Eyelid, conjunctiva, orbit

• Eyelids provide a protective covering for the eye and spread precorneal tear film over the ocular surface
• Cilia on the eyelids protect the eye from dust
• Eyelids comprise:

  • An anterior layer of skin
  • Orbicularis muscle innervated by the seventh nerve
  • A tough collagenous layer (tarsal plate) housing oil glands
• Lids are closed by the orbicularis oculi muscle, which is innervated by the facial nerve
• The orbicularis oculi muscle is composed of three parts: orbital, palpebral, and lacrimal
• Contraction of peripheral fibers of the orbicularis results in protective forced closure, while inner palpebral muscle fibers result in blinking
• The lacrimal part of the orbicularis is responsible for suctioning tears
• Lids are opened with Levator palpebrae, Muller's muscle, and lower lid retractor
• In 3rd nerve palsy, ptosis occurs due to levator palpebrae muscle paralysis
• Tarsus is a thick fibrous tissue that acts as a skeleton for the lid
• Meibomian gland produces meibum, an oily substance that prevents evaporation of the eye's tear film
• Entropion is when the lid margin folds inward
• Ectropion is when the lid margin turns outward
• Lagophthalmos is the inability to close the eyelids completely
• Functions of eyelids include:

  • Protection of the globe
  • Distribution of tear film over the ocular surface to prevent dryness
  • Secretion of an oily layer of tear film (Meibomian glands) to prevent evaporation and decrease surface tension of the aqueous layer
  • Protection from dust particles (eyelashes)
• Ptosis is an abnormally low lid position or droopy lid, usually involving the upper lid
• Etiology of ptosis can be mechanical, neurological (like 3rd nerve palsy), aponeurotic, myogenic (like myasthenia gravis), or congenital
• Treatment of ptosis is needed when visual access is occluded or to avoid the risk of amblyopia in young children; otherwise, treatment is cosmetic with levator resection or frontalis suspension
• Entropion is the inversion of the lid margin toward the globe, causing irritation and foreign body sensation
• Entropion treatment can be temporary with artificial tears to alleviate symptoms or permanent with surgical correction
• Ectropion is the eversion of the eyelid, causing excessive tearing or epiphora
• Ectropion can be caused by age-related laxity of the lower lid, facial nerve palsy, or cicatricial changes in the facial skin due to scarring
• Ectropion treatment requires surgical correction
• Trichiasis is the misdirection of growth of normal eyelashes toward the globe, causing irritation and foreign body sensation
• Trichiasis can be caused by chronic blepharitis, trachoma, or chemical burn and can be treated by epilation with forceps, electrolysis, laser, or cryotherapy
• Distichiasis is the growth of eyelashes from an abnormal place, particularly from the Meibomian gland due to metaplasia of Meibomian gland epithelium
• Distichiasis treatment requires surgical correction and can be inherited or part of a syndrome
• External Hordeolum (Stye) is inflammation and infection of the eyelash follicle and gland of Zeis, caused by Staphylococcus
• External Hordeolum is treated by the removal of the affected lash and topical antibiotics
• Internal Hordeolum is an acute infection and inflammation of the Meibomian gland, requiring topical antibiotics and drainage of abscess if present
• Chalazion is chronic granulomatous sterile inflammation of the Meibomian gland, resulting in a painless lid mass
• Chalazion treatment starts with warm compresses and may require incision and curettage if it persists
• Blepharitis is inflammation of the eyelid margin, which can be anterior or posterior
• Anterior blepharitis is around eyelashes, caused by Staphylococcus, and is treated with topical antibiotics and lid hygiene
• Posterior blepharitis, also called Meibomian gland dysfunction, involves chronic inflammation, plugging, and abnormal secretions of the Meibomian gland, leading to chronic irritation, dryness, and tired sore eyes
• Basal cell carcinoma, also called rodent ulcer, is related to ultraviolet exposure, does not metastasize, and requires complete excision with a safety margin
• Squamous cell carcinoma is more aggressive, can metastasize via lymphatics, and is also related to UV exposure, requiring surgical excision with a safety margin, cryotherapy, radiotherapy, and chemotherapy in cases of metastasis
• Sebaceous gland carcinoma can mimic chronic blepharitis and requires surgical correction
• In multicellular organisms, the distance for substances to enter cells is larger due to a higher surface area to volume ratio
• Multicellular organisms require specialised exchange surfaces for efficient gas exchange of carbon dioxide and oxygen