nerve, the optic canal also conducts the ophthalmic artery and the postganglionic sympathetic fibers that arise from the carotid plexus. All three of these structures feed through the annulus of Zinn, a surrounding ring of connective tissue that anchors the origin of all four rectus muscles (■ Fig. 9.3).
Superior ophthalmic vein
Fig. 9.3. Coronal plane section of the right orbital apex as seen from within the orbit and the various structures that pass through the optic canal, the superior orbital fissure, and the inferior orbital fissure. The optic nerve is at the center, emerging from the canal to pass through the annulus of Zinn. Superolateral to the canal lies the superior orbital fissure. Structures passing through the fissure (ordered from cranial to caudal) include the lacrimal nerve, the superior ophthalmic vein, a branch of the middle meningeal artery, the frontal nerve, and the trochlear nerve. Farther below, the oculomotor nerve, the nasociliary nerve, and sympathetic nerve fibers pass through the tendinous annulus of Zinn. The ophthalmic artery also passes through the canal and the annulus, along a course located inferior to the optic nerve (modified from Stewart; see Further Reading)
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