Kenneth B. Gum

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).

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 thyroidstimulating 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.

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.

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2  OPHTHALMOLOGY SECRETS IN COLOR

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.)

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.

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B

A

C

H

J I

M N K G D

O K

L L

Q P

W

R P

E

Q

Q F

R

S

W

T H U V G D

F

C

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.)

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: BR UCH’S MEMBRANE

1. Composed of five layers.

2. Spontaneous breaks can occur in pseudoxanthoma elasticum and myopia.

3. Defect in Bruch’s membrane in age-related macular degeneration may lead to the exudative form. 4. Trauma may cause a break in the membrane, leading to a choroidal neovascular membrane.

4  OPHTHALMOLOGY SECRETS IN COLOR

14.Less laser power is required for photocoagulation in darkly pigmented fundi. What determines this pigmentation?

The pigmentation of the fundus seen ophthalmoscopically is largely determined by the number of melanosomes in the choroid. The darker macular area results from taller pigment epithelial cells that contain more and larger melanosomes than the periphery.

15.What is the blood–retinal barrier?

The inner blood–retinal barrier consists of the retinal vascular endothelium, which is nonfenestrated and contains tight junctions. The outer blood–retinal barrier is the retinal pigment epithelium. Bruch’s membrane is permeable to small molecules.

16.Name the 10 classically described anatomic layers of the retina and the cells that make up the retina.

The retina may be divided into 10 layers, starting just above the choroids and extending to the vitreous:

•Retinal pigment epithelium

•Outer segments of the photoreceptors

•External limiting membrane

•Outer nuclear layer

•Outer plexiform layer

•Inner nuclear layer

•Inner plexiform layer

•Ganglion cell layer

•Nerve fiber layer

•Internal limiting membrane

Within these layers lie the photoreceptors, horizontal cells, bipolar cells, amacrine cells, retinal interneurons, ganglion cells, and the glial cells of the retina, Müller cells.

17.Which retinal layer is referred to as the fiber layer of Henle in the macular region?

The outer plexiform layer, which is made up of connections between photoreceptor synaptic bodies and horizontal and bipolar cells, becomes thicker and more oblique in orientation as it deviates away from the fovea. At the fovea this layer becomes nearly parallel to the retinal surface and accounts for the radial, or star-shaped, patterns of exudate in the extracellular spaces under pathologic conditions causing vascular compromise, such as hypertension.

18.What are three clinically recognized remnants of the fetal hyaloid vasculature?

Mittendorf’s dot, Bergmeister’s papilla, and vascular loops (95% of which are arterial).

19.A patient presents with a central retinal artery occlusion and 20/20 visual acuity. How do you explain this finding?

Fifteen percent of people have a cilioretinal artery that supplies the macular region. Thirty percent of eyes have a cilioretinal artery supplying some portion of the retina. These are perfused by the choroidal vessels, which are fed by the ophthalmic artery and thus are not affected by central retinal artery circulation.

20.Where do branch retinal vein occlusions occur? Which quadrant of the retina is most commonly affected?

Branch retinal vein occlusions occur at arteriovenous crossings, most commonly where the vein lies posterior to the artery. The superotemporal quadrant is most often affected because of a higher number of arteriovenous crossings on average.

21.Discuss the organization of crossed and uncrossed fibers in the optic chiasm.

Inferonasal extramacular fibers cross in the anterior chiasm and bulge into the contralateral optic nerve (Willebrand’s knee). Superonasal extramacular fibers cross directly to the opposite optic tract. Macular fibers are located in the center of the optic nerve. Temporal macular fibers pass uncrossed through the chiasm, whereas nasal macular fibers cross posteriorly. However, in albinism, many temporal fibers also cross.

22.Describe the location of the visual cortex.

The visual cortex is situated along the superior and inferior lips of the calcarine fissure. This area is called the striate cortex because of the prominent band of geniculocalcarine fibers, termed the stria of Gennari after its discoverer.

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23.What is the most likely anatomic location of pathology associated with downbeat nystagmus?

Downbeat nystagmus is usually indicative of cervicomedullary structural disease. The most common causes are Arnold-Chiari malformation, stroke, multiple sclerosis, and platybasia. Any patient with this finding should have neuroimaging studies done.

24.A patient presents with a chief complaint of tearing and ocular irritation. As she dumps the plethora of eyedrops from her purse, she explains that she has seen seven different doctors and none has been able to help her. The exam shows mild inferior punctate keratopathy but a normal tear lake and normal Schirmer’s test. Of interest, she had blepharoplasty surgery 6 months previously. What is the diagnosis?

You are already patting yourself on the back as you ask if the irritation is worse in the morning or evening. She replies emphatically that it is much more severe upon awakening. You ask her to close her eyes gently and see 2 mm of lagophthalmos in each eye. This is a frequently overlooked cause of tearing in otherwise normal eyes.

25.During orbital surgery, a patient’s lacrimal gland is removed. Afterward, there is no evidence of tear deficiency. Why not?

Basal tear production is provided by the accessory lacrimal glands of Krause and Wolfring. Krause’s glands are located in the superior fornix, and the glands of Wolfring are located above the superior tarsal border. They are cytologically identical to the main lacrimal gland.

26.Describe the anatomy of the macula and fovea.

The macula is defined as the area of the posterior retina that contains xanthophyllic pigment and two or more layers of ganglion cells. It is centered approximately 4 mm temporal and 0.8 mm inferior to the center of the optic disc. The fovea is a central depression of the inner retinal surface and is approximately 1.5 mm in diameter.

27.Fluorescein angiography typically shows perfusion of the choroid and any cilioretinal arteries prior to visualization of the dye in the retinal circulation. Why?

Fluorescein enters the choroid via the short posterior ciliary arteries, which are branches of the ophthalmic artery. The central retinal artery, also a branch of the ophthalmic artery, provides a more circuitous route for the dye to travel, resulting in dye appearance in the retinal circulation 1 to 2 seconds later.

28.Explain why visual acuity in infants does not reach adult levels until approximately 6 months of age, based on retinal differentiation.

The differentiation of the macula is not complete until 4 to 6 months after birth. Ganglion cell nuclei are initially found directly over the foveola and gradually are displaced peripherally, leaving this area devoid of accessory neural elements and blood vessels as neural organization develops to adult levels by age 6 months. This delay in macular development is one factor in the inability of newborns to fixate, and improvement in visual activity parallels macular development.

29.A neonate presents with an opacification in her left cornea. What is the differential diagnosis?

Neonatal cloudy cornea usually falls into one of the following categories (which can easily be recalled by using the mnemonic STUMPED ): sclerocornea, trauma, ulcers, metabolic disorder, Peters’ anomaly, endothelial dystrophy, and dermoid.

30.Describe the innervation of the lens.

The lens is anatomically unique because it lacks innervation and vascularization. It depends entirely on the aqueous and vitreous humors for nourishment.

31.Describe the innervation of the cornea.

The long posterior ciliary nerves branch from the ophthalmic division of the trigeminal nerve and penetrate the cornea. Peripherally, 70 to 80 branches enter the cornea in conjunctival, episcleral, and scleral planes. They lose their myelin sheath 1 to 2 mm from the limbus. The network just posterior to the Bowman’s layer sends branches anteriorly into the epithelium.

32.What are the three layers of the tear film? Where do they originate?

•The mucoid layer coats the superficial corneal epithelial cells and creates a hydrophilic layer that allows for spontaneous, even distribution of the aqueous layer of the tear film. Mucin is secreted principally by the conjunctival goblet cells but also from the lacrimal gland.

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