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Anatomy and Physiology of the Eyelids and Adnexa

Anatomy of the Eyelids

Figure 1: Anatomy of the eyelid

  • The eyelids are composed of several layers of tissue which protect the anterior eyeball.
  • From superficial to deep, they are the skin layer, a layer of striated muscle (orbicularis oculi), areolar tissue, fibrous tissue (tarsal plates), and a layer of mucous membrane (palpebral conjunctiva).

Figure 2: Layers of the eyelid

  • The function of orbicularis oculi (a muscle of protraction) is to close the eyelids; it is supplied by the facial nerve.
  • The muscles of retraction (which open the eyelids) include the levator palpebrae superioris supplied by the oculomotor nerve, and Muller's muscle, which is supplied by postganglionic sympathetic fibres originating from the superior cervical ganglion.

Figure 3: Muscles of the eyelid

  • The main supporting structure of the eyelids is a dense fibrous tissue layer, called the tarsal plate. The palpebral conjunctiva is the mucous membrane which adheres firmly to the tarsal plates and lines the posterior eyelids. 
  • The sensory nerve supply to the eyelids is derived from the first and second divisions of the trigeminal nerve (V).
  • The blood supply to the lids is derived from the lacrimal and ophthalmic arteries by their lateral and medial palpebral branches.

The Glands of the Eyelid

Figure 4: Glands of the eyelid

  • The lid margins are divided into the anterior and posterior margins at the mucocutaneous junction.
  • Glands of Zeis and glands of Moll are both found on the anterior margin. Glands of Zeis are small, modified sebaceous glands that open into the hair follicles at the base of the eyelashes; whereas glands of Moll are modified sweat glands that open in a row near the base of the eyelashes.
  • The posterior lid margin is in close contact with the globe, and along this margin are the small orifices of modified sebaceous glands, called meibomian glands.

Anatomy of the Lacrimal Apparatus

Figure 5: Anatomy of the lacrimal gland

Lacrimal Apparatus

  1. The lacrimal apparatus consists of the lacrimal gland, accessory lacrimal gland, lacrimal puncta, lacrimal canaliculi, lacrimal sac and nasolacrimal duct.
  2. These structures are involved mainly in the production and drainage of tears.
  3. The blood supply of the lacrimal gland is derived from the lacrimal artery. The vein that drains the gland joins the ophthalmic vein. The lymphatic drainage joins with the conjunctival lymphatics to drain into the preauricular lymph nodes.
  4. The nerve supply of the lacrimal gland is by the lacrimal nerve (sensory), a branch of the trigeminal first division, the great petrosal nerve (parasympathetic secretory), which comes from the superior salivary nucleus and is a branch of the facial nerve; and sympathetic nerves accompanying the lacrimal artery and the lacrimal nerve.

Tear Film

  1. The tear film is composed of three layers, namely the outer lipid layer, intermediate aqueous layer and the inner mucous layer.  
  2. Any abnormality of one of the three layers produces an unstable tear film and the symptoms of dry eyes.
  3. A tear layer frequently affected is the aqueous layer, resulting in aqueous tear deficiency (ATD) or lacrimal hyposecretion.

 

Tear Film Component

Table 1: Tear Film Component

The Lacrimal Secretory System

  1. The secretory system is composed of the main lacrimal gland, which is located in the temporal outer portion of the orbit, and accessory glands (also known as the glands of Krause and Wolfring), located in the substantia propria of the palpebral conjunctiva.
  2. Secretions from the lacrimal gland are triggered by emotion or physical irritation and cause tears to flow copiously over the lid margin (epiphora). The afferent pathway of the reflex arc is the ophthalmic branch of the trigeminal nerve. The efferent pathway is comprised of parasympathetic and sympathetic contributions. Parasympathetic innervation, originating from the pontine lacrimal (superior salivary) nucleus, is conveyed by the greater petrosal nerve, which is contained in the separate part of the facial nerve known as the nervus intermedius, the nerve of the pterygoid canal (Vidian nerve), synapsing in the pterygopalatine ganglion, and then via an uncertain route to the lacrimal gland.
  3. The parasympathetic innervation is clinically relevant because a patient may complain of dry eye symptoms while taking medications with anticholinergic side effects, such as tricyclic antidepressants, antihistamines, and drugs used in the management of Parkinson's disease.

The Lacrimal Drainage System

  1. The lacrimal puncta, canaliculi, and sac, and the nasolacrimal duct form the drainage system, ultimately draining into the nose.
  2. With each blink, the eyelids close and distribute tears evenly across the cornea. When tears flood the conjunctival sac, they enter the puncta partially by capillary attraction. With lid closure, the specialised portion of pretarsal orbicularis surrounding the ampulla tightens to prevent their escape. Simultaneously, the lid is drawn toward the posterior lacrimal crest and traction is placed on the fascia surrounding the lacrimal sac, causing the canaliculi to shorten; this results in the creation of a negative pressure within the sac. This dynamic pumping action draws tears into the sac. The tears then pass by gravity and tissue elasticity through the nasolacrimal duct to exit beneath the inferior meatus of the nose.
  3. The nose is lined by a highly vascular mucosal epithelium; consequently, topically applied medications that pass through this nasolacrimal system have direct access to the systemic circulation.
  4. Valve-like folds of the epithelial lining of the duct, called the valve of Hasner tend to resist the retrograde flow of tears and air. When the structure is imperforate, it is the most common cause of congenital nasolacrimal duct obstruction, resulting in epiphora and chronic dacryocystitis.

References

  1. Vaughan & Asbury's General Ophthalmology 18th Edition
  2. Oxford Handbook of Ophthalmology 2nd Edition

 

Images

Figure 1: http://one.aao.org/lms/courses/cosmetic_facial_surgery/images/LO11_fig1.jpg

Figure 4: http://atlas-emergency-medicine.org.ua/ch.2_files/image013.gif

Figure 5: http://www.lhwl.lhsa.com/Courses/Adv%20Bio/special_senses.htm

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