The conjunctiva is typically divided into three parts:
Part
Area
Palpebral or tarsal conjunctiva
Lines the eyelids
Bulbar or ocular conjunctiva
Covers the eyeball, over the anteriorsclera: This region of the conjunctiva is tightly bound to the underlying sclera byTenon's capsule and moves with the eyeball movements. The average thickness of the bulbar conjunctival membrane is 33 microns.[2]
Fornix conjunctiva
Forms the junction between the bulbar and palpebral conjunctivas: It is loose and flexible, allowing the free movement of the lids and eyeball.[3]
Blood to the bulbar conjunctiva is primarily derived from the ophthalmic artery. The blood supply to the palpebral conjunctiva (the eyelid) is derived from theexternal carotid artery. However, the circulations of the bulbar conjunctiva and palpebral conjunctiva are linked, so both bulbar conjunctival and palpebral conjunctival vessels are supplied by both the ophthalmic artery and the external carotid artery, to varying extents.[4]
The conjunctiva consists of unkeratinized, both stratified squamous and stratified columnar epithelium, with interspersedgoblet cells.[6] The epithelial layer contains blood vessels, fibrous tissue, and lymphatic channels.[6]Accessory lacrimal glands in the conjunctiva constantly produce the aqueous portion oftears.[6] Additional cells present in the conjunctival epithelium includemelanocytes,T and B cell lymphocytes.[6]
The conjunctiva helps lubricate the eye by producingmucus andtears, although a smaller volume of tears than thelacrimal gland.[7]It also contributes toimmune surveillance and helps to prevent the entrance ofmicrobes into the eye.
Disorders of the conjunctiva andcornea are common sources of eye complaints, in particular because the surface of the eye is exposed to various external influences and is especially susceptible totrauma,infections, chemical irritation,allergic reactions, anddryness.
The conjunctival microvascular hemodynamics are affected bydiabetic retinopathy (DR), hence can be useful for DR diagnosis and monitoring,[8] and discriminating stages of DR.[9]
Hypertension is associated with an increase in thetortuosity of bulbar conjunctival blood vessels and capillary and arteriole loss.[17][18]
Carotid artery occlusion is associated with slower conjunctival blood flow and apparent capillary loss.[4]
With age, the conjunctiva can stretch and loosen from the underlying sclera, leading to the formation of conjunctival folds, a condition known asconjunctivochalasis.[19][20]
The conjunctiva can be affected bytumors which can be benign, pre-malignant or malignant.[21]
Leptospirosis, an infection with Leptospira, can causeconjunctival suffusion, which is characterized by chemosis, and redness without exudates.
The bulbar conjunctivalmicrovasculature containsarterioles,meta-arterioles,venules,capillaries, and communicating vessels. Vessel morphology varies greatly between subjects and even between regions of the individual eyes. In some subjects, arterioles and venules can be seen to run parallel with each other. Paired arterioles are generally smaller than corresponding venules.[22] The average bulbar conjunctival vessel has been reported to be 15.1 microns, which reflects the high number of small capillaries, which are typically <10 microns in diameter.[23]
The bulbar conjunctival microvasculature is in close proximity to ambient air, thusoxygen diffusion from ambient air strongly influences theirblood oxygen saturation. Because of oxygen diffusion,hypoxic bulbar conjunctival vessels will rapidly reoxygenate (in under 10 seconds) when exposed to ambient air (i.e. when the eyelid is open). Closing the eyelid stops this oxygen diffusion by placing a barrier between the bulbar conjunctival microvessels and ambient air.[24]
The bulbar conjunctival microvessels are typically imaged with a high-magnificationslit lamp with green filters.[25][26][27] With such high-magnification imaging systems, it is possible to see groups of individual red blood cells flowing in vivo.[25] Fundus cameras may also be used for low-magnification wide field-of-view imaging of the bulbar conjunctival microvasculature. Modified fundus cameras have been used to measure conjunctival blood flow[28] and to measureblood oxygen saturation.[24]Fluorescein angiography has been used to study the blood flow of the bulbar conjunctiva and to differentiate the bulbar conjunctival andepiscleral microcirculation.[29][30][31]
The bulbar conjunctival microvasculature is known to dilate in response to several stimuli and external conditions, including allergens (e.g. pollen),[32] temperature,[33] time-of-day,[33]contact-lens wear,[13] and acute mild hypoxia.[24] Bulbar conjunctival vasodilation has also been shown to correlate changes in emotional state.[34]
Type 2 diabetes is associated with an increase in average bulbar conjunctival vessel diameter and capillary loss.[11][12]Sickle-cell anemia is associated with altered average vessel diameter.[14]
^abPavlou AT, Wolff HG (July 1959). "The bulbar conjunctival vessels in occlusion of the internal carotid artery".A.M.A. Archives of Internal Medicine.104 (1):53–60.doi:10.1001/archinte.1959.00270070055007.PMID13660526.
^Isenberg SJ, McRee WE, Jedrzynski MS (October 1986). "Conjunctival hypoxia in diabetes mellitus".Investigative Ophthalmology & Visual Science.27 (10):1512–5.PMID3759367.
^abFenton BM, Zweifach BW, Worthen DM (September 1979). "Quantitative morphometry of conjunctival microcirculation in diabetes mellitus".Microvascular Research.18 (2):153–66.doi:10.1016/0026-2862(79)90025-6.PMID491983.
^abDitzel J (1967-01-12). "The in vivo reactions of the small blood vessels to diabetes mellitus".Acta Medica Scandinavica. Supplementum.476 (S476):123–34.doi:10.1111/j.0954-6820.1967.tb12691.x.PMID5236035.
^abCheung AT, Ramanujam S, Greer DA, Kumagai LF, Aoki TT (2001-10-01). "Microvascular abnormalities in the bulbar conjunctiva of patients with type 2 diabetes mellitus".Endocrine Practice.7 (5):358–63.doi:10.4158/EP.7.5.358.PMID11585371.
^Isenberg SJ, McRee WE, Jedrzynski MS, Gange SN, Gange SL (January 1987). "Effects of sickle cell anemia on conjunctival oxygen tension and temperature".Archives of Internal Medicine.147 (1):67–9.doi:10.1001/archinte.147.1.67.PMID3800533.
^Harper RN, Moore MA, Marr MC, Watts LE, Hutchins PM (November 1978). "Arteriolar rarefaction in the conjunctiva of human essential hypertensives".Microvascular Research.16 (3):369–72.doi:10.1016/0026-2862(78)90070-5.PMID748720.
^Horak F, Berger U, Menapace R, Schuster N (September 1996). "Quantification of conjunctival vascular reaction by digital imaging".The Journal of Allergy and Clinical Immunology.98 (3):495–500.doi:10.1016/S0091-6749(96)70081-7.PMID8828525.
^abDuench S, Simpson T, Jones LW, Flanagan JG, Fonn D (June 2007). "Assessment of variation in bulbar conjunctival redness, temperature, and blood flow".Optometry and Vision Science.84 (6):511–6.doi:10.1097/OPX.0b013e318073c304.PMID17568321.S2CID943038.
