Ординатура / Офтальмология / Английские материалы / Pediatric Ophthalmology for Primary Care 3rd edition_Wright, Farzavandi_2008

Vitreous

The vitreous is a transparent gel that fills the posterior chamber. Collagen type 2 is the major structural protein. The vitreous adheres most to the retina around the optic nerve and to the peripheral retina close to the ciliary body. The composition of the vitreous gel changes during the aging process. As a person ages, the gel becomes liquefied and loses its “jellylike” consis tency. Parts of the vitreous may break away and float around in the posterior aspect of the eye. Floaters are fairly common and do not necessarily indicate a retinal problem unless the patient also experiences the sensation of flash ing lights.

Fundus

The retina is a highly organized structure consisting of alternating layers of neuron cell bodies and synaptic processes (Figure 1 7). The outer layer of the retina (layer closest to the sclera) consists of photoreceptors that are responsible for changing light energy into neuronal activity. There are 2 types of photoreceptors: rods, which are responsible for vision under dim illumination, and cones, which are responsible for fine, high resolution, and color vision. The ends of the rods and cones interdigitate with a basal single cell layer called the retinal pigment epithelium (RPE). The RPE separates the retina from the vascular choroid. This single cell layer has tight junctions and apical microvilli that extend around the tips of the rods and cones. The RPE functions to maintain a blood retinal barrier, separat ing the retina from the choroid and choriocapillaris. The RPE selectively

transports nutrients from the choriocapillaris to the outer retina. This active transport process maintains appropriate retina hydration. Breakdown of the RPE barrier results in fluid exudates within the retina causing retinal edema and decreased vision. The RPE cells also function to rejuvenate the rods and cones by phagocytizing debris from the tips of these photoreceptors. Rods and cones synapse with bipolar cells that in turn synapse with ganglion cells.

Axons of the ganglion cells stream through the nerve fiber layer to exit the optic nerve. These same axons proceed uninterrupted to synapse with neu rons in the lateral geniculate nucleus of the brain. Processes that damage the optic nerve actually damage the axons and subsequently affect the neurons in the inner aspect of the retina.

Figure 1 7.

Schematic diagram of the cell types and histologic layers in the human retina. Also shown are the Bruch membrane and the edge of the vitreous. The basic relationship between rod

(R) and cone (C) photoreceptors as well as bipolar (B), horizontal (H), amacrine (Am), inner plexiform cell (I), and ganglion (G) neurons are depicted. The Muller cell (M) extends almost the entire width of the retina. Astrocytes (As) are found primarily in the nerve fiber layer (NFL). Modified from Dowling JE, Boycott BB. Proc R Soc Land (Biol). 1966;166:104. In: Blanks JC. Morphology of the retina. In: Ryan S, et al. Retina. 2nd ed. St Louis, MO: Mosby; 1994

The macula is the central aspect of the retina, located within the vascular arcades, and the fovea centralis is the small pinpoint reflex in the center of the macula (Figure 1 8). The macula and fovea are almost entirely popu lated by cones, whereas the peripheral retina is populated mostly by rods.

Figure 1 8.

Clinical fundus photograph showing normal optic disc, macula, and retina vessels. The macula is surrounded by the temporal retinal vascular arcade. The clinical macula is relatively ill-defined and represents the area inside the temporal arcade, which can be seen by the circular light reflex.

The fovea provides us with clear, central 20/20 vision. Loss of the fovea and macular retina results in legal blindness even though the peripheral retina is intact. The macula is an area of increased yellow pigmentation and is called the macula lutea (yellow spot). The macula can be visualized during direct ophthalmoscopy by having the patient look directly at the light. In the center of the macula, the fovea is identified as a small indentation and may be visu alized with a small light reflex.

The optic disc, which is the anterior aspect of the optic nerve, is located nasal to the macula. The optic disc is the exit point for the 1 million axons of the retinal ganglion cells that continue on to synapse with the lateral geniculate nucleus. There are no photoreceptors in the area of the optic disc. Therefore, the optic disc represents a blind spot of approximately

5 degrees in the visual field. The central aspect of the optic disc is excavated and is called the optic cup (Figure 1 9). The cup to disc ratio is the ratio of the diameter of the optic cup to the diameter of the optic disc. The normal cup to disc ratio is 0.3. A large cup may indicate glaucoma and increased intraocular pressure.

Figure 1 9.

Normal optic nerve (on face or ophthalmoscopic view on top and transverse view on bottom); note the central cup is actually a central indentation of the nerve at the point of exit of the retinal vessels. The lamina cribrosa is the structural divide between the optic nerve and the optic disc. RPE: retinal pigment epithelium.

Retinal Vessels

Retinal vessels (ie, arteries, veins) emanate from the center of the optic disc and divide into the superior and inferior temporal arcades and the superior and inferior nasal arcades. The retinal vessels supply oxygen and nutrients to the inner layers of the retina while the choriocapillaris of the choroid supply the outer layers. Embryologic retinal vessel growth starts at the optic nerve and the retinal vessels grow out from the optic nerve into the periph eral retina. Fetal retinal growth is dependent on stimulation from vascular endothelial growth factor (VEGF). VEGF is up regulated in the fetus because of physiologic fetal hypoxia. If the fetus is born severely premature

and exposed to high oxygen, VEGF is down regulated and the retinal vessels stop growing. The vessels can actually undergo vaso obliteration and this is the first phase (not stage) of retinopathy of prematurity (ROP) (Pierce EA et al; Chow LC, Wright KW et al; Wright KW et al) (see Chapter 19 on ROP).

Extraocular Muscles

There are 6 extraocular muscles—4 rectus muscles and 2 oblique muscles (figures 1 1 and 1 10). The 4 rectus muscles (superior, inferior, medial, and lateral recti) originate at the orbital apex and extend anteriorly to insert on the back of the globe. Horizontal rectus muscles (ie, medial and lateral recti) have relatively simple functions and pull the eye in the direction of the contracting muscle (Figure 1 11). Adduction is movement toward the midline (to the nose) and abduction is movement away from the midline (toward the ear). The medial rectus muscle is an adductor so it rotates the eye in toward the nose, and the lateral rectus muscle, an abductor, rotates the eye out toward the ear. Note that the vertical rectus muscles (ie, supe rior and inferior rectus muscles) and the oblique muscles (ie, superior and inferior oblique muscles) have more than one function, as the muscle axis is different than the visual axis of the eye (Figure 1 10 and Table 1 2). Even so the major function of the vertical rectus muscles is vertical with the superior rectus muscle being an elevator and the inferior rectus muscle a depressor. Figure 1 11 shows the field of action of the extraocular muscles.

Table 1-2. Extraocular Muscles

Figure 1 11.

Field of action of the extraocular muscles. Diagram shows right eye, and the arrows point to the direction of gaze where a specific muscle is the major mover. To test a specific muscle, have the patient look in the direction of the arrow. SR, superior rectus; IO, inferior oblique; MR, medial rectus; SO, superior oblique; IR, inferior rectus; LR, lateral rectus.

The field of action is the gaze position where a specific muscle contributes most and is the major mover. For example, the superior oblique muscle is the major contributor in moving the eye down and in. Therefore, as noted in Figure 1 11, the arrow relating to the superior oblique points down and in.

Table 1 3 lists the agonist antagonist relationship of the muscles. Agonist antagonist muscles work against each other to maintain smooth eye movements; as one muscle contracts (agonist) the opposite muscle

(antagonist) is inhibited so it will relax. For example, when the medial rectus contracts to adduct the eye, the lateral rectus muscle is inhibited so it relaxes, allowing the eye to rotate in (Figure 1 12). For example, when the eye moves inward toward the nose (adduction), the medial rectus contracts and the lateral rectus is inhibited.

The superior oblique muscle originates at the orbital apex and courses supranasally to enter the trochlea and then reflects posteriorly to insert under the superior rectus onto the globe. The functional origin of the

Table 1-3. Monocular Movements

Agonist Antagonist

Figure 1 12.

Diagram shows lateral rectus muscle (LR) and medial rectus muscle (MR) connected to a electromyogram (EMG) that records electrical potential changes from muscle contracture. The diagram shows the eye adducting (turning to the nose) with the medial rectus contracting and lateral rectus muscle relaxing. Note that the EMG recordings shown in the shaded box show high electrical activity (lots of squiggly black lines) as the medial rectus muscle contracts, and little to no activity for the lateral rectus muscle as it relaxes.

superior oblique muscle is the trochlea, located in the superior nasal quad rant of the orbit. When the superior oblique muscle contracts, it pulls the back of the eye up and in, thereby depressing the front of the eye. The infe rior oblique muscle originates in the lacrimal fossa located in the inferior nasal quadrant of the anterior aspect of the orbit. The inferior oblique paral lels the course of the superior oblique tendon and inserts on the posterior temporal aspect of the globe. When the inferior oblique contracts, it pulls the back of the eye down and in, thus elevating and abducting the eye.

The third cranial nerve innervates the superior rectus, inferior rectus, medial rectus, and inferior oblique muscles. The lateral rectus is innervated by the sixth cranial nerve and the superior oblique is innervated by the fourth cranial nerve. A congenital paresis of the fourth cranial nerve is fairly common, and affected children present with torticollis (head tilt) to com pensate for the weak superior oblique muscle.