I
General
- 1.
Name the seven bones that make up the bony orbit and describe which location is most prone to damage in an orbital blow-out fracture.
The seven orbital bones are the frontal, zygoma, maxillary, sphenoid, ethmoid, palatine, and lacrimal. A true blow-out fracture most commonly affects the orbital floor posteriorly and medially to the infraorbital nerve. The ethmoid bone of the medial wall is often broken.
- 2.
Which nerves and vessels pass through the superior orbital fissure? Which motor nerve to the eye lies outside the annulus of Zinn, leaving it unaffected by retrobulbar injection of anesthetic?
The superior orbital fissure transmits the third, fourth, and sixth cranial nerves as well as the first division of the fifth cranial nerve, which has already divided into frontal and lacrimal branches. The superior ophthalmic vein and sympathetic nerves also pass through this fissure. The fourth cranial nerve, supplying the superior oblique muscle, lies outside the annulus. This position accounts for residual intorsion of the eye sometimes seen during retrobulbar anesthesia ( Fig. 1-1 ).
Figure 1-1
The annulus of Zinn and surrounding structures.
(From Campolattaro BN, Wang FM: Anatomy and physiology of the extraocular muscles and surrounding tissues. In Yanoff M, Duker JS [eds]: Ophthalmology, ed 2, St. Louis, Mosby, 2004.)
- 3.
A 3-year-old is referred for evaluation of consecutive exotropia after initial bimedial rectus recessions for esotropia performed elsewhere. Review of the operative notes discloses that each muscle was recessed 4.5 mm for a 30-prism diopter deviation. Unfortunately, the child had mild developmental delay and presents with a 25-prism diopter exotropia. You decide to advance the recessed medial rectus of each eye back to its original insertion site. Where is this site in relation to the limbus? Identify the location of each of the rectus muscle insertion sites relative to the limbus.
Reattach each medial rectus muscle 5.5 mm from the limbus. Insertion of the inferior rectus is 6.5 mm from the limbus, the lateral rectus is 6.9 mm from the limbus, and the superior rectus, 7.7 mm. The differing distances of rectus-muscle insertions from the limbus make up the spiral of Tillaux. An important caveat in developmentally delayed children is to postpone muscle surgery until much later, treating any amblyopia in the interim. Early surgery frequently leads to overcorrection.
- 4.
What is the most common cause of both unilateral and bilateral proptosis in adults?
Thyroid orbitopathy is the most common cause. Many signs are associated with thyroid eye disease, which is probably caused by an autoimmune reactivity toward the epitope of thyroid-stimulating hormone receptors in the thyroid and orbit. The order of frequency of extraocular muscle involvement in thyroid orbitopathy is as follows: inferior rectus, medial rectus, lateral rectus, superior rectus, and obliques. There is enlargement of the muscle belly with sparing of the tendons.
- 5.
You have just begun a ptosis procedure. A lid crease incision was made, and the orbital septum has been isolated and opened horizontally. What important landmark should be readily apparent? Describe its relation to other important structures.
The orbital fat lies directly behind the orbital septum and directly on the muscular portion of the levator ( Fig. 1-2 ). A separate medial fat pad often herniates through the septum in later years.
Figure 1-2
Schematic cross-section of eyelids and anterior orbit. A, Skin; B, frontalis muscle; C, orbicularis muscle (orbital portion); D, orbicularis muscle (preseptal portion); E, orbicularis muscle (pretarsal portion); F, orbicularis muscle (muscle of Riolan); G, orbital septum; H, orbital fat; I, superior transverse ligament; J, levator muscle; K, levator aponeurosis; L, Müller’s muscle; M, superior rectus muscle; N, superior oblique tendon; O, gland of Krause; P, gland of Wolfring; Q, conjunctiva; R, tarsus; S, inferior rectus muscle; T, inferior oblique muscle; U, inferior tarsal muscle; V, capsulopalpebral ascia; W, peripheral arterial arcade.
(From Beard C: Ptosis, ed 3, St. Louis, Mosby, 1981.)
- 6.
To what glands do the lymphatics of the orbit drain?
There are no lymphatic vessels or nodes within the orbit. Lymphatics from the conjunctivae and lids drain medially to the submandibular glands and laterally to the superficial preauricular nodes.
- 7.
What is the orbital septum?
The septum is a thin sheet of connective tissue that defines the anterior limit of the orbit. In the upper lid it extends from the periosteum of the superior orbital rim to insert at the levator aponeurosis, slightly above the superior tarsal border (see Fig. 1-2 ). The lower lid septum extends from the periosteum of the inferior orbital rim to insert directly on the inferior tarsal border.
- 8.
A 70-year-old patient presents with herpes zoster lesions in the trigeminal nerve distribution. Classic lesions on the side and tip of the nose increase your concern about ocular involvement. Why?
This sign, called Hutchinson’s sign, results from involvement of the infratrochlear nerve. The infratrochlear nerve is the terminal branch of the nasociliary nerve, which gives off the long ciliary nerves (usually two) that supply the globe.
- 9.
Where is the sclera the thinnest? Where are globe ruptures after blunt trauma most likely to occur?
The sclera is thinnest just behind the insertion of the rectus muscles (0.3 mm). Scleral rupture usually occurs opposite the site of impact and in an arc parallel to the limbus at the insertion of the rectus muscles or at the equator. The most common site of rupture is near the superonasal limbus.
- 10.
Describe the surgical limbus and Schwalbe’s line.
The surgical limbus can be differentiated into an anterior bluish zone that extends from the termination of Bowman’s layer to Schwalbe’s line, which is the termination of Descemet’s membrane. The posterior white zone overlies the trabecular meshwork and extends from the Schwalbe’s line to the scleral spur.
- 11.
You are preparing to do an argon laser trabeculoplasty. Describe the gonioscopic appearance of the anterior chamber angle.
The ciliary body is a visible concavity anterior to the iris root. The scleral spur appears as a white line anterior to the ciliary body. Above this are the trabecular meshwork and canal of Schlemm. Treatment is applied to the anterior trabecular meshwork.
- 12.
After a filtering procedure, your patient develops choroidal effusions. Explain the distribution of these fluid accumulations based on uveal attachments to the sclera.
The uveal tract is attached to the sclera at the scleral spur, the optic nerve, and the exit sites of the vortex veins. The fluid dissects the choroid from the underlying sclera but retains these connections.
- 13.
Describe the structure of Bruch’s membrane. Name two conditions in which defects develop in this structure spontaneously.
The Bruch’s membrane consists of five layers: internally, the basement membrane of the pigment epithelium, the inner collagenous zone, a central band of elastic fibers, and the outer collagenous zone; externally, the basement membrane of the choriocapillaris. Pseudoxanthoma elasticum and myopia may cause spontaneous defects in this membrane, making the patient prone to development of choroidal neovascularization.
Key Points: Bruch’s Membrane
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