Ординатура / Офтальмология / Английские материалы / Basic Sciences in Ophthalmology_Velayutham_2009
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The central retinal artery then bends forwards, and with the vein on its lateral side pierces the sclera at the lamina cribrosa to reach the retina. It then divides in to superior and inferior retinal vessels to supply the retina.
Branches
a.When the vessel is within the optic nerve branches are given off to pial plexus.
b.A collateral branch, which joins the pial network, gives a branch, which runs backwards and supplies the macular fibers in the optic nerve.
c. The two terminal branches superior and inferior seen in the retina divide in to a smaller nasal and a bigger temporal vessel. This happens either at the margin of the disc or in the nerve itself. The nasal vessels run radially while the temporal vessels take an arcuate course. The retinal vessels divide dichotomously and do not have any anastomosis. They supply the retina up to the inner nuclear layer. The retina behind this is avascular and is supplied by the choroidal vessels. The capillaries are present in the inner nuclear and ganglion cell layers. The capillaries are two-layered in most part of the retina. Close to the disc they are four layered, as the retina is thick at this level. The endothelium of retinal capillaries is non-fenestrated.
The circle of Zinn-Haller is situated close to the optic nerve. It is formed by the anastomosis between the short ciliary arteries after they have pierced the sclera. From this plexus vessels pass to the choroid, optic nerve at the level of lamina cribrosa and to the pial plexus. It also supplies the retina around the optic nerve head.
The cilio retinal artery a branch of this anastomosis enters the eye on the temporal side of the disc close to its edge and supplies the macula.
The retinal veins follow the pattern of the arteries and the central retinal vein is formed at the level of the lamina cribrosa posterior to the division of the artery. It lies temporal to the artery and drains either directly in to the cavernous sinus or in to the superior ophthalmic vein.
2.Posterior ciliary arteries: When the ophthalmic artery is still below the optic nerve it gives off two posterior arteries. It divides into 10-20 branches. Of these two large branches called the long posterior ciliary arteries pierce the sclera on the medial and lateral side of the nerve. They run forwards in the supra choroidal space and supply the ciliary body and forms the major arterial circle of the iris by anastomosing with the anterior ciliary arteries. The other branches called short ciliary arteries pierce the eyeball around the optic nerve and supply the choroid.
3.Lacrimal artery: Runs along the upper border of the lateral rectus and supplies the lacrimal gland.
4.Lateral palpebral: Branches arise from the lacrimal artery and supply the conjunctiva and eyelids on the lateral side.
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5.Recurrent meningeal artery: Anastomoses with the middle meningeal artery, which belongs to the external carotid system.
6.Muscular branches: Are two in number. The lateral branch supplies the lateral rectus, superior rectus, levator and the superior oblique. The medial branch supplies the inferior and medial recti and the inferior oblique.
7.Anterior ciliary arteries: Arise from the muscular branches. They form a subconjunctival network around the cornea and enter the suprachoroidal space and anastomose with the posterior ciliary arteries.
8.Supra orbital artery: Branches off the ophthalmic artery above the optic nerve and along with the supraorbital nerve reaches the space under the frontalis muscle by passing through the supraorbital foramen. This vessel supplies the upper lid, scalp, levator, periorbita and the diploe of the frontal bone.
9.Medial palpebral arteries: The palpebral arteries of the upper and lower lid anastomose with the lateral palpebral vessels and form arcades in the submuscular layer of the lids. They supply the structures in the lid and the conjunctiva.
10.Posterior ethmoidal artery: Supplies the posterior ethmoidal air sinus.
11.Anterior ethmoidal artery: Supplies the anterior ethmoidal air sinus, dura mater of the anterior cranial fossa and enter the face between the nasal cartilage and the nasal bone to supply the skin of the nose.
12.Dorsalis nasal artery: Is situated above the medial palpebral ligament and supplies the skin over the root of the nose and lacrimal sac. It anastomoses with the facial artery.
13.Supratrochlear artery: It reaches the forehead by curving around the medial end of the superior orbital margin and supplies the medial part of the forehead.
14.Episcleral and conjunctival arteries and other small branches arise from the larger branches.
CEREBRAL VESSELS SUPPLYING THE VISUAL PATHWAY
Branches of Internal Carotid
1.The anterior cerebral artery supplies the intracranial portion of the optic nerve and the upper surface of the chiasma.
2.Middle cerebral artery supplies the infero lateral aspect of the chiasma and the anterior part of the optic tract. The deep optic branch supplies the optic radiation close to the internal capsule. Its terminal branches supply the cerebral cortex representing the macular area.
3.Posterior communicating artery runs backwards from its origin and joins the posterior cerebral artery a branch of the basilar artery (anastomosis between the internal carotid and vertebral system). It supplies the under surface of the chiasma and anterior 1/3rd of the optic tract.
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4.Anterior choroidal artery supplies the internal capsule, chiasma, posterior 2/3rd of optic tract, anterior and lateral aspect of lateral geniculate body and optic radiation at its origin.
Circle of Willis is formed by the anastomosis of the two internal carotids with the basilar. It lies in the subarachnoid space around the structures in the interpeduncular cistern.
Basilar artery is formed by the union of the two vertebral arteries. It gives off pontine branches supplying the cranial nerve nuclei present in the pons and the posterior cerebral arteries formed by the bifurcation of the basilar artery. These vessels supply the lateral geniculate ganglion, visual cortex and the posterior part of the optic radiation.
Venous Drainage
The blood from the orbit and its contents is drained into the cavernous sinus through the superior and inferior ophthalmic veins.
Superior ophthalmic vein: Starts at the root of the nose. After the supra-orbital vein joins the angular vein, it gives a communication which joins the superior ophthalmic vein. Its tributaries are
a.Inferior ophthalmic vein
b.Anterior and posterior ethmoidal veins
c. Muscular veins
d.Lacrimal veins
e.Central retinal veins (may drain into the cavernous sinus)
f.Anterior ciliary veins
g.Venae vorticosae
The inferior ophthalmic vein starts in the floor of the orbit in its anterior
part. It runs on the lateral rectus and joins the cavernous sinus either on its own or after joining with the superior ophthalmic vein. It receives blood from the inferior vortex veins, lacrimal sac and the inferior and lateral rectus. It communicates with the pterygoid plexus through the inferior orbital fissure.
Vortex veins: The four venae vorticosae drain blood from the choroid. There is no corresponding veins to posterior ciliary arteries. The small veins from the optic nerve head also join the choroidal veins. The veins that join together to form the vortex veins have radial and curved branches, which give it a whorled (vortex) appearance. These veins drain blood from the choroid, optic nerve head, iris and ciliary body. When they enter the sclera the veins develop an ampulliform dilatation. Then they pierce the sclera on either side of superior and inferior rectus. They are 6mm or more behind the equator. The superior branches are more behind the equator. The lateral branches are closer to the mid-vertical plane compared to the medial. The superior lateral vein is 8 mm behind the equator and is the most posterior. It is close to the tendon of the superior oblique. The inferior lateral vein is the most anterior and is 5.5 mm
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behind the equator. As they pierce the sclera obliquely, they form a canal which is 4 mm long. The canal runs posteriorly towards the midvertical plane of the eye. The veins can be seen through the sclera as dark lines. The superior branches drain into the superior ophthalmic vein and the inferior vessels drain into the inferior ophthalmic vein.
Angular Vein
This vein is situated about 8 mm medial to the medial canthus. It is close to the medial edge of the medial canthal ligament. It is frequently encountered during surgery for dacryocystitis. It continues below as the facial vein. It receives blood from the supraorbital vein, supra trochlear, the superficial veins from the skin of the nose and the superior and inferior palpebral veins. The superior palpebral vein crosses the medial palpebral ligament between the angular vein and medial canthus. The facial vein accompanies the artery and runs downwards and backwards across the face. It joins the retromandibular vein to form the common facial vein, which joins the internal jugular sinus.
Cavernous Sinus
The cavernous sinuses are situated on either side of the hypophysis and body of the sphenoid in the middle cranial fossa. They extend from the medial end of the superior orbital fissure to the petrous part of the temporal bone. It is formed by the splitting of the dura mater and is lined by endothelium. The vein is traversed by fibrous tissue giving it a spongy or cavernous appearance.
The internal carotid passes through the carotid canal and lies between the lingula and petrosal process of the sphenoid. It runs forwards in the groove on the body of the sphenoid and reaches the medial side of the anterior clinoid process. It then pierces the roof of the cavernous sinus. In the sinus, it is surrounded by the sympathetic plexus and is closely related to the sixth cranial nerve, which passes with in the sinus to reach the superior orbital fissure.
The 3rd and 4th cranial nerve and the ophthalmic branch of the fifth nerve travel in the lateral wall of the sinus to reach the superior orbital fissure. The maxillary branch is also present in the lateral wall but it exits the skull through the foramen rotundum. The temporal lobe is present lateral to the sinuses. The trigeminal ganglion is also present on the lateral side posteriorly.
Both sinuses are connected to each other by the intercavernous plexes, which are situated in front and behind the pituitary gland. Any infection in the face can spread to the cavernous sinus and produce thrombosis as the ophthalmic vein, which drains into the cavernous sinus is connected to the angular vein. Because of the intercavernous plexes the infection spreads to the other sinus also.
The ophthalmic veins, sphenoparietal sinus, inferior cerebral vein, central retinal vein and the emissary veins drain into the sinus.
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The cavernous sinus drains into the superior petrosal sinus, which drains into the transverse sinus. The inferior petrosal sinus connects the cavernous sinus to the internal jugular vein below the base of skull. As this sinus drains blood from the inner ear, infection of this region can affect the cavernous sinus. As the mastoid emissary veins are affected edema is seen in the mastoid region.
The pterygoid plexus also drains blood from the cavernous sinus.
Development of the Vascular System
During the fourth week of gestation vascular channels develop in the mesenchyme around the optic vesicle. The vessels develop from the internal carotid. Initially, a dorsal and a ventral artery develop and join the capillaries developing around the optic vesicle. Capillary plexes drain into the developing carotid sinus. A transient vessel called stapedial artery develop from the carotid to supply the orbit. This joins the ophthalmic artery.
The hyaloid artery develop from the dorsal ophthalmic artery when the embryonic fissure starts closing. At the level of the rim of the optic cup both dorsal and ventral vessels give branches and develop an annular vessel. The hyaloid artery grows forward and around the lens to join the annular vessel and supply the different parts of the eye. The dorsal ophthalmic artery becomes the ophthalmic artery. During the sixth week the temporal long ciliary artery, short ciliary arteries and the central retinal artery develop.
An arterial circle forms in front of the annular vessel from the long ciliary artery. This forms the major arterial circle of the iris. Radial vessels develop from these two circles to form the pupillary membrane over the iris and lens. Central part of this membrane disappears. The peripheral part forms the minor arterial circle of the iris.
The retinal vessels develop from the hyaloid artery during the sixteenth week (fourth month) of gestation and grows centripetally. Vascularisation of the nasal retina is complete before the temporal as the nasal ora is nearer to the optic disc. The capillaries reach the ora by the eight month but maturation of vessels continues up to three months after birth. The hyaloid system and tunica vasculosa lentis disappear in the third trimester.
PATHWAYS CONNECTED TO THE VISUAL SYSTEM
Light Reflex
When light falls on the retina the outer portions of the rods and cones are stimulated. These are the receptor organs for the light reflex. The impulses are then sent to the bipolar cells and then to the ganglion cells of the retina. From ganglion cells the reflex passes through the optic nerve to the optic tract and then they reach the visual cortex through the optic radiations.
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Pupillary reflex: The fibers for the pupillary reflex leave the posterior part of the tract and without entering the lateral geniculate body run superficially in the superior brachium and reach the pre tectal nucleus. The fibers for pupillary reflex terminate here. Some of the fibers emerging from the nucleus cross in the posterior commissure ventral to the aqueduct. The fibers run along with the medial longitudinal fasciculus to reach the Edinger Westpal nucleus which controls the sphincter pupillae muscle. From here the fibers for the pupillary reflex passes along with the third nerve. In the orbit the fibers travel with the branch to the inferior oblique. The preganglionic, para-sympathetic myelinated fibers separate from this branch and enter the ciliary ganglion. From this ganglion the fibers reach the eyeball through the short ciliary nerves to innervate the sphincter pupillae and ciliary muscle. These postganglionic fibers are myelinated. This helps in getting quick reactions.
Pathway for Dilatation
The dilator fibers leave the lateral column of the spinal cord at the junction of the thoracic and cervical regions through the four upper thoracic nerves. They reach the cervical sympathetic chain and travel upwards. The fibers synapse in the superior cervical sympathetic ganglion. The postganglionic fibers run upwards with the sympathetic plexus around the internal carotid artery. The fibers for the pupil leave the plexus to reach the trigeminal ganglion. They then run with the nasociliary nerve and enter via the long ciliary nerves to innervate the dilator muscle fibers.
Paralysis of sympathetic fibers will cause ptosis, miosis and enophthalmos. This is called Horner’s syndrome. In this condition both the pupils will be equal in size in bright light. But in the dark the affected pupil will not dilate giving rise to anisocoria.
The sympathetic fibers control the blood circulation in the eye.
Accommodation Reflex
The power of the human lens has to change for us to see clearly at various distances. Besides this to see the near objects the eyes must converge to fixate on them. The pupils also must constrict. These three changes bring about accommodation.
The afferent pathway for this is the visual reflex. The reflex for convergence, which may involve the visual cortex, stimulates the proprioceptive impulses in the medial recti muscle. Through the inferior division of the V nerve it reaches the mesencephalic nucleus of the trigeminal nerve. From here the fibers pass to the center for constriction of the pupil, i.e. the EdingerWestpal nucleus. The fibers travel with the oculomotor nerve and then without entering the ciliary ganglion it reach the accessory ganglion and then the sphincter pupillae.
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Clinical Aspects
a.Lesions at the level of the superior brachium will result in loss of light reflex but accommodation reflex will be spared. (AR pupil)
b.AR pupil can result due to lesions affecting the pupillary fibers before they reach the III nerve nucleus.
Total pupillary paralysis can result from supranuclear lesion, nuclear or infranuclear lesion in the nerve, in the ciliary ganglion or short ciliary nerves.
Embryogenesis
Development from the Neural Plate
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Organo Genesis of the Eye
Neural Crest Cells
Neural crest cells make a major contribution to the connective tissue components of the eye and orbit. But the extraocular muscles and the blood vessels develop from the mesoderm only. Neural crest cells arise from the neurectoderm located at the crest of the neural folds just before the folds fuse to form the neural tube. They migrate to different parts of the embryo where differentiation will occur due to local factors.
Mesenchymal cells of the facial primordia are derived from the neural crest cells. The neural crest cells in the mesenchephalic region move forward to form the maxilla and mandible. The cells from the diencephalon form the frontonasal mass. These cells along with the cells from the anterior midbrain settle around the optic vesicles. The migration of neural crest cells are promoted by fibronectin and inhibited by proteoglycons.
Any abnormalities that involves the tissues that develop from the neural crest cells are called neurocristopathy. These congenital anomalies result either from faults in migration or differentiation. Anomalies in the anterior segment are found to be associated with dental malformation, middle ear deafness, malformations of the skull, shoulder girdle and upper spine. Malformations of
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the median face cause hypertelorism due to impaired midline coalescence of the frontonasal process.
Initially, when the globe develops the axes between the two eyes are at an angle of 180°. By 12th week, it becomes 105. As the head develops, it reduces further to become 71° just before birth. By the age of three it is 68.
CONGENITAL ANOMALIES (Fig. 11.3)
Fig. 11.3: Congenital anomalies
As in any organ teratogenic influence affecting the foetus at a very stage of development will cause severe damage to the eye. In these cases, the surrounding tissues also will be affected. It the damage occurs later the lesion will be more localized.
Developmental anomalies fall into three categories. Developmental arrest, abnormal differentiation or both.
If the development is affected very early the optic vesicle will not grow fully. This will cause gross anomalies.
E.g. Anophthalmos: though the term means total absence of the globe very often a rudimentary eyeball will be seen. This condition is bilateral.
Microphthalmos: This may be associated with other anomalies like persistence of primary vitreous or incomplete closure of the embryonic fissure. Leucoma, cataract and poorly developed retina, and optic nerve also will be seen.
Nanophthalmos: Here the size of the eye is less than normal. But the eyes will otherwise be normal except for hypermetropia, thick sclera and deep-set eyes. Mode of inheritance: Both autosomal dominant and recessive.
Cyclopia: Here, there is a single eye in the middle of the face. There are gross deformities and the child will not survive. In synophthalmia two incomplete globes are seen fused together.
Cystic eye: In true cystic eye, the optic vesicles remain as cysts without any further development. The skin covering the cyst does not show any eyelid formation.
In cystic coloboma, when there is incomplete closure of fetal fissure. Hence, there is a cyst with a small, deformed eyeball.
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Orbital teratomas as in other teratomas contains all the three germinal layers. Single or multiple cysts are also found.
An encephalocoele also may occupy the orbit but this lesion will be pulsatile. Though the bony defect is present at birth the encephalocoele may develop later.
In cryptophthalmos, the eyelids do not develop properly. This defect may be complete or partial. The eyeballs are covered by skin. As the lids fail to form the exposed corneal and conjunctival epithelium undergo metaplasia into skin. There is absence of lashes, brows and sometimes even the lacrimal gland. The cornea is absent and the lens will be poorly developed. The retina and optic nerve are likely to be developed normally.
In an abortive form, the upper lid is replaced by a fold of skin that is adherent to the cornea. The lower lid will be normal in these cases.
Though the eyeball appears to move if an incision is made into the skin the eyeball will be opened up. This condition can occur unilaterally also.
Inheritance: Autosomal recessive.
In ankyloblepharon the upper and lower lids are formed but fused. This may be complete or partial. The cornea is normal in these cases. This can be differentiated from cryptophthalmos by the presence of lashes and brows. If the lids are incised the patient can get some useful vision but the lids may fuse together again.
Differentiation of anterior segment structures occurs mainly between sixth and sixteenth week. Any factors affecting growth during this period will cause anterior segment anomalies.
Congenital Anomalies of the Cornea
Any cornea that is smaller than 9 mm at birth in an otherwise normal eye, is called microcornea. Inheritance is autosomal dominant or recessive. Unlike sclerocornea and cornea plana a normal limbus is present here. If it is associated with a small eye, it is called microphthalmos.
If the cornea is larger than 12 mm it is called megalocornea. This condition is not associated with glaucoma and the cornea is histologically normal. 90% of the affected persons are males. This condition is usually X linked recessive but may be inherited as autosomal dominant or recessive manner. The gene is located in the X12-q 26 region. This anomaly is due to inadequate growth of the anterior tip of the optic cup. This leaves a large gap, which is filled by the cornea.
Sometimes, the peripheral part of the cornea appears like sclera with a variable size of clear cornea in the centre. This is called sclero cornea.
If the corneal curvature is very low, around 38 D it is called cornea plana. Here the curvature is similar to sclera. The anterior chamber will be shallow but glaucoma is not common. During the sixteenth week of gestation the