Anatomy Topics

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LACRIMAL GLAND

The ocular surface is protected by tear fluid supplied from the gland system comprising the main and accessory lacrimal gland, meibomian gland, and goblet cell of conjunctiva. The lacrimal gland is important as it has a significant role in maintaining the stability of the microenvironment of the ocular surface. When a loss of function occurs in the LG, a significant reduction in tear production and dry eye disease (DED) may occur.

ANATOMY OF THE HUMAN LACRIMAL GLAND
The human lacrimal gland consists of the main lacrimal gland and the accessory lacrimal gland. The main lacrimal gland comprises palpebral and orbital lobes, which are continuous with each other at the lateral edge of the aponeurosis of levator palpebrae superioris. The orbital lobe is
approximately twice as large as the palpebral lobe and lies in the lacrimal fossa on the anterior lateral area of the orbit. The palpebral lobe lies below the aponeurosis of levator palpebrae superioris and is in contact with the superior and lateral fornix of conjunctiva. Excretory ducts coming from the palpebral and orbital lobes open into the superior conjunctival fornix. The main lacrimal gland measures approximately 20 X 25 mm and has a thickness of 3 mm in the palpebral lobe and 5 mm in the orbital lobe. The size of the main lacrimal gland varies individually to some degree.
The accessory lacrimal gland is a small or mini lacrimal gland located in the lamina propria of conjunctiva and is divided into 2 anatomic groups: the glands of Krause and glands of Wolfring. Both types of glands are too small to identify in living individuals. The glands of Krause are located in the lamina propria of fornix and the glands of Wolfring are in the edge of the tarsus. The ducts of both glands open on the conjunctival surface.

INNERVATION OF THE LACRIMAL GLAND
The lacrimal gland receives multiple types of nerve fibers originating from several complex pathways involving cranial nerves and the autonomic nervous system. These nerve fibers include sensory, parasympathetic, and sympathetic fibers. The parasympathetic innervation of the lacrimal gland arises from fibers conveyed in the facial nerve (VII). Preganglionic parasympathetic fibers arise from the superior salivatory nucleus and travel with the facial nerve (greater petrosal branch), which eventually synapses in the pterygopalatine ganglion. Postsynaptic fibers join the maxillary division (V2) of the trigeminal nerve and travel in its zygomatic branch, which divides into the zygomaticofacial and zygomaticotemporal nerves (ZTNs). The ZTN then sends a communicating branch to the lacrimal nerve (LN) posterior to the gland and the postsynaptic parasympathetic fibers from the ZTN travel forward in the orbit with sensory fibers of the LN entering the gland and ending in the skin of the upper eyelid. Since the LN merely passes through the lacrimal gland on its way to reach the skin of the upper eyelid, it called into question why the ZTN would communicate with it before entering the gland!??

The fact ZTN takes a different course than that which is described in most anatomic texts. Ruskell (2004) mentioned an alternative route for the efferent nerve fibers which involves them arising from a structure referred to as the retro-orbital plexus as rami lacrimales, which then proceed to the lacrimal gland. Ruskell (2004) mentioned that some efferent nerve fibers may travel along the middle meningeal artery when it supplies an accessory ophthalmic artery. Rusu (2010) reported similar findings involving the retro-orbital plexus. Tubbs et al. (2012) found that the ZTN communicated with the LN in 8.7% of specimens. Scott (2014) found a communicating branch between the ZTN and LN In less than 40% of the cases they studied. However, further studies are needed for a greater understanding of the typical and atypical courses of these nerves which will help surgeons identify them more easily and avoid damaging them.

Clinically important problems associated with lacrimal gland innervation involve syndromes such as keratoconjunctivitis sicca and epiphora when due to hyperproduction of tears. These syndromes can be caused by damage to the parasympathetic innervation of the lacrimal gland. Thats why knowledge of the precise innervation of the lacrimal gland is important for safe surgical procedures involving the orbit. This lnowlege would be most useful in surgeries involving the upper eyelid, such as ptosis surgery, where damage to the lacrimal gland is at risk. Due to the ZTN’s common location at the lateral pole of the lacrimal gland, upper eyelid blepharoplasty techniques should take care to avoid damage of the nerve at this site.

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