Chapter 9 Ocular Adnexa and Lacrimal System
Eyebrows
The eyebrows consist of thick skin covered by characteristic short, prominent hairs extending across the superior orbital margin, usually arching slightly but sometimes merely running horizontally. Generally, in men the brows run along the orbital margin, whereas in women the brows run above the margin.1 The first body hairs produced during embryologic development are those of the eyebrow.2
The muscles located in the forehead—the frontalis, procerus, corrugator superciliaris, and orbicularis oculi—produce eyebrow movements, an important element in facial expression (Figure 9-1). The frontalis muscle originates high on the scalp and inserts into connective tissue near the superior orbital rim. The fibers are oriented vertically and raise the eyebrow, causing a look of surprise or attention. The corrugator originates on the frontal bone and inserts into skin superior to the medial eyebrow. It is characterized as the muscle of trouble or concentration, and its fibers are oriented obliquely; it moves the brow medially, toward the nose, creating vertical furrows between the brows. The procerus, the muscle of menace or aggression, originates on the nasal bone and inserts into the medial side of the frontalis. It pulls the medial portion of the eyebrow inferiorly and produces horizontal furrows over the bridge of the nose.2,3 The orbicularis oculi (described later) lowers the entire brow. The fibers of these muscles blend with one another and are difficult to separate.2 All are innervated by the facial nerve—cranial nerve VII.
Eyelids
Clinical Comment: Lagophthalmos
LAGOPHTHALMOS refers to incomplete closure of the eyelids (Figure 9-2). Its cause may be physiologic, mechanical (e.g., scarring), or paralytic. Lagophthalmos is most evident during sleep, when drying of the inferior cornea may result. Scratchy, irritated eyes are evident on awakening, and punctate keratitis can occur.4–6 Clinical assessment of the inferior cornea will show varying degrees of epithelial disruption, manifested as staining with fluorescein dye.
Palpebral Fissure
The palpebral fissure is the area between the open eyelids. Although numerous variations exist in the positional relationship of the lid margins to the limbus, generally the upper lid just covers the superior limbus when one’s eyes are open and looking straight ahead. The lower lid position is more variable, usually lying within 1 mm of the inferior limbus.7–9
The upper and lower eyelids meet at the corners of the palpebral fissure in the lateral and medial canthi. The lateral canthus is located approximately 5 to 7 mm medial to the bony orbital margin and lies directly on the globe.9 The medial canthus is at the medial orbital margin but is separated from the globe by a reservoir for the pooling of tears, the lacrimal lake. The floor of the lacrimal lake is the plica semilunaris (Figure 9-3). This narrow, crescent-shaped fold of conjunctiva, located in the medial canthus, allows for lateral movement of the eye without stretching the bulbar conjunctiva. The caruncle is a small, pink mass of modified skin located just medial to the plica. It is covered with epithelium that contains goblet cells and fine hairs and their associated sweat and sebaceous glands.
Eyelid Topography
The upper eyelid extends to the eyebrow and is divided into the tarsal and the orbital (or preseptal) parts. The tarsal portion lies closest to the lid margin, rests on the globe, and contains the tarsal plate. The skin is thin, and the underlying loose connective tissue is devoid of adipose tissue. The orbital portion extends from the tarsus to the eyebrow, and a furrow—the superior palpebral sulcus—separates the tarsal portion from the orbital portion (Figure 9-4). This sulcus separates the pretarsal skin, which is tightly adherent to the underlying tissue, from the preseptal skin, which is only loosely adherent to its underlying tissue, which contains a cushion of fat. In the eyelids of those of Eastern Asian descent, the orbital septum fuses with the levator aponeurosis below the upper tarsal border, allowing the fat to descend further into the lid10 and eliminating the superior palpebral sulcus.2,11–13
In the lower eyelid the inferior palpebral sulcus, which separates the lower lid into tarsal and orbital parts, is often not very distinct. The tarsal portion rests against the globe, and the orbital portion extends from the lower border of the tarsus onto the cheek, extending just past the inferior orbital margin to the nasojugal and malar sulci (see Figure 9-4). These furrows occur at the attachment of the skin to the underlying connective tissue and become more prominent with age.
Eyelid Margin
The eyelid margin rests against the globe and contains the eyelashes and the pores of the meibomian glands. The cilia (eyelashes) are arranged at the lid margin in a double or triple row, with approximately 150 in the upper eyelid and 75 in the lower lid.14 The lashes curl upward on the upper and downward on the lower lid. Replacement lashes grow to full size in approximately 10 weeks, and each lash is replaced approximately every 5 months.9 The eyelashes are richly supplied with nerves, causing them to be sensitive to even the slightest unexpected touch, which will elicit a protective response—a blink.
Clinical Comment: Abnormalities Affecting the Cilia
Various epithelial diseases can cause madarosis (loss of eyelashes) or trichiasis (misdirected growth of eyelashes), in which the eyelashes grow toward rather than away from the palpebral fissure. Contact with the cornea can cause irritation and painful abrasions and can lead to ulceration.4 The problem lashes can be removed by epilation.
The pores of the meibomian glands are located posterior to the cilia (see Figure 9-3), and the transition from skin to conjunctiva, the mucocutaneous junction, occurs just posterior to these openings.15 A groove called the gray line runs along the eyelid margin between the cilia insertions and the pores of the meibomian glands. This groove is the location of a surgical plane that divides the lid into anterior and posterior portions.2
The eyelid margin can be divided into two parts: the medial one sixth is the lacrimal portion, and the lateral five sixths is the ciliary portion. The division occurs at the lacrimal papilla, a small elevation containing the lacrimal punctum, the opening that carries the tears into the nasolacrimal drainage system (see Figure 9-3). Usually, no cilia or meibomian pores are found medial to the punctum, in the lacrimal portion of the lid margin.
Clinical Comment: Epicanthus
EPICANTHUS is a vertical fold of skin at the nasal canthus, arising in the medial area of the upper eyelid and terminating in the nasal canthal area. It is common in the newborn and may cause the appearance of esotropia (Figure 9-5). A parent of an infant with epicanthus might worry that the child’s eyes are crossed; however, a cover test will identify a true esotropia. As the bridge of the nose develops, epicanthus gradually disappears. A form of epicanthus arising from the tarsal fold and extending into the medial canthal area is common in those of Eastern Asian descent.2
Eyelid Structures
Orbicularis Oculi Muscle
Palpebral Portion
The palpebral portion of the orbicularis oculi muscle occupies the area of the eyelid that rests on the globe and is closest to the eyelid margin. It sometimes is divided further into pretarsal and preseptal parts. The palpebral portion is composed of semicircles of muscle fibers that run from the medial orbital margin and the medial palpebral ligament16 to the lateral palpebral raphe, where the superior and inferior fibers interdigitate with one another (see Figure 9-1). The lateral palpebral raphe overlies the lateral palpebral ligament.
Some fibers arise from deeper attachments on the posterior lacrimal crest. This section of the palpebral part of the orbicularis, the muscle of Horner or lacrimal part (pars lacrimalis), encircles the lacrimal canaliculi.9,11,16,17 Contraction of the orbicularis assists in moving tears through the canaliculi into the nasolacrimal drainage system.18 Another section of the palpebral orbicularis, the muscle of Riolan or ciliary part (pars ciliaris), lies near the lid margin on both sides of the meibomian gland openings; it maintains the lid margins close to the globe.19
Clinical Comment: Ectropion and Entropion
Eversion of the eyelid margin is called ectropion (Figure 9-6), the common cause of which is loss of muscle tone, a normal occurrence in the aging process. As the lid margin falls away from its position against the globe, the lacrimal punctum is no longer in position to drain the tears from the lacrimal lake. Epiphora, an overflow of tears onto the cheek, may occur, causing maceration of the delicate skin in this area.
FIGURE 9-6 Severe involutional (senile) ectropion.
(From Kanski JJ: Clinical ophthalmology: a systematic approach, ed 5, Oxford, UK, 2003, Butterworth-Heinemann.)
Inversion of the lid margin is called entropion and may result from spasm of the orbicularis oculi muscle causing the lid margin to turn inward (Figure 9-7). This inward turning puts the eyelashes in contact with the globe and, unless relieved, can cause corneal abrasion. Scarring of the lid after trauma or disease may also cause entropion. Both ectropion and entropion are more common in the lower lid and can be corrected surgically, if necessary. The anatomic relationship of the muscular and connective tissue components is an important consideration when repair is done.20–22
Orbital Portion
The orbital portion of the orbicularis oculi muscle is attached superiorly to the orbital margin, medial to the supraorbital notch. The fibers encircle the area outer to the palpebral portion and attach inferiorly to the orbital margin, medial to the infraorbital foramen.2 These concentric circular fibers extend throughout the rest of the lid and over the orbital rim.
Orbicularis Action
The orbicularis oculi muscle is innervated by cranial nerve VII (the facial nerve). Contraction of the palpebral portion closes the eyelid gently, and the palpebral orbicularis is the muscle of action in an involuntary blink and a voluntary wink; relaxation of the levator muscle follows.23 Spontaneous involuntary blinking renews the precorneal tear film. A reflex blink is protective and may be elicited by a number of stimuli—a loud noise; corneal, conjunctival, or cilial touch; or the sudden approach of an object. When the orbital portion of the orbicularis contracts, the eye is closed tightly, and the areas surrounding the lids—the forehead, temple, and cheek—are involved in the contraction. Such eyelid closure is often a protective mechanism against ocular pain or after injury, and is called reflex blepharospasm. If the lids are closed tightly in a strong contraction, forces compressing the orbital contents can significantly increase the intraocular pressure.24
Superior Palpebral Levator Muscle
The superior palpebral levator muscle, the retractor of the upper eyelid, is located within the orbit above the globe and extends into the upper lid. It originates on the lesser wing of the sphenoid bone above and in front of the optic foramen, and its sheath blends with the sheath of the superior rectus muscle. As the levator approaches the eyelid from its posterior origin at the orbital apex, a ligament, the superior transverse ligament (Whitnall’s ligament) may act as a fulcrum, changing the anteroposterior direction of the levator to superoinferior10,12,25–28 (Figure 9-8). The superior transverse ligament is a fibrous band that spans the anterior superior orbit from the trochlea to the lacrimal gland fascia. It provides support for the upper lid and orbital structures as well as acting as a fulcrum. The ligament is located at the point where the levator muscle fibers end and the aponeurosis begins.29
Levator Aponeurosis
As it enters the eyelid, the levator becomes a fan-shaped tendinous expansion, the levator aponeurosis. Unlike a typical tendon, the aponeurosis spreads out into an extensive sheet posterior to the orbital septum. The fibers of the aponeurosis penetrate the orbital septum and extend into the upper lid, fanning out across its entire width. These tendinous fibers pass through the submuscular connective tissue; the posterior fibers insert into the lower anterior surface of the tarsal plate, and the anterior fibers run between the muscle bundles of the orbicularis to insert primarily into the skin of the eyelid, although some insert into the intermuscular septa of the orbicularis9,11,25–28 (see Figure 9-8). This attachment of the fibers from the levator aponeurosis anchors the skin to the underlying tissues in the pretarsal area of the eyelid and creates the palpebral sulcus. In those of Eastern Asian descent, the orbital septum attaches to the tarsal plate more inferiorly, and the aponeurotic fibers do not attach as extensively to the cutaneous tissue.2,11
The two side extensions of the aponeurosis are referred to as horns. The lateral horn helps to support the lacrimal gland by holding it against the orbital roof, dividing the gland into orbital and palpebral lobes (Figure 9-9). The lateral horn then attaches to the lateral palpebral ligament and lateral orbital tubercle. The medial horn is attached to the medial palpebral ligament and medial orbital rim.
Levator Action
Contraction of the levator muscle causes elevation of the eyelid. The connection between the sheath of the levator and sheath of the superior rectus coordinates eyelid position with eye position so that as the eye is elevated, the lid is raised.27,28 The levator is innervated by the superior division of the oculomotor nerve, cranial nerve III.
The eyelids are closed by relaxation of the levator and contraction of the orbicularis oculi muscles. The tonic activity of the levator and the relaxation of the orbicularis holds the eyelid open. In a blink, tonic activity of the levator is suspended, and with a burst of activity the orbicularis rapidly lowers the lid, followed by a cessation of orbicularis activity and resumption of levator tonicity.30
Retractor of Lower Eyelid
The retractor of the lower lid is the capsulopalpebral fascia (lower eyelid aponeurosis).2,10 The capsulopalpebral fascia, an anterior extension from the sheath of the inferior rectus muscle and the suspensory ligament, inserts into the inferior edge of the tarsal plate.11,27,28 This insertion coordinates lid position with globe movement. The lower eyelid is depressed on globe depression, and the lower eyelid elevates slightly on upward movement of the globe.8 The capsulopalpebral fascia also fuses with the orbital septum and sends some fibers to insert into the inferior fornix.2,21,28
Tarsal Muscle (of Müller)
The superior tarsal muscle (muscle of Müller) is composed of smooth muscle and originates on the posteroinferior aspect of the levator muscle. These smooth muscle fibers begin to appear within the striated muscle at the point at which the muscle becomes aponeurotic.2,9,27,28 The superior tarsal muscle inserts on the superior edge of the tarsal plate (see Figures 9-8 and 9-9). Contraction of Müller’s muscle can provide 2 mm of additional lid elevation.10
A similar smooth muscle, the inferior tarsal muscle, is found in the lower eyelid. It arises from the inferior rectus muscle sheath and inserts into the lower conjunctiva and lower border of the tarsal plate.2,9,24 Investigators disagree about whether the inferior tarsal muscle actually inserts into the tarsal plate2,9,27 or inserts into the tissue below the tarsal plate.31 Both tarsal muscles are innervated by sympathetic fibers that widen the palpebral fissure when activated (as in situations associated with fear or surprise).9,28
Clinical Comment: Ptosis
PTOSIS is a condition in which the upper eyelid droops or sags. It can be caused by weakness or paralysis of the levator or Müller’s muscle. If Müller’s muscle alone is affected, a less noticeable form of ptosis occurs than when the levator is involved.32 An individual with ptosis might attempt to raise the lid by using the frontalis muscle, which results in elevation of the eyebrow and wrinkling of the forehead (Figure 9-10).
Tarsal Plate
Each eyelid contains a tarsal plate (tarsus) that gives the lid rigidity and structure and shapes it to the curvature of the globe. The tarsal plate in the upper lid is approximately 11 mm high, and the inferior tarsal plate is approximately 5 mm high.9 The anterior surface is adjacent to the submuscular connective tissue. The posterior surface is adherent to the palpebral conjunctiva. The orbital border of the tarsus is attached to the orbital septum, whereas the marginal border lies at the lid margin. The sides of the tarsal plates are attached to the bony orbital margin by the palpebral or tarsal ligaments (see Figure 9-9).
Palpebral Ligaments
The palpebral or tarsal ligaments are bands of dense connective tissue connecting the tarsal plates to the orbital rim and holding the tarsal plates in position against the globe during eye and lid movements. The medial palpebral ligament runs from the medial edge of each tarsal plate to the medial orbital rim, where it divides into two limbs. One limb attaches to the posterior lacrimal crest and the other to the anterior lacrimal crest. Both limbs lie anterior to the orbital septum33 (see Figure 8-18).
The lateral palpebral ligament is located posterior to the orbital septum and attaches the lateral edges of the tarsal plates to the lateral orbital margin at the lateral orbital tubercle34 (see Figure 8-18). Fibrous connections between the lateral palpebral ligament and the check ligament for the lateral rectus muscle allow a slight lateral displacement of the lateral canthus with extreme abduction.35
The upper borders of both the medial and the lateral ligaments are joined to the expansion of the levator tendon, and their lower borders are joined to an expansion of the ligament of Lockwood.9 The connective tissue structures at the canthi have been described as a retinaculum that is made up of the palpebral ligaments, the horns of the levator aponeurosis, the suspensory ligament, the check ligaments, and the superior transverse ligament.2
Glands of the Lids
The meibomian glands (tarsal glands) are sebaceous glands embedded in the tarsal plate. These long, multilobed glands resemble a large bunch of grapes and are arranged vertically such that their openings are located in a row along the lid margin posterior to the cilia (Figure 9-11). Approximately 30 to 40 meibomian glands are found in the upper lid and 20 to 30 in the lower lid.14 On eyelid eversion the vertical rows of the meibomian glands can sometimes be seen as yellow streaks through the palpebral conjunctiva. These glands secrete the outer lipid layer of the tear film.
Clinical Comment: Contact Lens Wear
Some studies have identified a loss in both the number and the length of meibomian glands in the contact lens wearer (Figure 9-12). Loss does not appear to be dependent on the type of lens but rather on the duration of wear and is speculated to be due to chronic irritation.36
The sebaceous glands of Zeis secrete sebum into the hair follicle of the cilia, coating the eyelash shaft to keep it from becoming brittle.9
The glands of Moll have been called modified sweat glands but are more accurately described as specialized apocrine glands.37 They are located near the lid margin and their ducts empty into the hair follicle, into the Zeis gland duct, or directly onto the lid margin. Similar glands found in the axillae are scent organs, but that is likely not the function of the Moll glands.9,14
The accessory lacrimal glands of Krause are located in the stroma of the conjunctival fornix, and the accessory lacrimal glands of Wolfring are located along the orbital border of the tarsal plate2,9 (see Figure 9-11). These glands are oval and display numerous acini. In the upper fornix, 20 to 40 glands of Krause are found, although only six to eight such glands appear in the lower fornix.2 The glands of Wolfring are less numerous. The secretion of the accessory lacrimal glands appears similar to that of the main lacrimal gland and contributes to the aqueous layer of the tear film.
Histologic Features
Skin
The skin of the eyelid contains many fine hairs, sebaceous glands, and sweat glands. It is the thinnest skin in the body, easily forms folds and wrinkles, and is almost transparent in the very young.2 The epidermal layer consists of a basal germinal layer, a granular layer, and a superficial layer that is cornified. The underlying dermis is abundant in elastic fibers. A very sparse areolar connective tissue layer, the subcutaneous tissue, lies below the dermis. This thin layer is devoid of adipose tissue in the tarsal portion. A pad of fat often is located in this region in the orbital portion that separates the orbicularis from the skin.9
Muscles
The orbicularis oculi lies deep to the subcutaneous layer. These striated muscle bundles run throughout the eyelid. In a sagittal section of the lid prepared for microscopic examination, the orbicularis bundles are cut in cross section (Figure 9-13). Along the lid margin, small muscle bundles located on both sides of the meibomian gland represent a specific part of the orbicularis, the ciliary part (Riolan’s muscle), which holds the lid margin against the globe (see Figure 9-11).
Posterior to the orbicularis lies another layer of loose connective tissue, the submuscular areolar layer, which separates the muscle from the tarsal plate. Between this layer and the tarsal plate is a potential space, the pretarsal space, that contains the vessels of the palpebral arcades. The preseptal space is located between the orbicularis and the orbital septum; directly above is the preseptal cushion of fat.9
Tendinous fibers of the levator aponeurosis run through the submuscular tissue layer between the orbicularis and the superior tarsal muscle to insert into the tarsal plate and the skin of the lid (see Figure 9-11). It is this insertion of fibers that anchors the skin so firmly in the tarsal portion of the lid. There is no such attachment of the aponeurosis in the preseptal area. The smooth muscle fibers of the superior tarsal muscle are located above the superior tarsal plate and insert into its upper edge. Both the aponeurosis and the superior tarsal muscle are cut longitudinally in a microscope slide of a sagittal section of the lid.
