Conjunctiva

Structure

The conjunctiva is typically divided into three parts:

Blood supply

Blood to the bulbar conjunctiva is primarily derived from the ophthalmic artery. The blood supply to the palpebral conjunctiva (the eyelid) is derived from the external 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.

Nerve supply

Sensory innervation of the conjunctiva is divided into four parts:

Microanatomy

The conjunctiva consists of unkeratinized, both stratified squamous and stratified columnar epithelium, with interspersed goblet cells. The epithelial layer contains blood vessels, fibrous tissue, and lymphatic channels. Accessory lacrimal glands in the conjunctiva constantly produce the aqueous portion of tears. Additional cells present in the conjunctival epithelium include melanocytes, T and B cell lymphocytes.

Function

The conjunctiva helps lubricate the eye by producing mucus and tears, although a smaller volume of tears than the lacrimal gland. It also contributes to immune surveillance and helps to prevent the entrance of microbes into the eye.

Clinical significance

Disorders of the conjunctiva and cornea are common sources of eye complaints, in particular because the surface of the eye is exposed to various external influences and is especially susceptible to trauma, infections, chemical irritation, allergic reactions, and dryness.

• The conjunctival microvascular hemodynamics are affected by diabetic retinopathy (DR), hence can be useful for DR diagnosis and monitoring, and discriminating stages of DR.
• Type II diabetes is associated with conjunctival hypoxia, increased average blood vessel diameter, and capillary loss.
• Sickle-cell anemia is associated with blood vessel sludging, altered blood flow and blood vessel diameter, and capillary micro-haemorrhages.
• Hypertension is associated with an increase in the tortuosity of bulbar conjunctival blood vessels and capillary and arteriole loss.
• Carotid artery occlusion is associated with slower conjunctival blood flow and apparent capillary loss.
• With age, the conjunctiva can stretch and loosen from the underlying sclera, leading to the formation of conjunctival folds, a condition known as conjunctivochalasis.
• The conjunctiva can be affected by tumors which can be benign, pre-malignant or malignant.
• Leptospirosis, an infection with Leptospira, can cause conjunctival suffusion, which is characterized by chemosis, and redness without exudates.

Bulbar conjunctival microvasculature

Vessel morphology

The bulbar conjunctival microvasculature contains arterioles, 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. The average bulbar conjunctival vessel has been reported to be 15.1 microns, which reflects the high number of small capillaries, which are typically

Blood oxygen dynamics

The bulbar conjunctival microvasculature is in close proximity to ambient air, thus oxygen diffusion from ambient air strongly influences their blood 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.

Blood vessel imaging methods

The bulbar conjunctival microvessels are typically imaged with a high-magnification slit lamp with green filters. With such high-magnification imaging systems, it is possible to see groups of individual red blood cells flowing in vivo. 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 and to measure blood oxygen saturation. Fluorescein angiography has been used to study the blood flow of the bulbar conjunctiva and to differentiate the bulbar conjunctival and episcleral microcirculation.

Vasodilation

The bulbar conjunctival microvasculature is known to dilate in response to several stimuli and external conditions, including allergens (e.g. pollen), temperature, time-of-day, contact-lens wear, and acute mild hypoxia. Bulbar conjunctival vasodilation has also been shown to correlate changes in emotional state.

Type 2 diabetes is associated with an increase in average bulbar conjunctival vessel diameter and capillary loss. Sickle-cell anemia is associated with altered average vessel diameter.

Additional images

Image:Gray893.png|Sagittal section through the upper eyelid File:Slide2www.JPG|Extrinsic eye muscle. Nerves of orbita. Deep dissection.

References

References

1. (2017). "Taylor and Hoyt's Pediatric Ophthalmology and Strabismus".
2. (May 2009). "In vivo confocal microscopy of the bulbar conjunctiva". Clinical & Experimental Ophthalmology.
3. Eye, human Encyclopædia Britannica
4. (July 1959). "The bulbar conjunctival vessels in occlusion of the internal carotid artery". A.M.A. Archives of Internal Medicine.
5. "Table 1: Summary of sensory nerve supply".
6. (2012). "Goldman's Cecil Medicine". Elsevier Saunders.
7. London Place Eye Center (2003). [http://www.lasereye.com/conjuc.htm Conjunctivitis] {{Webarchive. link. (2004-08-08 . Retrieved July 25, 2004.)
8. (April 2017). "Assessment of Conjunctival Microvascular Hemodynamics in Stages of Diabetic Microvasculopathy". Scientific Reports.
9. (July 2016). "Automated fine structure image analysis method for discrimination of diabetic retinopathy stage using conjunctival microvasculature images". Biomedical Optics Express.
10. (October 1986). "Conjunctival hypoxia in diabetes mellitus". Investigative Ophthalmology & Visual Science.
11. (September 1979). "Quantitative morphometry of conjunctival microcirculation in diabetes mellitus". Microvascular Research.
12. (1967-01-12). "The in vivo reactions of the small blood vessels to diabetes mellitus". Acta Medica Scandinavica. Supplementum.
13. (2001-10-01). "Microvascular abnormalities in the bulbar conjunctiva of patients with type 2 diabetes mellitus". Endocrine Practice.
14. (1968-01-01). "Vascular changes in the bulbar conjunctiva associated with sickle-cell disease: some observations on fine structure". Transactions of the American Ophthalmological Society.
15. (January 1987). "Effects of sickle cell anemia on conjunctival oxygen tension and temperature". Archives of Internal Medicine.
16. (August 2013). "Human bulbar conjunctival hemodynamics in hemoglobin SS and SC disease". American Journal of Hematology.
17. (November 1978). "Arteriolar rarefaction in the conjunctiva of human essential hypertensives". Microvascular Research.
18. (August 1955). "Anatomical and physiological aspects of the capillary bed in the bulbar conjunctiva of man in health and disease". Angiology.
19. "Conjunctivochalasis - Medical Definition". Medilexicon.com.
20. (January 1942). "Conjunctivochalasis". American Journal of Ophthalmology.
21. (January 2009). "Ocular surface tumors". Oman Journal of Ophthalmology.
22. (September 1956). "Blood vessels of the bulbar conjunctiva in man". The British Journal of Ophthalmology.
23. (March 2010). "Quantitative assessment of conjunctival microvascular circulation of the human eye". Microvascular Research.
24. (August 2016). "In vivo oximetry of human bulbar conjunctival and episcleral microvasculature using snapshot multispectral imaging". Experimental Eye Research.
25. (May 2014). "Orthogonal polarization spectral imaging of conjunctival microcirculation". Graefe's Archive for Clinical and Experimental Ophthalmology.
26. (July 2016). "Automated fine structure image analysis method for discrimination of diabetic retinopathy stage using conjunctival microvasculature images". Biomedical Optics Express.
27. (February 2016). "Automated Assessment of Hemodynamics in the Conjunctival Microvasculature Network". IEEE Transactions on Medical Imaging.
28. (January 2013). "Human conjunctival microvasculature assessed with a retinal function imager (RFI)". Microvascular Research.
29. (1988-01-01). "Patterns of blood flow in episcleral vessels studied by low-dose fluorescein videoangiography". Eye.
30. (1995-01-01). "Dynamics of external ocular blood flow studied by scanning angiographic microscopy". Eye.
31. (January 1987). "Low dose fluorescein angiography of the conjunctiva and episclera". The British Journal of Ophthalmology.
32. (September 1996). "Quantification of conjunctival vascular reaction by digital imaging". The Journal of Allergy and Clinical Immunology.
33. (June 2007). "Assessment of variation in bulbar conjunctival redness, temperature, and blood flow". Optometry and Vision Science.
34. (2013-11-01). "The Emotional Eye: Red Sclera as a Uniquely Human Cue of Emotion". Ethology.