Lower lid ectropion is a very common condition in older persons. Frequency increases steadily with age.  Defined as an eversion of the eyelid away from the globe, the condition is classified according to its anatomic features as involutional, cicatricial, tarsal, congenital, or neurogenic/paralytic. Surgical approaches are directed toward the underlying etiologic factors.
See the image below.
Ectropion is an outward turning of the eyelid margin. It occurs most often in the lower eyelid. The condition may be mild or severe and may involve all or part of the eyelid margin.
Ectropion is a very common condition in older persons. It is more frequently found in men than in women, which may be related to men generally having larger tarsal plates than women.
Ectropion is most commonly observed as an involutional change associated with horizontal laxity of the involved eyelid. Ectropion may be classified into the following 5 types, ordered in decreasing frequency: involutional (senile), paralytic (neurogenic), cicatricial, mechanical, and congenital. 
Involutional ectropion is the most common form of ectropion. A major factor is horizontal lid laxity, which is usually due to age-related (most patients are older persons) weakness of the canthal ligaments and pretarsal orbicularis.
For many years, physicians have questioned why some patients develop ectropion and others develop entropion, when both conditions seem to share the same etiologic factors. Bashour and Harvey recently answered this question.  They report the following:
Patients with involutional entropion have tarsal plates that are smaller than the normal average for age. Involutional entropion results from the vector mechanical effect of an atrophied or smaller than normal for age partially or fully disinserted tarsal plate being overcome by the normal or increased tone of the preseptal/pretarsal orbicularis muscle.
Patients with involutional ectropion have tarsal plates that are larger than the normal average for age. Involutional ectropion results from normal or larger-than-normal for age tarsal plate vector mechanical effects overcoming the normal or decreased tone of the preseptal/pretarsal orbicularis muscle in combination with medial/lateral canthal tendon laxity.
Patients with an anophthalmic socket may have involutional ectropion due to chronic pressure of the ocular prosthesis.
Disinsertion of the capsulopalpebral fascia may lead to severe tarsal ectropion.
Paralytic ectropion may occur with seventh nerve palsy from diverse causes such as Bell palsy, cerebellopontine angle tumors, herpes zoster oticus, and infiltrations or tumors of the parotid gland.
Cicatricial ectropion occurs from scarring of the anterior lamella by conditions such as facial burns, trauma, chronic dermatitis, or excessive skin excision (or laser) with blepharoplasty. Ectropion is not uncommon after orbital fracture repair with a transcutaneous approach. Less common causes of cicatricial ectropion include cutaneous T-cell lymphoma.
Mechanical ectropion may occur with lid tumors, such as neurofibromas, that evert the lower lid.
Congenital ectropion is rare and usually involves the lower lid. The cause is often a vertical deficiency of the anterior lamella. Although congenital ectropion is rarely an isolated anomaly, it may be associated with blepharophimosis syndrome, microphthalmos, buphthalmos, orbital cysts, Down syndrome, and ichthyosis (collodion baby). Occasionally, cases of congenital ectropion are paralytic.
Causal factors leading to ectropion include horizontal laxity of the eyelid (universal), dehiscence of the lower eyelid retractors, vertical shortening of the anterior lamella of the eyelid, paralysis of the orbicularis oculi muscle causing loss of eyelid muscular tone, and neoplasia within the lower eyelid pulling or forcing the eyelid away from the globe.
A study by Michels et al indicated that risk factors for either entropion or ectropion of the eyelid include dehiscence of the eyelid retractors, presence of a "white line," orbital fat prolapse in the cul-de-sac, decreased excursion and increased distraction of the lower eyelid, and increased laxity of the eyelid (as measured by the snapback test). Eyelids with ectropion were reported to have less elasticity than those with entropion. 
Patients usually present because of epiphora, ocular irritation, or cosmesis.
Generally, the progression observed is from eyelid laxity to punctal ectropion, to medial ectropion, and then to generalized ectropion. If the punctum is everted slightly from the lacus lacrimalis, tears cannot effectively drain into the canalicular system. Also, horizontal eyelid laxity may produce a flaccid canalicular syndrome or poor lacrimal pump so that tears are not siphoned from the lacus lacrimalis, producing epiphora.
This tearing malfunction is aggravated by the chronic ectropion and eyelid retraction that produce lagophthalmos and secondary exposure keratopathy. With time, the exposed conjunctiva thickens and keratinizes, producing further ocular irritation.
Tearing (epiphora) is probably the most common indication for surgical correction, but ocular irritation and cosmesis are also frequently encountered and may be indications for surgery.
The orbicularis oculi muscle is the sphincter of the eyelids. It spreads over the eyelids and out onto the forehead, temple, and cheeks. It is divided into orbital and palpebral portions, with the palpebral portion subdivided into preseptal and pretarsal parts.
The pretarsal part is attached laterally to the Whitnall tubercle by the lateral canthal tendon. This tendon (which is actually just a band of connective tissue) is weak in ectropion. Medially, the pretarsal orbicularis forms the anterior crus of the medial canthal tendon that inserts into the frontal process of the maxillary bone. The posterior pretarsal orbicularis inserts into the posterior lacrimal crest. The small strip of pretarsal muscle at the lid margin forms the gray line and is called the Riolan muscle. The images below demonstrate these relationships through horizontal and sagittal sectioning of the orbit.
Surgery to correct ectropion is contraindicated in patients who are unable to tolerate the procedure.
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