Modern Ophthalmology (3 Volumes) LC Dutta, Nitin K Dutta
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DISORDERS OF THE EYELIDS

Anatomy of the EyelidsChapter 1

Bakulesh Khamar,
Mayuri Khamar,
Nitin Trivedi,
Usha H Vyas
 
DEVELOPMENT OF THE EYELIDS
Both ectodermal and endodermal tissue take part in the development of the lid. The lid fold starts at the 7th week of intrauterine life. The upper lid is formed from the lateral and medial aspect of the frontonasal process and lower lid from the maxillary process. The margins of the two lids unite with flimsy tissue by the 9th week of gestation. The two lids again separate at 5th month of gestation. Failure of separation results in cryptophthalmos, incomplete or abnormal separation results in ankyloblepharon or ankyloblepharon filiforme adnatum. Failure of union of the frontal and maxillary processes leads to coloboma of the lower lid and failure of union between lateral and medial frontonasal processes results in coloboma of the upper lid.
Two lids are the movable folds of skin which close the eyes for protection and rest. The upper lid is much more mobile, the lower lid being more or less static. The upper eyelid extends upward up to eyebrow whereas the lower one passes on to cheek without much demarcation except in the presence of malar and nasojugal folds.
Palpebral fissure is an almond shaped opening bounded by the two lids. In an adult, the average height and width of lids are 10 mm and 25-30 mm respectively. Normally in straight gaze, upper lid covers about 2 mm of upper part of cornea and cuts it at 11 and 1 O'clock position.
Medial and lateral canthi are the angles formed at the junctions of lids. Lateral angle is more acute than the medial one. Medial angle lies below the lateral canthus and so the fissure has an upward slant away from midline. Increase in this obliquity is a mongoloid slant whereas reduction or reversal is called an antimongoloid slant.
Lid fold or crease is a fold of overhanging skin in upper lid and is situated about 7-8 mm from lid margin. It almost disappears on looking down and is accentuated in upgaze. Presence of the fold indicates the action of levator palpebrae superioris.
 
Structure of Eyelids
Structurally, the lid can be grossly divided into two laminae, viz. anterior lamina containing skin and orbicularis, and posterior lamina containing tarsal plate and conjunctiva. Layer wise, it is composed of the followings (Fig.1.1):
  1. Skin
  2. Subcutaneous tissue,
  3. Orbicularis muscle,
  4. Submuscular areolar tissue,
  5. Fibrous layer,
  6. Mucous membrane with Muller's muscle, and
  7. Cilia.
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Fig. 1.1: Schematic diagram of the cut section of the upper lid
2Skin Skin of the lid, about the thinnest in the body, is highly elastic with sparse hairs.
Subcutaneous areolar tissue It contains loose connective tissue and no fat.
Orbicularis oculi It is a striated muscle, runs circularly around palpebral fissure. A narrow strip of the muscle near the lid margin is known as Riolan's muscle. The muscle is supplied by the 7th cranial nerve and its main function is normal closure of the lids. The adhesion of the muscle to the tarsus helps in maintaining the shape of the lid (Lamination effect).
Submuscular areolar tissue It is a potential space in front of the tarsal plate. It is continuous above with the subaponeurotic layer of the scalp and so the blood or fluid in the space above trickles down into this plane. It contains the major nerve plexus supplying sensory nerves to the lid. By entering this space at gray line on lid margin, the two laminae of the lid can be easily separated.
Fibrous layer It consists of the orbital septum and the tarsal plates. The orbital septum is a curtain like structure initiated from the orbital rim and extended up to the borders of tarsal plates. In upper lid, the definition is disturbed as it is massively perforated by levator muscle, which it lines anteriorly. In Caucasians, it fuses with the front surface of the levator muscle at about 10 mm above the upper border of the tarsal plate, but in Orientals, it comes down low allowing the orbital fat to prolapse and giving the typical oriental look. In older people, the septum weakens and so pouches of fat are seen under the skin. Tarsal plate, one in each lid, is a fibrous structure containing sebaceous glands called the Meibomian glands. The free or ciliary border of the tarsus is thickened and its flanging effect, helps in the stability of the lid margin and supports the lid. Medial and lateral ends of the tarsal plates are anchored to the bony orbit by thickened bands of fascia known as palpebral or check ligaments. Medial palpebral ligament is a tough triangular structure passing through the medial ends of the upper and lower tarsal plates to the anterior lacrimal crest. A posterior extension from the ligament bridges across the fossa and fuses with the fascia of the sac. The two limbs of the Y shaped anterior portion form the medial canthus and are very close to the canaliculi.
Levator palpebrae superioris is about 60 mm long striated muscle running along the roof of the orbit. It originates from the under surface of the lesser wing of sphenoid in front of the optic foramen by a small tendon. At about 1 cm behind the orbital septum it is converted into a vertical aponeurotic part. It inserts into many structures, i.e. (1) lower part of the front and upper border of the tarsal plate, (2) skin through orbicularis muscle (3) fornix through the common tendinous sheath with superior rectus muscle, and (4) fuses with lacrimal fascia laterally and superior oblique pulley medially. Horns of levator are the expansions at the insertion which, check the movement of the muscles (Fig.1.2). The medial horn fuses with the medial palpebral ligament and the tendon of the superior oblique muscle whereas the lateral horn passes between the two parts of the lacrimal gland. Superior transverse (Whitnall's) ligament is a fascial condensation about 14-20 mm above the upper border of the tarsal plate; it works as a sling for the muscle. The muscle is supplied by the superior division of the 3rd cranial nerve by a branch which passes through superior rectus or curves round its lateral border to reach the muscle.
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Fig. 1.2: Schematic diagram of the levator aponeurosis with Whitnall's ligament
Conjunctiva lines the back of the lid and continues to the globe forming fornices. A nonstriated muscle lies between the conjunctiva and the levator, called the Muller's muscle supplied by the sympathetic nerve. This involuntary muscle is about 20 mm long and helps in elevation of the lid by about 2-3 mm.
As the levator-Muller complex maintains the mobility of the upper eyelid, capsulopalpebral ligament present in the lower lid, helps in its downward retraction during downgaze (Fig 1.3). It is a continuation of the inferior rectus muscle sheath and attaches to the lower border of tarsal plate. It has three components i.e. Tenon's capsule, inferior tarsal muscle, and lower lid aponeurosis.
3
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Fig. 1.3: Schematic diagram of the cut section of the lower lid to show the retractors of the lower lid (capsulopalpebral ligament)
It keeps the lower lid margin in proper position, and its weakness in old age or due to other reasons, leads to lower lid entropion.
Lid margin is a 2 mm wide strip with a rounded anterior and square sharp posterior border. It has a row of anteriorly curving cilia at the front. Gray line lies between the cilia in front and openings of Meibomian glands on the back. There is a zone of transitional epithelium behind the openings of Meibomian glands where keratinized epithelium of skin changes over to nonkeratinized stratified epithelium of the conjunctiva. Modified sebaceous glands of Zeis are attached to the roots of cilia.
Cilia These are highly sensitive specialized hairs about 150 in upper lid and its half the number in lower lid. Unlike other hair follicles, they are devoid of piloerector muscle. Glands of Moll, modified sweat glands, communicate with roots of cilia.
 
Blood Supply
Eyelid is richly supplied by the vascular plexus formed by marginal tarsal arcades. They are formed by lateral palpebral branches of lacrimal artery on the temporal side and medial palpebral artery on the nasal side. Some branches from the surrounding area also contribute to it. Upper lid has another arcade at the upper border of the tarsal plate. Branches are given off from the plexus to supply skin and orbicularis superficially and tarsus and conjunctiva deeply.
Main venous drainage is into ophthalmic vein, but a small amount also drains into the veins of forehead and temple.
 
Lymphatics
The lymphatics of the lids are arranged in pre and posttarsal plexuses that communicate by cross channels. The pretarsal group drains the dermis, pretarsal orbicularis muscle, and adjacent structures, and post tarsus group drains the palpabral conjunctiva and tarsal glands. Medial side of both the lids drain into the submandibular glands. Lateral parts empty into the parotid and preauricular group.
 
Nerve Supply of the Eyelids
Orbicularis oculi muscle is supplied by the seventh cranial nerve (Facial nerve-supplying all the muscles of facial expression) through its temporal and zygomatic branches. The facial nerve divides into two divisions—upper division temporofacial and lower division cervico-facial (The upper division further subdivides into the temporal and zygomatic branches for supply to the frontalis and orbicularis oculi muscles respectively. The lower division subdivides to supply the buccal and cervical muscles. Sensory supply to the eyelids comes from the ophthalmic and maxillary division of the trigeminal nerve through the supraorbital, supratrochlear and lacrimal branches. Zygomaticotemporal branch of the maxillary nerve supplies the lateral part of the upper eyelid, brow and forehead and the skin over the bridge of the nose. Infratrochlear branch of the maxillary nerve carries the sensation of the lower eyelid. The zygomatico-facial branch from the maxillary nerve innervates the skin of the lateral portion of the lower lid.
FURTHER READING
  1. Anderson RL, Beard C. The levator aponeurosis:Attachments and their clinical significance. Arch Ophthalmol 1979; 95: 1129–31.
  1. Dixon RS. The role of Whitnall's ligament in ptosis surgery. Arch Ophthalmol 1979; 97: 705–07.
  1. Malone B, Maisel RH. Anatomy of the facial nerve. Am J Otolaryngol 1988; 9: 144.
  1. Mustarde JC. New horizons in eyelid reconstruction. Int Ophth Clinics 1989; 29: 4; 237–46.
  1. Warwick R. Wolfe's anatomy of the eye and orbit. HK Lewis & Co. Ltd:  London,  1976.