Inflammatory Diseases

Hordeolum/Chalazion

A hordeolum is an acute bacterial infection of the sebaceous glands of the eyelid. If the sebaceous glands adjoining the ciliary follicles are infected, it is termed Hordeolum externum, which differentiates itself from Hordeolum internum, where the meibomian glands are infected. Clinically, acute onset, erythematous, painful swelling of the lid margin can be seen. Local application of antibiotics and warm compresses is the treatment of choice.

A chalazion represents a chronic noninfectious inflammation, mainly of the meibomian glands ( ▶ Fig. 5.1). It is often associated with meibomian gland dysfunction or blepharitis. A well-circumscribed indolent swelling of the tarsal plate can be seen, sometimes associated with a granuloma on the tarsal conjunctiva. Treatment involves local application of warmth and lid hygiene measures to address the underlying blockage of the meibomian gland orifices, along with topical application of steroids and antibiotics. Incision and curettage is required where conservative treatment is unsuccessful. A routine histologic examination of the specimens obtained is recommended to exclude sebaceous gland carcinoma, which may masquerade as a chalazion. Histology of a chalazion shows a typical granulomatous picture with abundant giant cells.

Lid Abscess

Abscesses in the periocular region can occur as in any other region of the body. The underlying cause is often a folliculitis involving the eyelids or eyebrow cilia. Clinically localized pain, redness, and swelling of the involved lid are seen. Incision and drainage of the abscess may often be needed as conservative treatment alone may not suffice. Daily wound toilette, local and systemic antibiotic administration similar to abscess treatment in general are recommended.

Preseptal Cellulitis

Preseptal cellulitis is an infection of the eyelids superficial to the orbital septum. It can be caused by local skin infections, but also by sinusitis or septic dental foci. Clinical signs are lid swelling and redness as well as mechanical ptosis. However, ocular motility is not affected, neither will there be signs of compressive optic neuropathy (e.g., afferent pupillary defect, vision loss) or exophthalmos. Treatment is targeted to the underlying disease as well as administration of antibiotics. Oral antibiotics would suffice in most cases, but in severe cases, especially in children, intravenous administration of antibiotics is required. A preseptal cellulitis can breach the septum and progress to orbital cellulitis (see Chapter ▶ 6).

5.1.2 Malignant Tumors of the Eyelids

Basal Cell Carcinoma

Basal cell carcinoma (BCC) is a malignant tumor arising from the basal cells of the epidermis. These tumors are the most common malignant tumors in humans and predominantly occur in aged, light-skinned individuals. The underlying cause is mainly DNA damage due to ultraviolet radiation. Thus, BCCs frequently occur in sun-exposed areas, particularly the face and can appear multilocularly, often involving the eyelids ( ▶ Fig. 5.2). BCCs account for nearly 90% of all malignant eyelid tumors amongst Caucasians, whereas they are rarely found amongst Asians and Africans. BCCs can also affect younger individuals, especially immunocompromised patients, patients with xeroderma pigmentosum or Gorlin–Goltz-syndrome. ¹

The tumors usually display an infiltrative growth pattern with a high recurrence rate in cases of incomplete tumor excision, though metastases occur rarely and late.

Common localizations for basal cell carcinoma are the lower lid as well as the medial and lateral canthus. Involvement of the medial canthus carries a poor prognosis, since orbital infiltration from here is more likely.

Clinically and histologically BCC can be divided into several subgroups, such as nodular (solid), pigmented, and sclerodermiform tumors. Sclerodermiform tumors show an occult growth pattern and often a lack of ulceration. Since the diagnosis is often delayed, there is a higher prevalence for deep infiltration of orbital structures. Accordingly, prognosis is worse in these cases.

Complete micrographically controlled surgical excision followed by reconstruction of the area is the treatment of choice. In rare cases, orbital exenteration may be required to achieve margin clearance, especially in cases with orbital infiltration. Local radiotherapy can be considered in patients with nonresectable tumors or as adjuvant therapy. Metastasizing BCCs have a very limited prognosis. Since 2013, the Hedgehog-signal pathway inhibitor vismodegib (Erivedge, Roche) is available for treatment of locally advanced tumors not amenable to surgery or radiotherapy and of metastasized BCC.

Squamous Cell Carcinoma

Squamous cell carcinomas (SCCs) arise from the skin or conjunctiva either de novo or from a precancerous lesion. About 10% of malignant eyelid tumors are SCCs. They also occur most commonly in elderly, light-skinned individuals; similarly to BCC, ultraviolet radiation exposure may be a risk factor for SCC. Other risk factors include mechanical and chemical irritants as well as infection with human papilloma virus (conjunctival SCC) ¹ ( ▶ Fig. 5.3).

SCC usually demonstrates infiltrative growth and a tendency for lymphogenous and hematogenous metastasis.

Treatment of choice is complete excision with histologically clear margins. In some cases of SCC, adjuvant or primary radiotherapy might be indicated. Conjunctival SCC often requires adjuvant chemotherapy with mitomycin C, interferon alfa, or 5-fluorouracil. When regional lymph node metastases occur, an additional neck dissection is indicated.

The prognosis of this tumor depends on the tumor stage, with a 5-year mortality of 0 to 40%. ²

Sebaceous Gland Carcinoma

Sebaceous gland carcinomas (SGCs) arise from the epithelial cells of the meibomian, Zeiss, or other sebaceous glands located in the tarsus, caruncle, cilia, and eyebrows. Chalazion and unilateral chronic blepharitis should be considered important ophthalmologic differential diagnoses. SGC is mainly seen in middle-aged or elderly patients. While this tumor is very common in Asia, it only rarely occurs in whites. In contrast to BCC and SCC, sebaceous gland carcinoma is frequently localized in the upper lid ( ▶ Fig. 5.4).

Histologically, SGCs are characterized by a pagetoid growth pattern, with diffuse intraepithelial spread which is clinically not readily apparent. Thus, map biopsies are mandatory in excising SGCs.

SGCs can show lymphogenous as well as hematogenous metastasis. The local recurrence is approximately 30% ¹ and depends on the clearance as well as adjuvant therapy. The prognosis depends largely on tumor stage, underlining the importance of an early diagnosis. Since SGC can mimic inflammatory diseases of the eyelids and ocular surface, the diagnosis is frequently delayed. This is one of the main reasons for a relatively high mortality of 5 to 40%. ^{2,​ 3}

Recommended therapeutic options include complete surgical excision with histologically clear margins, sometimes needing orbital exenteration. Adjuvant cryotherapy or chemotherapy may also be indicated.

Malignant Melanoma of the Eyelids

Malignant melanoma of the eyelids may be cutaneous, conjunctival, or combined. The main risk factor for the development of malignant melanoma of the eyelid skin is UV light exposure, which does not seem to be of such impact in malignant melanoma of the conjunctiva. Melanomas of the eyelids can appear as lentigo maligna melanoma ( ▶ Fig. 5.5), superficial spreading melanoma, or nodular melanoma. They can develop de novo or from precancerous lesions. The prognosis is worse when the tumor has infiltrated the lid margin and the conjunctiva. As well as advanced tumor stage, another negative prognostic factor is infiltration of the palpebral conjunctiva and caruncle.

Sentinel node biopsy (SNLB) ⁴ is required for accurate staging of cutaneous malignant melanoma of the eyelids; the diagnostic accuracy of this method in conjunctival melanoma is currently unclear.

Depending on tumor stage, mortality can be as high as 25%. ¹ To improve the prognosis, early diagnosis and complete tumor excision is recommended.

Adjuvant treatment procedures, i.e. radiotherapy, brachytherapy, or local application of chemotherapeutics, are appropriate, particularly in conjunctival melanomas and their precancerous lesions. The latest agents for adjuvant systemic therapy are targeting specific genetic mutations in melanoma cells (e.g., vemurafenib) or have an immunomodulatory effect (e.g., ipilimumab). The prognosis in metastasizing melanomas is guarded.

Cutaneous Neuroendocrine Carcinoma (Merkel Cell Carcinoma)

Merkel cell carcinoma (MCC) originates from Merkel cells that represent stem cells of the epidermis associated with tactile sensitivity ( ▶ Fig. 5.6). It is a rare form of tumor, but often involves the eyelids (10% of all cases) ^{5,​ 6} and commonly affects elderly light-skinned individuals. Virus infection with the so-called Merkel cell polyomavirus is discussed as an important etiology, comparable to squamous cell carcinoma due to HPV-infection.

Merkel cell carcinomas grow rapidly, with a tendency for local tumor recurrences as well as lymph node and distant metastasis. In a recently published study, a 5-year survival rate of 50% was found. ⁷ The prognosis depends significantly on the tumor stage.

Treatment of choice is a complete tumor excision with histologically clear margins, and combination with adjuvant radiotherapy. Other therapeutic strategies comprise immunomodulation with interferon alfa or tumor necrosis factor α. ^{8,​ 9}

5.2 Diseases of the Eye

B. Wiechens

5.2.1 Glaucoma

Glaucoma is a group of eye diseases leading to damage of the optic nerve that results in irreversible visual field defects and can lead to blindness. The etiology is multifactorial. The main mechanisms involved are a reduction in optic nerve perfusion and increased intraocular pressure.

Orbital diseases may lead to a deterioration of underlying glaucoma or to a development of secondary glaucoma. This section will give only a brief overview on the systematology of glaucoma. For in-depth information about the disease, consult the standard text books.

Clinical Classification

Glaucoma can be differentiated as open-angle or angle-closure glaucoma. Depending on the pathogenesis, it can be primary, secondary, or congenital/juvenile. While primary glaucoma develops without any underlying ocular pathology, secondary glaucoma is usually related to an ocular condition or to a side effect of systemic medication.

Primary Glaucoma

Primary Open-angle Glaucoma (POAG)

Epidemiology

POAG represents nearly 90% of glaucoma in adults. The incidence of the disease increases significantly after the age of 40 years, reaching a peak between 60 and 70 years. Some studies suggest a genetic predisposition as POAG is frequently seen to accumulate in families.

Etiology and Pathogenesis

One of the main causes for the disease is an increased outflow resistance for the aqueous humor in the trabecular meshwork, the reason for which is yet unknown. The resulting increase of intraocular pressure leads to damage of the retinal ganglion cells and consequently of the optic nerve.

Clinical Symptoms

POAG does not cause any symptoms until it is well advanced. General complaints of patients, such as burning sensation, feeling of pressure behind the eye, redness, or temporary blurred vision are not usually associated with chronic open-angle glaucoma. The damage to the optic nerve causes slow progressive visual field loss, which can become significant before patients go to see an ophthalmologist.

Gonioscopy reveals the iridocorneal angle to be open and without any pathologic findings. Depending on the stage of the disease, the optic nerve shows more or less pronounced cupping on ophthalmoscopy ( ▶ Fig. 5.7). It is important to detect asymmetry. In some cases viewing the fundus in red-free light will detect focal defects of the nerve fiber layer. Splinter-shaped hemorrhages at the optic disc margin can be indicative of glaucoma progression.

Diagnosis and Differential Diagnosis

For the diagnosis of glaucoma, measurement of the intraocular pressure (IOP), gonioscopy, ophthalmoscopy, perimetry ( ▶ Fig. 5.8), and results of a diurnal variation profile are important issues. Nowadays, photographs of the optic disc and tomography of the optic nerve and nerve fiber layer (i.e., optical coherence tomography, OCT/Heidelberg retinal tomography) are employed for clinical follow-up.

However, increased IOP is not always seen in glaucoma. Hence it is necessary to distinguish two other entities closely linked to glaucoma when contemplating differential diagnosis: normal-tension glaucoma and ocular hypertension.

Normal-tension Glaucoma

In this disease the typical findings for glaucoma— progressive optic nerve neuropathy and glaucomatous visual field defects—are present but the intraocular pressure is normal. These patients commonly are found to have low blood pressure (especially during the night) and vasospasms of peripheral blood vessels of the hands and feet. Hence, in addition to ophthalmologic diagnosis, systemic investigations to exclude rheologic risk factors and 24-hour blood pressure profile measurements are recommended.

Ocular Hypertension

In patients with this disease, increased intraocular pressure can be present for years without any pathologic findings of the optic disc or of the visual field. For this reason, periodic ophthalmologic examinations are recommended in patients with ocular hypertension. The risk for development of a primary chronic open-angle glaucoma requiring therapy increases with lower age of the patient (about 2% per year) and a higher intraocular pressure (7% in cases of intraocular pressure of more than 26 mm Hg) (Ocular Hypertension Treatment Study). After 5 years the cumulative conversion rate into chronic open-angle glaucoma is about 9.5%. ¹⁰ Positive family history also increases the risk for glaucoma development.

Treatment

Decrease of the intraocular pressure is the most important aim of any therapy in primary open-angle glaucoma. The IOP has to be adjusted for any individual patient to prevent deterioration of optic neuropathy. A glaucomatous optic disc, visual field defect, asymmetry of optic nerve excavation exceeding 20%, and intraocular pressure of more than normal values (21 mm Hg) are considered indicators for the initiation of treatment. Basically an intraocular pressure of 30% below the initial values should be achieved, with pressures being within the normal range (10–21 mm Hg).

Decrease of intraocular pressure by medical treatment can be achieved through inhibition of aqueous humor production or enhancement of trabecular or uveoscleral drainage. Several drugs are available: parasympathomimetics, sympathomimetics, sympatholytics, prostaglandin analogues, carbonic anhydrase inhibitors, and—for acute cases—also osmotic agents such as mannitol 20% or Glycerosteril 10% (Fresenius). Treatment should be initiated with only one drug (e.g., beta blockers, if no contraindications) or prostaglandin analogues. When sufficient control of intraocular pressure is not achieved, a change of the substance class is indicated. If needed, combination therapy can be used. If progression of the glaucoma occurs despite reduction in IOP, the target IOP has to be further reduced (below 15 mm Hg or even below 12 mm Hg).

Surgical treatment is indicated if the intraocular pressure cannot be reduced to target levels despite multidrug treatment or if the patient cannot tolerate drug treatment due to allergic reaction or other side effects. The following surgical procedures are available:

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  Laser-assisted trabeculoplasty.

  
  
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  Trabeculectomy (filtration surgery).

  
  
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  Deep sclerectomy.

  
  
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  Canaloplasty.

  
  
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  Cyclocryocoagulation/cyclophotocoagulation.

  
  
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  Glaucoma drainage implants and shunts.

For further details, refer to textbooks of surgical procedures in ophthalmology.

Primary Angle-closure Glaucoma

Primary angle-closure glaucoma is less common than open-angle glaucoma, with a frequency of only about 5% of all glaucoma cases. ¹¹

Etiology and Pathogenesis

Angle-closure glaucoma is caused by a shallow anterior chamber with a narrow iridocorneal angle. Hyperopia, thickened lens (in cases with progressive cataract), and previous trauma with anterior displacement of the lens are risk factors. These mechanisms lead to an impairment of drainage of intraocular fluid from the anterior chamber. In addition, accumulation of aqueous humor in the posterior chamber can increase the anterior vaulting of the iris, causing additional angle-closure. As a result, there is an acute or intermittent increase of intraocular pressure (acute angle-closure glaucoma). In cases with anatomical disposition, acute angle-closure glaucoma can be triggered by pupillary dilatation leading to blockage of the iridocorneal angle with excessive increase of intraocular pressure.

Clinical Symptoms

The severity of clinical symptoms depends on the rate and level of rise in IOP. In severe cases, patients present with sudden redness and intense pain of the affected eye with radiation to the periorbital region. There can be additional symptoms such as upper abdominal pain with nausea and vomiting, headache, thoracic pain, and general feeling of illness. Acute visual loss might be masked by these general symptoms and lead to delayed diagnosis and treatment.

Ophthalmologic findings of acute glaucoma, which usually occurs unilaterally, are mixed conjunctival and ciliary injection, corneal edema, unreactive mid-dilated pupil (pressure-related damage of the sphincter pupillae muscle), and high intraocular pressure (palpation often showing a “rock-hard” globe) ( ▶ Fig. 5.9). Funduscopy is usually difficult due to corneal opacity.

Patients with chronic or intermittent angle-closure glaucoma commonly report transient blurred vision and perception of colored halos around light sources, which are due to the prismatic effect of the corneal epithelial edema.

Examination with a slit lamp reveals a shallow anterior chamber. In the acute setting gonioscopy is usually not possible because of the significant corneal epithelial edema. In cases of previous angle-closure, spot-shaped opacities in the anterior aspect of the lens might be seen.

Diagnosis

Diagnosis of angle-closure glaucoma is similar to that of open-angle glaucoma.

Therapy

Acute angle-closure glaucoma is an emergency and requires immediate ophthalmologic attention, initiated by medical and followed by surgical treatment. The aim of immediate medical therapy is to decrease the intraocular pressure and open the iridocorneal angle. Contraction of the pupil by topical administration of pilocarpine 1 to 2% is a mainstay of treatment. Additional systemic administration of acetazolamide (10 mg/kg body weight, intravenously) reduces the production of aqueous humor. After cardiopulmonary risk factors have been excluded, an osmotic reduction of the vitreous can be achieved by additional systemic administration of hyperosmolar solutions (e.g. mannitol 1–1.5 g/kg body weight). With supplemental topical medication, that is, beta-blockers, alpha-2 agonists, and carbonic anhydrase inhibitors, a further decrease of intraocular pressure can be achieved. Severe pain during acute angle-closure is treated with analgesics.

Causative treatment of acute angle-closure glaucoma is surgical iridectomy (in cases with corneal edema) or YAG-laser iridotomy (in cases with clear media). As the nonaffected eye is often at risk for developing angle-closure glaucoma as well, prophylactic YAG-laser iridotomy should also be performed.

Further surgical therapeutic options depend on the factors underlying the disease. In cases, of anterior lens vaulting, a lens extraction should be performed, because medical glaucoma therapy alone is unsuccessful in the long term, and filtration surgery may lead to malignant glaucoma. ¹¹ Additional medical treatment after control of the intraocular pressure is usually not necessary. However, in chronic cases, goniosynechiae can occur. In patients with suspected combination with primary open-angle glaucoma, further local glaucoma therapy is recommended.

Secondary Glaucoma

Secondary glaucoma encompasses glaucoma for which other eye conditions (e.g., pseudoexfoliation, pigment dispersion, intraocular inflammation, hemorrhage, and tumors), systemic diseases (e.g., diabetes mellitus, atherosclerosis), or side effects of systemic medication (especially steroids) are causative. Similarly to primary glaucoma, secondary open-angle glaucoma is distinguished from secondary angle-closure glaucoma.

Secondary Open-angle Glaucoma

Under normal anatomical conditions this type of glaucoma is caused by increase of ocular outflow resistance due to deposition of different substances in the trabecular meshwork. Pseudoexfoliation glaucoma belongs to this group of diseases. It is fairly common and is characterized by excessive production and deposition of extracellular matrix fibrils on the lens, zonular fibers, and trabecular meshwork. The origin of these proteins is still unclear, but it has been shown that these matrix proteins are deposited in other organs of the body too and that there is a genetic predisposition.

Pigment dispersion glaucoma, which is common in young myopic males, is characterized by release of iris pigment with subsequent deposition on the back surface of the cornea, on the iris, and the trabecular meshwork. In some patients a concave iris configuration can be recognized as the underlying pathomechanism.

Inflammatory glaucoma is characterized by sedimentation of proteins or by primary inflammation of cellular structures in the trabecular meshwork with consequent increase in intraocular pressure.

Phacolytic Glaucoma

Leaking of lens proteins through capsular defects in mature cataracts causes this rare condition. The proteins as well as macrophages lead to a blockage of the trabecular meshwork.

Steroid Response Glaucoma

Topical or systemic glucocorticoid therapy can induce deposition of mucopolysaccharides into the trabecular meshwork with consecutive increase of intraocular pressure. About 20 to 25% of the population are “steroid-responders.” Usually, the IOP comes back to normal after discontinuation of treatment, but temporary medication may be required.

Secondary Angle-closure Glaucoma

Secondary angle-closure glaucoma is characterized by a closure due to scarring of the iridocorneal angle. Neovascularization glaucoma belongs to this group. Ischemic retinal diseases (e.g., advanced diabetic retinopathy or ischemic retinal vein occlusion) can lead to neovascularization of the iris through release of VEGF and other growth factors with consecutive scarring and blockage of the iridocorneal angle. Secondary angle-closure glaucoma can also occur in orbital diseases ¹² (see the subsection “Glaucoma in Orbital Diseases” later).

Posttraumatic glaucoma

Direct injuries with resulting cicatrization of the iridocorneal angle or following sedimentation of blood or exudate can lead to an increase in intraocular pressure.

Tumor related glaucoma

Tumors located in the iris or ciliary body (e.g., melanoma or metastases) can lead to a blockage of the iridocorneal angle with development of secondary glaucoma.

Treatment

The underlying cause should be the mainstay of treatment of secondary glaucoma, but often, the treatment is similar to that of primary glaucoma. If sufficient control of intraocular pressure is not achieved, filtration or cyclodestructive surgery may be indicated.

Congenital Glaucoma

Primary and secondary congenital glaucoma can be distinguished. From data for Germany, the incidence of primary glaucoma is as high as 1:15,000 to 1:20,000 births per year. It most commonly (>70% of cases) occurs bilaterally and manifests usually before the 6th to 12th months of life.

Secondary glaucoma can present as part of systemic diseases (metabolic diseases, neurofibromatosis, von Hippel-Lindau angiomatosis, and other phakomatoses). Secondary glaucomas can present and manifest during childhood.

Classic symptoms of congenital glaucoma are increased sensitivity to light and epiphora. Typical clinical signs are increased corneal diameter with large eye (buphthalmus), conjunctival redness, and corneal edema and breaks in Descemet’s membrane (Haab’s striae). Sometimes reversible optic disc cupping is also seen.

Diagnosis can be challenging. It includes measurement of intraocular pressure, gonioscopy, and examination of the anterior and posterior aspect of the eye as well as ultrasound.

Treatment is usually surgical (goniotomy, trabeculotomy). Life-long ophthalmologic follow-up and possibly topical antiglaucomatous medications are required.

Glaucoma in Orbital Diseases

Changes of intraocular pressure can appear either as coincidentally with an orbital disease or as a side effect of it. This phenomenon was described fairly early in the ophthalmic literature. ¹³

Intraocular pressure can be influenced by orbital entities through four primary pathophysiologic mechanisms ¹⁴:

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  Structural bony anomalies in congenital defects, posttraumatically and following surgery.

  
  
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  Mass effects due to tumor growth or infiltrative disease.

  
  
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  Vascular alterations leading to increased venous pressure in the orbit (e.g., certain arteriovenous malformations, carotid cavernous sinus fistulas).

  
  
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  Inflammation and infection with alteration of vascular blood supply in orbital cellulitis or Graves’s orbitopathy.

Although there is no direct correlation between disease severity, localization, and nature of orbital diseases with variations of the intraocular pressure, it is reasonable to conclude that orbital pathology can lead to a rise in IOP. Due to special anatomical conditions of the orbit (see Chapters ▶ 1 and ▶ 2) inflammatory diseases of the orbit or tumors may lead to increased hydrostatic pressure in the orbit. This leads to a disturbance of blood flow in the orbital veins and, since venous valves are not present in the orbit, an increase of the episcleral venous pressure. As a consequence, these conditions can bring about structural changes affecting Schlemm’s canal and an increase of intraocular pressure.

Numerous orbital diseases can increase intraocular pressure (see ▶ Table 5.1). This list of orbital diseases underlines that a thorough ophthalmologic examination, including measurement of intraocular pressure, should be done in all patients with orbital diseases. It is important to note that the intraocular pressure is evaluated not only in the primary position of the eye but also in upgaze and downgaze. Compression of the globe in upgaze can lead to an increase of IOP in case of inferior rectus muscle fibrosis (common in Graves’s orbitopathy). Regular perimetry should also be performed in all patients with orbital diseases, not only because of possible development of glaucoma but also to exclude visual field changes due to optic nerve compression.

5.2.2 Corneal Diseases

This section focuses on the description of corneal diseases that occur in conjunction with orbital diseases or that are of differential diagnostic importance. Essentially, infectious keratitis and exposure keratitis as well as neurotrophic keratopathy will be subjects of discussion.

Pathogenesis

The cornea together with the sclera is the envelope of the globe. Its water content is between 70 and 80% with a protein content of 18–20% and a polysaccharide content of about 4%. Due to its avascular, highly organized multilayered architecture (corneal epithelium, basement membrane, Bowman’s layer, corneal stroma, Descemet’s membrane, and corneal endothelium), the cornea is translucent for the passage and refraction of light ( ▶ Fig. 5.10). With a diameter of about 11.7 mm (range 10–13 mm)—the horizontal diameter being slightly larger than the vertical—and a radius of 7.7–7.8 mm, the cornea has a refractive power of about 43 diopters, accounting for 70% of the refractive power of the eye. ¹⁵ The tear film and the aqueous humor provide the nutrition of the avascular cornea. The limbal conjunctival vessels supply the stem cells that provide the epithelial surface. The tear film is important for wetting the corneal surface. Its components (lysozyme, lactoferrin, betalysin, and antibodies) also play an important role in the defense against environmental toxins and pathogenic organisms.

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