|CHAPTER||21||The Lacrimal Apparatus|
▃Anatomy of Lacrimal Apparatus
Lacrimal apparatus consists of tear-secreting glands and the lacrimal drainage system. Tear-secreting glands include lacrimal gland and accessory lacrimal glands (glands of Krause and Wolfring). Lacrimal drainage system includes puncta, canaliculi, lacrimal sac, and nasolacrimal duct (NLD).
Tear-secreting glands comprise of lacrimal glands and accessory lacrimal glands.
In the orbit, it is situated in lacrimal fossa at the outer part of the orbital plate of frontal bone. The lateral aponeurosis of levator muscle tendon produces an indentation in the gland, so that the gland appears to be composed of two separate lobes: An orbital portion lying above the aponeurosis and a palpebral portion lying below the aponeurosis.
Ducts of Lacrimal Glands
About 10 to 12 ducts from both parts open in the lateral part of the upper fornix. The ducts from the orbital lobe pass through the palpebral lobe. Thus, surgical removal of the palpebral lobe will abolish secretion from the entire lacrimal gland.
Nerve supply to lacrimal gland involves three components (Fig. 21.1):
Sensory innervation: The principal afferent pathway for reflex lacrimal secretion comes from lacrimal nerve (branch of 1st division of Vth CN).
Parasympathetic innervation: It is secretomotor and forms the efferent pathway of the reflex lacrimal secretion mechanism. It originates from superior salivary nucleus in pons, and travels via VIIth nerve and the greater superficial petrosal nerve. The fibers synapse in the pterygopalatine ganglion. The postganglionic fibers are carried by zygomatic nerve (branch of IInd division of Vth nerve) and lacrimal nerve (branch of 1st division of Vth nerve).
Sympathetic innervation: It is vasomotor in function. Postganglionic sympathetic fibers originate in the superior cervical ganglion and travel along the plexus surrounding the internal carotid artery. They travel via deep petrosal nerve and zygomatic nerve to the lacrimal gland.
It is supplied by lacrimal artery (a branch of ophthalmic artery).
Accessory Lacrimal Glands
These include glands of Krause and Wolfring. Glands of Krause are microscopic groups of acini. They lie below the surface of conjunctiva between fornix and the edge of tarsus. There are approximately 42 in the upper fornix and 6 to 8 in the lower fornix. Numerous acini open by a common duct into their respective fornix. Glands of Wolfring are present near the upper border of superior tarsal plate and the lower border of inferior tarsal plate.
■Lacrimal Drainage System
Lacrimal drainage system consists of puncta, canaliculi, lacrimal sac and NLD (Fig. 21.2). Tears secreted by the lacrimal gland are pumped along the marginal tear stripes of upper and lower lids toward the lacrimal lake at the inner canthus. Tears pass from the lacrimal lake into the canaliculi through the puncta, which then flow to the common canaliculus and lacrimal sac. Valves within the drainage system permit only one-way flow of tears.
Number: These are two in number (upper and lower).
Location: These are situated approximately 6 mm from inner canthus near the posterior margin of each eyelid where eye lashes end. Each punctum is situated upon a slight elevation called lacrimal papilla. Puncta are visible when lids are slightly everted.
Number: These are two in number (upper canaliculus and lower canaliculus).
Parts: Each canaliculus consists of vertical portion (1–2 mm) and horizontal portion (approximately 8 mm).
Lined by: Canaliculi are lined by stratified squamous epithelium.
At the junction of two segments, the canaliculus widens into an ampulla. In 90% of cases, upper and lower canaliculi join to form a common canaliculus which opens into the lateral wall of lacrimal sac. A small fold of mucosa (valve of Rosenmuller) overhangs the entrance of common canaliculus into the lacrimal sac, preventing the reflux of tears from sac into canaliculi.
Location: It lies in lacrimal fossa and is formed by lacrimal bone and frontal process of maxilla.
Length: Approximately 10 mm long.
Parts: Upper portion, that is, fundus extends slightly above medial palpebral ligament and is surrounded by fibers of orbicularis muscle. Lower end narrows and is continuous with NLD.
Lined by: It is lined by columnar epithelium.
Nasolacrimal Duct (NLD)
Extent: It extends from lacrimal sac to inferior meatus of nose.
Length: It is approximately 12 to 18 mm in
Course: The upper end of duct is the narrowest part. It runs downward, slightly laterally (outward), and posteriorly to open into inferior nasal meatus below inferior turbinate.
Opening: Opening of NLD is partially covered by a mucosal fold, valve of Hasner. A congenital NLD obstruction is usually due to an imperforate valve of Hasner.
Lined by: It is lined by columnar epithelium.
Surface marking: A line from a point just outside the inner canthus to the groove between the ala of the nose and the cheek marks the course of NLD.
The optical integrity and normal function of the eye depends on an adequate supply of tear film. The maintenance of the tear film depends on:
•Secretory system (secretion of tears).
•Distributary system (spread of tear film).
•Excretory system (elimination of tears).
■Secretion of Tears
Secretory system includes lacrimal gland, accessory lacrimal glands of Krause and Wolfring, sebaceous glands of eyelids (Meibomian glands and glands of Zeiss), goblet cells, and other mucin-secreting elements of conjunctiva.
Tears secretion may be basic secretion of tears or reflex secretion of tears. Basic secretors of tear are accessory lacrimal glands of Krause and Wolfring (aqueous part), mucus glands, and sebaceous glands. These structures provide all three layers of tear film. Extirpation of lacrimal gland does not result in xerosis or dryness of conjunctiva as conjunctiva is kept moist by accessory lacrimal glands of Krause and Wolfring and its own mucous glands.
Reflex secretion of tears is the function of lacrimal gland which secretes aqueous part only. It may be of peripheral or central origin. Reflex secretion of peripheral origin is caused by 5th nerve stimulation in disorders of cornea, conjunctiva, uvea, or nose. Reflex secretion of central origin may be found in emotional states (such as weeping) or retinal stimulation with varying intensity of light.
Corneal irritation results in reflex tearing through 5th nerve stimulation. Surface anesthesia or paralysis of 1st division of 5th CN inhibit this reflex tearing but cannot abolish psychogenic (emotional) reflex tearing. Psychogenic reflex tearing can be abolished by blocking the pterygopalatine ganglion (efferent pathway).
While reflex secretion of lacrimal gland is controlled by the parasympathetic supply, sympathetic fibers may control the basic secretors in the lids and conjunctiva. Directly acting parasympathomimetic drugs such as pilocarpine produce an increase in lacrimal flow, whereas parasympatholytics such as atropine reduce tear flow.
After lesions of facial ganglion, regenerating salivary gland nerve fibers may be misdirected to the lacrimal gland. Therefore, mastication produces tearing as well as salivation–“crocodile tears.”
Pre-ocular Tear Film
It consists of three layers (Fig. 21.3):
•Superficial lipid layer (outermost).
•Middle aqueous layer.
•Deep mucin layer (innermost).
Lipid (oily) Layer
It is secreted by Meibomian glands and glands of Zeiss.
The lipid layer is composed of phospholipids (polar lipids), cholesterol esters, and triglycerides (nonpolar). Blinking causes lid movement, leading to release of lipids from lid glands. So, thickness of layer can be increased by forced blinking and reduced by infrequent blinking.
•It reduces rate of evaporation of underlying aqueous layer.
•It forms a barrier along lid margins and prevents overflow of aqueous strip onto the skin.
•It acts as a surfactant, allowing spread of tear film.
It is secreted by the main lacrimal gland and accessory lacrimal glands of Krause and Wolfring. It contains:
•Electrolytes: HCO–3, Cl–, Na+, K+
•Proteins: Albumin and globulins. Immunoglobulins (Ig) found in normal tear fluid are IgA, IgG, and IgM. IgA is dominant in tears; IgE increases in allergic conjunctivitis; and IgG is found in tears of patients with acute infections.
•Lysozyme enzyme, a high-molecular weight proteolytic enzyme is produced by lysosomes. It dissolves bacterial wall and considered to be the antibacterial substance in tears. The action of lysozyme depends on pH. The optimum pH for lysis lies between 6.0 and 7.4.
•It provides atmospheric O2 to corneal epithelium which is essential for normal corneal metabolism.
•It has antibacterial function due to presence of IgA and lysozyme.
•It washes away debris.
•It provides a smooth optical surface to cornea.
Mucins are high-molecular weight glycoproteins secreted mainly by goblet cells of conjunctiva and also by glands of Manz.
Mucin layer is spread over and adsorbs on hydrophobic corneal epithelium through the act of blinking. Thus, it converts hydrophobic corneal epithelium into a hydrophilic one over which tear fluid spreads evenly (Flowchart 21.1).
Properties of Tear Film
Characteristic features of tear film are:
•Thickness of tear film is 8 µm.
•Average pH of tears is 7.25 (slightly alkaline). Tear pH is lowest on awakening due to acid byproducts.
Prolonged lid closure during sleep creates anaerobic conditions and leads to acid byproducts. It results into low pH on awakening. As eyes are open, pH increases due to loss of .
When solutions having a pH <6.6 or >7.8 are instilled into conjunctival sac, subjective discomfort occurs.
•Refractive index of tear film– 1.336.
•Osmolality– 309 mosm/L (≡ 0.9% NaCl).
•Chemical composition is same as explained in aqueous layer. Rate of tear secretion– It is about 1.2 µL/min.
Functions of Tear Film
Tear film serves the following functions:
•Optical function: It maintains optically uniform corneal surface.
•Mechanical function: It flushes cellular debris and foreign matter.
•Nutritional function: It provides oxygen to cornea which is essential to normal corneal metabolism.
•Antibacterial function: Due to the presence of proteins such as IgA, lysozyme and lactoferrin tear film has antibacterial property.
■Spread of Tear Film
Tear film is spread over the ocular surface through blinking. Shear forces produced by the moving lids play a vital role in maintenance of normal tear film. Three factors are required for effective spreading of tear film, namely, normal blinking process, contact between lid and globe, and normal corneal epithelium.
■Elimination of Tears
Elimination of tears takes place by evaporation or through lacrimal drainage system. A variable amount of the component of tear film is lost by evaporation which is related to blinking rate, size of palpebral aperture, temperature, and humidity. The remainder of the tears are drained through lacrimal drainage system. About 70% of tears drain through lower canaliculus and remainder through the upper canaliculus. Proper tear elimination requires that the punctum, the entry point for lacrimal drainage, be apposed to the globe.
Blinking not only spreads the tear film over the eye but also moves the tears toward puncta with each blink. Orbicularis muscle is more firmly fixed at its nasal attachment and intimately disposed around the punctum and the canaliculus. Its upper preseptal (pretarsal) part inserts into the fascia overlying the fundus of the lacrimal sac.
Act of Blinking
The temporal part of orbicularis moves in a nasal direction due to its firm nasal attachment. Also, closure of eyelids occurs from temporal to nasal end. Therefore, tears move in the nasal direction toward the puncta with each blink. Punctum is drawn nasally, ampulla is compressed, and horizontal canaliculi shortens due to contraction of muscle with blinking. Contraction of the preseptal part, attached to the facia of the lacrimal sac, in blinking draws the lateral wall of the sac laterally. The sac expands, creating a negative pressure. The tears are sucked into the sac from the canaliculi.
Drainage of Tears
Tears are drained as follows:
•Tears secreted in the upper temporal fornix flow along upper and lower marginal strips and are conducted to puncta by capillarity and the act of blinking.
•Tears enter the upper and lower canaliculi and are driven into the lacrimal sac due to contraction of orbicularis with each blink, resulting in compression of ampullae, shortening of horizontal canaliculi, and expansion of sac creating negative pressure.
•The orbicularis muscle relaxes on opening the eye. The sac collapses and a positive pressure is created, which forces the tears down the NLD into the nose (lacrimal pump).
Excessive watering (overflow of tears) from the eye may be due to hypersecretion of tears (hyperlacrimation) or defective drainage (epiphora).
■Hypersecretion of Tears
It is seen in emotional states or secondary to stimulation of ophthalmic division of trigeminal nerve (V1) due to irritation of conjunctiva (in conjunctivitis), cornea (in abrasion, ulcer, and keratitis) and uvea (in uveitis). The watering is associated with the symptoms of the underlying cause. The treatment is usually medical.
■Defective Drainage (Epiphora)
Lacrimal drainage system consists of lid margins, puncta, canaliculi, lacrimal sac, and NLD. The term epiphora is reserved for overflow of tears due to defective drainage. It may be caused by the mechanical or functional disorder of lacrimal passages. Causes of epiphora may lie at any level of the lacrimal drainage system (Fig. 21.4).
Causes and Treatment of Epiphora
Malposition or eversion of lower punctum: It may be due to laxity of lids in old age or any cause of ectropion.
Occlusion of punctum: It may be congenital and acquired. Acquired causes include:
•Cicatricial due to injuries, burns, or infections.
•Due to prolonged use of idoxuridine drops.
•Due to antiglaucoma medications– pilocarpine, adrenaline, or ecothiophate.
Treatment of Punctal Causes
•Punctal occlusion due to complete absence usually requires placement of the Jones tube.
•Punctal occlusion due to membrane overlying punctal papilla is relieved by perforation of membrane with a 25-gauge needle.
•Punctal stenosis in absence of punctal eversion is treated by dilatation of punctum and punctoplasty.
Dilatation of punctum is carried out with punctum dilator (Nettleship dilator), which is introduced vertically and then pushed inward toward the canaliculus. If repeated dilatation to relieve the stenosis fails, punctoplasty is usually required. It involves excision of triangular flap of the posterior wall of the vertical and horizontal parts of canaliculus by the three-snip technique (Fig. 21.5).
•Punctal stenosis secondary to punctual eversion is treated by:
◊Retropunctal cautery (for pure punctal eversion). Punctum is pulled inward due to shrinkage of cauterized tissue.
◊Medial conjunctivoplasty (in medial ectropion not associated with lid laxity). A diamond-shaped tarsoconjunctival piece is excised (4 mm high and 8 mm in length) parallel and inferolateral to canaliculus. Approximation of wound margins with sutures pulls the punctum inward.
◊Lateral canthal sling (it tightens the lower lid to correct lower lid laxity) (Fig. 21.6).