Ординатура / Офтальмология / Английские материалы / Basic Sciences in Ophthalmology_Velayutham_2009
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Extraocular Muscles (Extrinsic Muscles) |
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Nerve supply: The superior oblique is supplied by the trochlear nerve, i.e. the fourth cranial nerve.
The superior muscular branch of the ophthalmic artery supplies the muscle.
Inferior Oblique
This is the shortest extraocular muscle. It is 37 mm long. If a line is drawn perpendicularly down from the supraorbital notch it will pass through the origin of this muscle which is just lateral to the upper most part of the nasolacrimal duct. From its origin the muscle goes backwards, upwards and laterally passing below the inferior rectus. Its tendon, which is just 1-2 mm long gets inserted into the posterior and temporal aspect of the eyeball. The width of the tendon is 9 mm. The anterior end of the insertion is 10 mm behind the lateral rectus. The posterior end is 1mm below and in front of the macula. It is close to the inferior vortex vein. The muscle forms an angle of 51 mm with the vertical plane of the globe.
Action
The inferior oblique is the elevator of the eye on adduction, opposite to that of superior oblique. It also abducts and extorts the eyeball. Along with superior rectus this muscle elevates the eyeball upwards.
The muscle is supplied by the inferior division of the oculomotor nerve and by the infraorbital artery and the medial muscular branch of the ophthalmic artery.
Fascias
Tenon’s capsule: The Tenon’s capsule is a condensation of fibrous tissue that covers the eyeball from the optic nerve to the cornea. At the limbus it is fused with the conjunctiva. In other areas there is a space between the two called the subconjunctival space. The space between the Tenon’s and the sclera is called the episcleral space. The capsule fuses with the orbital reticular tissue and the orbital fat. It is pierced by the extraocular muscles before their insertion where the Tenon’s capsule reflects over the muscle.
Development: The extraocular muscle develop from the myotonic cells of the mesoderm around the optic vesicle during the 5th week. The tendons fuse with the sclera by the 12th week.
Muscle sheaths and their extensions: Each muscle has an extracapsular portion and an intracapsular portion. The extracapsular portion is enveloped by a sheath. It is a reflection of the Tenon’s capsule. The sheaths of the four recti are connected by the intermuscular membrane. These sheaths form fibrous attachments to the orbit. They also support the globe and check ocular movements.
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The sheath of the superior rectus is attached to the levator.
Sheath of the inferior rectus has two layers. The upper one is attached to the Tenon’s. The lower one passes between the tarsus and orbicularis forming part of the Lockwood ligament.
Sheath of the superior oblique is attached to the levator, superior rectus and the Tenon’s.
The sheath of the inferior oblique fuses with that of the inferior rectus, lateral rectus and the optic nerve.
Suspensary ligament of lockwood: The sheaths of the inferior oblique and inferior rectus blend and join with the medial rectus and lateral rectus to form a hammock, which supports the eyeball. This tissue is called the ligament of Lockwood. Extensions from the ligament are to the:
•Tarsal plate
•Orbital septum
•Periosteum
•The floor of the orbit
Check ligaments: Connections are present from medial rectus and lateral rectus to the orbital walls. These are called the check ligaments. By this ligament the lateral rectus is attached to the zygomatic tubercle, to the lateral palpebral ligament and the lateral conjunctival fornix.
The check ligament of the medial rectus is attached to the lacrimal bone behind the posterior lacrimal crest and to the orbital septum. It is also connected to the sheath of the levator and superior rectus. Its inferior border fuses with extensions of inferior rectus and inferior oblique. The check ligaments of other muscles are not so well defined.
The episcleral tissue covers the intracapsular portion of the muscles. At the insertion this covering thickens to form the falciform fold of Guerin.
Role of fascial sheaths:
•To support the globe
•To protect the globe
•To form a smooth cavity with in which the eyeball can easily move
•To prevent retraction of globe
•To prevent movement of the globe in the direction of action of the muscle. (e.g. when the medial rectus contracts the visual axis must turn inward without the whole eyeball getting pulled towards the apex of the orbit)
•To keep the centre of rotation constant
•To limit the action of the contracting muscle and reduce the effect of relaxation of the opposing muscle
•To lessen the shaking up of the contents of the eyeball during sudden movements of the eye.
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Levator Palpebrae Superioris
The surface of lesser wing of sphenoid above and in front of the optic foramen gives origin to the levator. At its origin the tendon fuses with the superior rectus. The belly of the muscle is flat. It passes forward between the roof and the superior rectus to reach 1cm behind the septum orbitale. This position corresponds to the upper fornix and a few millimeters in front of the equator. The muscle now gives off a membranous expansion or aponeurosis which spreads like a fan and covers a wide area of the upper surface of the eye ball. The muscular part is horizontal whereas the tendinous part is vertical. Along with the upper lid the tendon moulds itself around the eyeball.
Insertion
a.Main insertion of the levator is to the skin of the upper lid at and below the upper palpebral sulcus. To reach the skin the muscle fibers naturally have to pierce the fibers of the orbicularis oculi.
b.Part of the aponeurosis is attached to the front and lower part of the tarsal plate. One must remember that the superior palpebral (Muller’s muscle) arising from the levator is attached to the upper border of the tarsus.
c. The facial sheathe of the muscle is attached to the upper fornix.
d.The ends of the aponeurosis are called the horns of the muscle. The lateral horn divides the lacrimal gland into its orbital and palpebral part and supports it. The aponeurosis is then attached to the orbital tubercle and to the upper aspect of the lateral palpebral ligament. The weaker medial horn is attached the bone below the fronto-lacrimal suture and to the medial palpebral ligament.
10 Cranial Nerves
Connected with the Eye
The optic nerve consists of:
a.Intraocular part
b.Intraorbital part
c. Intracanalicular part
d. Intracranial part (Figs 10.1 and 10.2)
Fig. 10.1: Optic chiasma
Intraocular part: The optic disc is the exit site of the axons of the ganglion cells of the retina. The portion that is visible ophthalmoscopically is the prelaminar portion of the optic nerve. It is situated 3 to 4 mm nasal to the fovea or the posterior pole. It is slightly vertically oval and is 1.5 mm/2 mm in size. The optic nerve head contains four types of cells, ganglion cell axons, astrocytes, capillary associated cells and fibroblasts. As there are no other layers of retina other than the nerve fiber layer it produces an absolute scotoma in the visual field called the blind spot of Mariotte. The central depression that is circular is called the optic cup. The larger the scleral canal larger will be the physiological cup. If the canal is small the cup also will be small. This type of disc is called ‘disc at risk’ because when the axons are affected due to any reason the resultant swelling will cause ischemia to a larger number of fibers. This is because whatever may be the size of the disc, the number of fibers
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Fig. 10.2: Relationship of optic tract
passing through are almost the same. The deepest axons, i.e. those close to the sclera occupy the peripheral optic nerve while the axons close to the vitreous occupy the center of the nerve.
The fibers of the optic nerve, which are one million in number, are grouped together into small bundles called the fascicles. These are axons of secondary neurons, i.e. part of the central nervous system. The axons are mostly concerned with vision. Few fibers are responsible for eye movements and pupillary reactions. A few fibers supply motor fibers to the blood vessels. They are supported by astrocytes. The Muller’s cells are replaced by astrocytes. The nerve fibers perforate the sclera making it sieve like. Hence, this part of the sclera is called lamina cribrosa. The nerve fibers become myelinated posterior to the lamina by the oligodendrocytes. This is similar to the Schwann cells of the peripheral nerves. But as the optic nerve is an extension of the white matter of the brain it is supported by oligodendrocytes. Sometimes the myelination of the optic nerve can extend on to the optic nerve head and rarely on to the retina also. This is more likely in premature births as light will help in myelination.
The laminar portion contains astrocytes along with elastic fibers and the collagenous connective tissue, which forms the scleral lamina. The lamina
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cribrosa present in this layer is a scaffold for the nerve fibers and a point of fixation for the retinal vessels. It also reinforces the posterior segment.
Blood Supply
The prelaminar portion and the lamina cribrosa are supplied by the branches of posterior ciliary arteries which are branches of ophthalmic artery. The optic nerve head is supplied by the retinal arterioles which are branches of the central retinal artery. The circle of Zinn-Haller which supplies blood to the intraocular portion of the optic nerve receives blood from choroidal feeder vessels, from short posterior ciliary arteries and a small contribution from the pial vessels. The radial branches from the circle of Zinn-Haller, that supply the laminar portion of the optic disc are autoregulated like the retinal blood vessels. Venous drainage is through the central retinal vein. Fluid can pass from within the eye to the subarachnoid space due to hydrostatic pressure. This is due to the fact the intraocular pressure is more than the optic nerve tissue pressure.
The disc vessels do not leak during fluorescein angiography. The staining of the disc seen during late phases of FFA is due to the choroidal capillaries, which are permeable to fluorescein and the scleral collagen, which will take up the leaked dye.
The posterior ciliary arteries are terminal branches. Hence, the ends of the capillaries do not anastomose with other capillaries. This creates a watershed zone between two capillary beds. When blood supply is affected this area is affected first. In cases like acute anterior segment ischemia of the optic nerve head portions of the nerve head which falls in this watershed area are affected.
The retrolaminar portion extends from the lamina cribrosa to the apex of the orbit.
Intraorbital Portion
This portion lies within the muscle cone. At the apex it is surrounded by the annulus of Zinn with the origin of the recti muscles. The length of the nerve in the orbit is about 3 cm, which is longer than the distance between the lamina cribrosa and the apex of the orbit. This is to facilitate free movement of the eyeball in various directions.
The orbital portion is supplied by the pial plexus and the intraneural branches of the central retinal artery. Ophthalmic artery lies below and lateral to the optic nerve. At the middle of the orbit it crosses under the optic nerve to lie medial to it.
Intracanalicular Portion
At the apex of the orbit the optic nerve enters the optic canal which is 5-10 mm long and 5-7 mm wide. Its bony wall is thinnest on the medial side. Here, the nerve is separated from the ethmoid and sphenoidal sinuses by this
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thin bone. The canal transmits the ophthalmic artery, which is on the lateral side, the filaments of the sympathetic plexus and the intracranial meninges around the optic nerve. The ophthalmic artery supplies the nerve. Small lesions arising within the canal will grossly affect the optic nerve. These lesions may not be picked up by even CT scans.
The dura and arachnoid around the optic nerve are attached to the periosteum making the nerve immobile. If any blunt injury occurs or, when there is edema of the optic nerve at this region the blood supply to the nerve is affected as this is a confined space.
Intracranial Portion
Posteriorly the nerve passes through an unyielding falciform fold and ascends at an angle of 45 degrees to join the optic chiasma.
The two optic nerves pass posteriorly over the cavernous sinus. Both nerves lie above the ophthalmic arteries and above and medial to internal carotid arteries. The anterior cerebral arteries, inferior surface of frontal lobe and olfactory tract lie above them. The two anterior cerebral arteries are joined together by the anterior communicating artery. The wall of the sphenoidal sinus, which is close to the inferomedial aspect of the nerve, is very weak. Hence, any inflammation of the sinus will affect the nerve, as the dura mater is absent here around the nerve.
This part of the optic nerve is supplied by the internal carotid and ophthalmic artery.
Meningeal sheathes: The pia mater covers the nerve from the retrolaminar portion, and ends in the intracranial part. It gives support to the nerve. The small arteriorles and venules present in the pia mater supply the nerve. The arachnoid mater continues with that of the brain. The dura mater is 0.3-0.5 mm thick. It has dense bundles of collagen and elastic tissue that fuse anteriorly with the outer layers of the sclera. The meninges have sensory nerve supply. Hence, optic neuritis can cause pain.
Development (Fig. 10.3)
The optic nerve develops from the optic stalk or the pedicle, which connects the optic vesicle and the forebrain. Its has a cavity which is connected to the cavity of the diencephalon posteriorly and the cavity of the optic vesicle anteriorly. The stalk contains neuroectodermal cells in the center and neural crest cells in the periphery. The stalk, which is initially short lengthens as the brain develops. As the fetal fissure closes the optic stalk becomes a rounded cord. The cavity is filled by the nerve fibers, which develop from the ganglion cells. Cells that were already in the stalk die to give room to these fibers. The nerve fibers first occupy the ventral and lateral portions. The glial tissue forms
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Fig. 10.3: Development of optic nerve head
around the 12th week. As the nerve fibers pass towards the optic stalk they displace some glial cells in the center of the disc. This is known as the Bergmeister’s primitive epithelial papilla. This is vascularized by the hyaloid artery. When this vessel atrophies the papilla also disappears. Remains of the papilla are seen very often.
Myelination of the nerve fibers start by the 28th week on the chiasmal side and proceeds towards the eye. It ceases at the lamina cribrosa at about 4 weeks after birth.
Chiasma: is situated just below the anterior part of the third ventricle, above the diaphragma sellae posterosuperior to the optic groove on the sphenoid. It is a flat oblong band 8 mm long, 12 mm wide and 4 mm thick. The posterior border of the chiasma is slightly higher than the anterior. In 80% of cases part of the hypophysial fossa is anterior to the chiasma. In 12% the fossa is behind the chiasma. When the intracranial portion of the optic nerve is shorter than 12 mm the chiasma will sit over the tuberculum sellae. Then it is called a prefixed chiasma. If the optic nerve is longer than 18 mm the chiasma will be over the dorsum sellae. Then it will be called postfixed chiasma.
The chiasma is about 10 mm above the sella. Hence, any tumor of the pituitary gland must be more than 10 mm in size to affect the visual fields.
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The anterior cerebral arteries lie anterior to the chiasma. The internal carotids lie on either side and the infundibulum (the stalk of the hypophysis) lies posterior. The third ventricle and the lamina terminalis lie above while the olfactory tract lies above and laterally. The hypophysis lies directly below and the cavernous sinus lies below and laterally. The chiasma is covered by the pia mater and the arachnoid is spread like an apron between the optic nerves.
55% of the fibers forming the chiasma decussate.
Optic Tract
It contains the uncrossed temporal and crossed nasal fibers from the optic nerves and the pupillary afferent fibers. The fibers from the upper part of the retina both crossed and uncrossed lie in the medial part of the tract. The macular fibers lie dorsolaterally. The tracts run backwards laterally and slightly upwards around the interpeduncular space. When the tracts go around the cerebral peduncles they turn slightly so that the part that was facing up faces upwards and medially. It is close to the anterior perforated substance and tuber cinereum. The tracts are separated from each other by the pituitary stalk inferiorly and the third ventricle superiorly. The tract then becomes part of the lateral surface of the cerebral peduncle. Here it lies between the internal capsule and basis pedunculi. The tract is attached to the brain by glial fibers. The anterior choroidal and posterior cerebral arteries lie below. The anterior choroidal artery crosses the tract twice. The optic tract crosses the third nerve and the pyramidal tract. Hence if there is a lesion in this region hemianopia is associated with motor and sensory loss also. As optic radiations are also close to the sensory and motor tracts in the posterior part of the internal capsule similar symptoms can occur. But the resultant hemianopia will be more congruous. Posteriorly the tract lies close to the roof of the inferior horn of the lateral ventricle. Here it develops a sulcus, which divides the tract into medial and lateral roots. The axons of the lateral root of the optic tracts terminate in the ipsilateral lateral geniculate ganglion.Asmall fascicle ends in the paraventricular nucleus of the hypothalamus. It is thought to send visual input to control diurnal rhythm. A slightly larger fascicle enters the medial geniculate nucleus and then terminates in the pretectal nucleus of the rostral midbrain and mediates the pupillomotor light reflex. The medial root enters the superior colliculus. This may be responsible for general reflex responses to light.
The optic tracts are supplied by the anterior choroidal arteries, branches of internal carotid arteries (Fig. 10.4).
Lateral Geniculate Nucleus
This is the thalamic visual center linking the retina and the striate cortex. It is situated in the lateral geniculate body. It is an oval or cap like structure resembling a closed fist situated in the posterior aspect of the thalamus, hidden by
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Fig. 10.4: Blood supply to visual pathway
the pulvinar. It is below and lateral to the pulvinar and above the lateral recess of the ambient cistern. Because of its location it is difficult to visualize LGN by neuroimaging. The optic radiations are situated dorsolaterally with the internal capsule on the lateral side. The acoustic radiations are present dorsomedially. The hippocampal gyrus and the inferior horn of the lateral ventricle are present posteriorly (Fig. 10.5).
Fig. 10.5: Lateral geniculate body