Eyelid

The human eyelid features a row of eyelashes along the eyelid margin, which serve to heighten the protection of the eye from dust and foreign debris.

Any condition that affects the eyelid is called eyelid disorder. The most common eyelid disorders, their causes, symptoms and treatments are the following:

Eyelid affected by stye

• Hordeolum (stye) is an infection of the sebaceous glands of Zeis usually caused by Staphylococcus aureus bacteria, similar to the more common condition Acne vulgaris. It is characterized by an acute onset of symptoms and it appears similar to a red bump placed underneath the eyelid. The main symptoms of styes include pain, redness of the eyelid and sometimes swollen eyelids. Styes usually disappear within a week without treatment. Otherwise, antibiotics may be prescribed and home remedies such as warm water compresses may be used to promote faster healing. Styes are normally harmless and do not cause long lasting damage.
• Chalazion (plural: chalazia) is caused by the obstruction of the oil glands and can occur in both upper and lower eyelids. Chalazia may be mistaken for styes due to the similar symptoms. This condition is however less painful and it tends to be chronic. Chalazia heal within a few months if treatment is administered and otherwise they can resorb within two years. Chalazia that do not respond to topical medication are usually treated with surgery as a last resort.
• Blepharitis is the irritation of the lid margin, where eyelashes join the eyelid. This is a common condition that causes inflammation of the eyelids and which is quite difficult to manage because it tends to recur.[3] This condition is mainly caused by staphylococcus infection and scalp dandruff. Blepharitis symptoms include burning sensation, the feeling that there is something in the eye, excessive tearing, blurred vision, redness of the eye, light sensitivity, red and swollen eyelids, dry eye and sometimes crusting of the eyelashes on awakening. Treatment normally consists in maintaining a good hygiene of the eye and holding warm compresses on the affected eyelid to remove the crusts. Gently scrubbing the eyelid with the warm compress is recommended as it eases the healing process. In more serious cases, antibiotics may be prescribed.
• Demodex mites are a genus of tiny mites that live as commensals in and around the hair follicles of numerous mammals including humans, cats and dogs. Human demodex mites typically live in the follicles of the eyebrows and eyelashes. While normally harmless, human demodex mites can sometimes cause irritation of the skin (demodicosis) in persons with weakened immune systems.
• Entropion usually results from aging, but sometimes can be due to a congenital defect, a spastic eyelid muscle, or a scar on the inside of the lid that could be from surgery, injury, or disease.[4] It is an asymptomatic condition that can, rarely, lead to trichiasis, which requires surgery. It mostly affects the lower lid, and is characterized by the turning inward of the lid, toward the globe.
• Ectropion is another aging-related eyelid condition that may lead to chronic eye irritation and scarring. It may also be the result of allergies and its main symptoms are pain, excessive tearing and hardening of the eyelid conjunctiva.
• Laxity is also another aging-related eyelid condition that can lead to dryness and irritation. Surgery may be necessary to repair the eyelid to its natural position. In certain instances, excessive lower lid laxity creates the Fornix of Reiss – a pocket between the lower eyelid and globe – which is the ideal location to administer topical ophthalmic medications.
• Eyelid edema is a condition in which the eyelids are swollen and tissues contain excess fluid. It may affect eye function when it increases the intraocular pressure. Eyelid edema is caused by allergy, trichiasis or infections.[5] The main symptoms are swollen red eyelids, pain, and itching. Chronic eyelid edema can lead to blepharochalasis.
• Eyelid tumors may also occur. Basal cell carcinomas are the most frequently encountered kind of cancer affecting the eyelid, making up 85% to 95% of all malignant eyelid tumors.[6] The tumors may be benign or malignant. Usually benign tumors are localized and removed before becoming a cancerous threat and before they become large enough to impair vision. Malignant tumors on the other hand tend to spread to surrounding areas and tissues.
• Blepharospasm (eyelid twitching) is an involuntary spasm of the eyelid muscle. The most common factors that make the muscle in the eyelid twitch are fatigue, stress, and caffeine.[7] Eyelid twitching is not considered a harmful condition and therefore there is no treatment available. Patients are however advised to get more sleep and drink less caffeine.
• Eyelid dermatitis is the inflammation of the eyelid skin. It is mostly a result of allergies or contact dermatitis of the eyelid. Symptoms include dry and flaky skin on the eyelids and swollen eyelids. The affected eyelid may itch. Treatment consists in proper eye hygiene and avoiding the allergens that trigger the condition. In rare cases, topical creams may be used but only under a doctor's supervision.
• Ptosis (drooping eyelid) is when the upper eyelid droops or sags due to weakness or paralysis of the levator muscle (responsible for raising the eyelid), or due to damage to nerves controlling the muscle. It can be a manifestation of the normal aging process, a congenital condition, or due to an injury or disease. Risk factors related to ptosis include diabetes, stroke, Horner syndrome, Bell's Palsy (compression/damage to Facial nerve), myasthenia gravis, brain tumor or other cancers that can affect nerve or muscle function.
• Ablepharia (ablepharon) Congenital absence of or reduction in the size of the eyelids.[8]

A South Korean woman, before (left) and after (right) undergoing double eyelid surgery. Note the presence of the epicanthic fold in both photographs.

An anatomical variation in humans occurs in the creases and folds of the upper eyelid.

An epicanthic fold, the skin fold of the upper eyelid covering the inner corner (medial canthus) of the eye, may be present based on various factors, including ancestry, age, and certain medical conditions. In some populations the trait is almost universal, specifically in East Asians and Southeast Asians, where a majority, up to 90% in some estimations, of adults have this feature.[9]

The upper eyelid crease is a common variation between people of Caucasian and East Asian (or Mongoloid) ethnicities.[10] Westerners commonly perceive the East Asian upper eyelid as a "single eyelid".[10] However, East Asian eyelids are divided into three types – single, low, and double – based on the presence or position of the lid crease.[11] Jeong Sang-ki et al. of Chonnam University, Kwangju, Korea, in a study using both Asian and Caucasian cadavers as well as four healthy young Korean men, said that "Asian eyelids" have more fat in them than those of Caucasians.[10]

• 
  Bloodvessels of the eyelids, front view
• 
  Horizontal section through the eye of an eighteen days' embryo rabbit. X 30
• 
  Sagittal section of right orbital cavity
• 
  Sagittal section through the upper eyelid
• 
  The tarsi and their ligaments. Right eye; front view
• 
  The lacrimal apparatus. Right side
• 
  Extrinsic eye muscle. Nerves of orbita. Deep dissection

• Cellulitis – a bacterial infection involving the inner layers of the skin
• Dermatochalasis – an excess of eyelid skin that may obstruct vision
• Gland of Moll – a modified sweat gland at the base of the eyelashes
• Hay-Wells syndrome – a disorder often causing fusion of the eyelids
• Nictitating membrane – a third eyelid present in some animals

• "eye, human."Encyclopædia Britannica from Encyclopædia Britannica 2006 Ultimate Reference Suite DVD 2009.