The eyelids are mobile flaps of tissue that aid in protection of the eyeball and assist in facial expression.
Structure
The upper and lower eyelids have a similar structure, although the upper eyelid is typically larger and more mobile due to increased muscle attachment.
The external surface of each eyelid is lined by thin skin which is attached to loose connective tissue, below which lies the palpebral part of orbicularis oculi. The internal surface is lined by the conjunctiva, a mucous membrane. At the margin of the eyelid there are numerous hairs (eyelashes). The internal structure of the eyelid contains the tarsal plate, a fibrous structure that gives the eyelid shape and provides an attachment for muscles. The tarsus is connected to the orbital septum superior/inferior. Each tarsus has an associated smooth muscle (superior/inferior tarsal muscle) which is under sympathetic control; the superior tarsus is the point of insertion of the levator palpebrae superioris.
The eyelids meet medially and laterally at the medial and lateral canthus. Near the medial canthus each eyelid has a small opening for the lacrimal canaliculi which drain tears to the nasolacrimal sac. Eyelashes are only found lateral to this point.
Microscopic Structure
The tarsus is comprised of dense connective tissue. The tarsal gland, a modified sebaceous gland, lies within the tarsus and secretes and oily substance that helps to prevent tears from escaping the conjunctival 'space'. This gland is lined by a single layer of cuboidal cells. Additional apocrine sweat and sebaceous glands are associated with the eyelids.
Relations
The eyelids are superficial to the eyeball and are surrounded superiorly and inferiorly by the orbital fascia.
Neurovascular Supply
Arterial Supply
The eyelids are supplied by branches of the ophthalmic artery, which form peripheral and marginal 'arcades'; these are vessels that run along the marginal and peripheral parts of the eyelids and anastamose centrally. The arterial supply is also contributed to by branches of the facial and superficial temporal arteries.
Venous Drainage
Venous drainage is mixed. Some blood returns posteriorly through ophthalmic veins to reach the cavernous sinus or pterygoid venous plexus. The remaining blood drains anteriorly to reach the facial and superficial temporal veins.
Lymphatics
The eyelids have a rich lymphatic supply, with a subcutaneous plexus and additional tarsal plexuses on the superficial and deep side of each tarsus. These plexuses freely communicate. From the medial side of the eyelid, lymph vessels generally accompany the facial vessels to reach submandibular lymph nodes. Those from the lateral side of the eyelid usually pass more posteriorly to reach parotid lymph nodes.
Innervation
The superior eyelid is innervated by branches of the ophthalmic nerve (V1). This includes the superior trochlear, supraorbital and lacrimal nerves. The inferior eyelid receives supply mostly from the infraorbital nerve (V2).
Potential Routes of Malignant Spread
Local Invasion
Locally invasive cancers can invade through the eyelid and tarsus, and spread posteriorly into the orbit. Medial tumours may obstruct the nasolacrimal duct.
Lymphatic Spread
Lymphatic spread may occur to either submandibular or pre-auricular/parotid nodes depending on the site of disease; well lateralised tumours will tend to pass to only one group whereas central eyelid tumours may pass to both.
Perineural Spread
Involvement of cranial nerves is possible. Upper eyelid tumours will usually involve the supraorbital nerve whereas lower eyelid tumours will typically spread along the infraorbital nerve. This can lead to involvement of the central nervous system in the absence of local invasion through the orbit.
