Ординатура / Офтальмология / Английские материалы / Basic Sciences in Ophthalmology_Velayutham_2009
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Cranial Nerves Connected with the Eye |
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In Raymond Cestan syndrome VI nerve is affected with hemiplegia. Lesions in the ventral paramedian pons affects the cortico spinal tract
along with VI and VII nerves causing ipsilateral lateral rectus palsy with contra lateral hemiplegia.
It exits the brain at the junction of the pons and medulla. The nerve travels upwards on the ventral surface of the pons lateral to the basilar artery. The anterior inferior cerebellar artery crosses the nerve. On the surface of the clivus, the nerve is surrounded by Batson’s venous plexus. As the nerve has got a long course it is frequently involved in many conditions. As the nerve is bound to the ventral surface of the pons by the anterior inferior cerebellar artery, it may be compressed by the vessel. Atherosclerotic, posterior inferior cerebellar artery and basilar artery also can affect the nerve.
As the nerve ascends vertically in the subarachnoid space it is vulnerable. Any descent of the brain stem following injury to the head, space-occupying lesions above the tentorium will cause sixth nerve palsy. The nerve gets stretched where the nerve exits from the pons and at the clivus. Meningitis also can affect the sixth nerve in this region along with other cranial nerves.
False localising sign follow increased intracranial tension due to the nerve being compressed between the pons and the basilar artery or the clivus or due stretching at the sharp edge of the petrous temporal bone.
About 2 cm below the posterior clinoid process and below the crest of the petrous part of the temporal bone it is closely connected with the inferior petrosal sinus. Thrombophlebitis of the lateral sinus can extend to the inferior petrosal sinus and affect the sixth nerve.
The nerve runs either through the sinus or around it and then under the petro clinoid (Gruber’s) ligament. Here it is close to the mastoid air cells and hence can be affected by mastoiditis and otitis media. The inflammation may cause meningitis at the level of the tip of the petrous part of the temporal bone causing Gradenigo’s syndrome. In this syndrome the Gasserian ganglion and the facial nerve are also affected. This area under the ligament is called the Dorello’s canal.
The nerve then enters the cavernous sinus and runs below and lateral to the internal carotid artery. The sympathetic fibers from the paracarotid plexus may run along with this nerve. As it is located close to the internal carotid vascular lesions affect the sixth nerve first. The sympathetic plexus around the carotid will also be affected. When lesions occur close to the spheno palatine fossa VI nerve, spheno palatine ganglion (loss of tears) and maxillary division of the fifth cranial nerve will be affected together.
It enters the orbit through the middle portion of the superior orbital fissure. After a short course in the orbit it supplies the lateral rectus. Due to the short course isolated lesions do not usually occur at this level. All the fibers ramify in the orbit and enter the muscle in a diffuse fashion.
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Supra Nuclear Connections of the Ocular Motor System
The normal smooth movement of the eyeballs involves the neurons of the third, fourth and sixth nerves on either side or the internuclear pathways in the brain stem. Conjugate gaze involves both visual and auditory systems.
The posterior part of the right middle frontal gyrus is responsible for conjugate deviation of the eyes to the left side. Stimulation of the area above the central part will result in downward movement and the area below will result in upward movement.
The occipital center is responsible for moving the eye to focus on the object of interest. Stimulation of this area will also result in conjugate deviation.
Inter Nuclear System
This is called the medial longitudinal fasciculus. This tract extends from the oculomotor nucleus to the spinal cord. It lies close to the median line beneath the aqueduct and the fourth ventricle. It also lies close to the oculomotor nuclei lateral to the third and fourth but medial to the sixth.
The fibers in the tract are:
a.the vestibular nuclei
b.ipsilateral sixth nerve connecting to the contra lateral subnucleus of the medial rectus.
All three oculomotor nuclei will be affected if the medial longitudinal bundle is affected.
The fibers of MLF synapses with nucleus of Darkschewitsch, interstitial nucleus of Cajal and rostral interstitial nucleus of MLF. Lesions of interstitial nucleus of Cajal will affect the vertical pursuit and vertical gaze holding.
Immediate Premotor Structures of the Brain Stem
Consists of the abducens nuclei and the paramedian pontine reticular formation for horizontal gaze and rostral mesencephalic reticular formation for vertical gaze.
Paramedian Pontine Reticular Formation
Is located near the abducens nucleus within the pons. Lesions of PPRF cause paralysis of conjugate gaze to the ipsilateral side. If the sixth nerve alone is affected abduction to the ipsilateral side only is affected. Interconnections, both afferents and efferents exist between PPRf and vestibular nuclei, reticular formation, spinal cord and cerebellum. The main projections are to ipsilateral abducens and to rostral intestitial nucleus of MLF, which is concerned with vertical gaze.
The premotor commands for horizontal gaze are sent from the abducens nuclei to ipsilateral lateral rectus and contralateral medial rectus through the internuclear neurons ascending in the contralateral MLF.
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Rostral Interstitial Nucleus of the MLF
Is a group of cells situated on either side of midline in the rostral mesencephalon. It has projections to oculomotor and trochlear nerve nuclei for vertical recti and oblique muscles. It causes vertical saccadic movements.
Nucleus of the Posterior Commissure
Lesions of this nucleus, which is situated in the caudal end of diencephalon, on its dorsal side will affect upward movements.
Cerebral Cortex
Movements both voluntary and involuntary are represented in the cerebral cortex. The cerebral regions involved are:
a.Frontal eye fields in the frontal cortex — posterior part of the second frontal convolution area 8 of Brodman. Stimulation of this area results in conjugate ocular movements to the opposite side. The fibers from the cortex pass through the posterior part of the anterior limb of the internal capsule near the genu to the medial part of the cerebral peduncle.
b.Internal sagittal stratum in the parieto occipital region.
Fibers from area 8 also project to layer IV of superior colliculus and pretectal
region, contra lateral pontine tegmentum, thalamic nuclei, medial pulvinar and PPRF and the oculomotor nuclei.
Parieto Occipital Eye Fields
Pursuit movements originate here. The eyes deviate to the opposite side when this area is stimulated.
Superior colliculus and thalamus: The superior colliculi project from the upper part of the mesencephalon. It contains alternating layers of cells. The superficial layers are involved in visual sensory function. The deep layers are involved with ocular movements. It receives projection from the other supranuclear structures associated with eye movements. The deepest layer consists of efferent fibers.
Vestibular System
The vestibular impulses that affect the vertical gaze fibers from ipsilateral vestibular nuclei pass to the brain stem on the opposite side. It then ascends in the medial longitudinal fasciculus to the oculomotor subnuclei involved in vertical movements.
For horizontal vestibular ocular impulses there is a projection to the contralateral abducens nucleus and ipsilateral medial rectus. This is through the ascending tract of Dieter’s and the contra lateral MLF.
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TRIGEMINAL NERVE
The trigeminal or the fifth cranial nerve has both sensory and motor fibers. It carries sensory nerve supply to the scalp, forehead, face, lids, eye, lacrimal gland, extraocular muscles, ear, dura mater and the tongue. The mandibular division supplies the muscles of mastication.
The nuclear complex is very long extending from midbrain to the upper cervical segments up to C4.
The mesencephalic nucleus: mediates proprioceptive and deep sensation from masticatory, facial and extraocular muscles.
Main Sensory Nucleus
Is situated in the pons lateral to the motor nucleus. It is continuous above with mesencephalic nucleus and below with the spinal nucleus. It serves light touch from skin and mucus membrane. The sensory root of the fifth cranial nerve divides into ascending and descending tracts at the level of the pons. Ascending root ends in the main sensory nucleus and the descending root ends in the spinal nucleus.
Spinal nucleus and tract: extends from the medulla to C4. It receives pain and temperature afferents. It also carries cutaneous components of VII, IX and X from the ear. The sensory fibers of ophthalmic division enter the ventral portion of the nucleus. Fibers from perioral and perinasal areas end rostrally.
Peripheral face and scalp end caudally.
Midface enters the central portion of the nucleus.
So if the sensory loss is bilateral and affects one of the concentric circles it must due to a lesion in the brain stem. If distribution of a particular division is affected the lesion must be after the exit of the nerve from the brain stem.
The axons from the nuclei terminate at the ventral postero medial nucleus of the thalamus after crossing at the level of pons. The axons from the nucleus then reach the postcentral gyrus of the cerebral cortex through the internal capsule.
Motor Nucleus
Lies medial to the sensory nucleus. It receives fibers from both the cerebral hemispheres, the reticular formation, red nucleus, tectum, medial longitudinal bundle and mesencephalic nucleus.
The fifth nerve emerges from the upper lateral portion of the ventral pons, passes over the petrous apex, forms the Gasserian ganglion and then divides in to three branches—ophthalmic, maxillary and the mandibular.
The Gasserian ganglion is situated in the Meckel’s cave, a recess in the duramater at the apex of the petrous part of the temporal bone, in the middle cranial fossa. The cavernous sinus lies anteromedial to the ganglion. The internal
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capsule lies close to the ganglion on its medial aspect. The ganglion contains the cells of origin of all the sensory axons of the fifth cranial nerve.
Ophthalmic Division
Travels along the lateral wall of the cavernous sinus lateral to internal carotid artery. At this position it lies beneath the third and fourth nerve. It supplies sensory nerve supply to the cerebral vessels, duramater of the anterior cranial fossa, cavernous sinus, sphenoidal wing, petrous apex, Meckel’s cave, tentorium cerebelli, falx cerebri and dural venous sinuses. It divides into three branches the frontal, lacrimal and nasociliary. The frontal and lacrimal enters the orbit through the lateral portion of the superior orbital fissure while the nasociliary enters through the central portion (Fig. 10.14).
Fig. 10.14A: Sensory distribution of supranuclear V nerve
Fig. 10.14B: Nucleus of V nerve
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Frontal Nerve
Divides into supraorbital and trochlear nerve and provides sensation for the medial portion of the upper lid and conjunctiva, forehead, scalp, frontal sinus and the side of the nose.
Lacrimal Branch
Supplies the lateral portion of the upper lid and conjunctiva, lacrimal gland and carries postganglionic parasympathetic fibers for reflex lacrimation.
Pathway for Reflex Tearing
The Naso Ciliary Nerve
Supplies sensation through nasal branches to the middle and inferior turbinates, septum, lateral nasal wall and tip of the nose.
Its supratrochlear branch supplies the conjunctiva and skin on the medial side besides the lacrimal sac and the nasolacrimal duct.
The long ciliary nerves carry sensory fibers from the ciliary body, iris, cornea and sympathetic innervation to the dilator muscle.
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The short ciliary nerves carry sensation from the globe. The ciliary nerves also carry postganglionic parasympathetic fibers from the ganglion to the pupillary sphincter and the ciliary muscle.
Hutchinson’s sign: If the tip of the nose is involved in Herpes zoster the eye is likely to be involved as the naso ciliary supplies the tip of the nose and the eye.
Maxillary Division
Exits the skull through the foramen rotundum. It reaches the infraorbital fissure after crossing the pterygo palatine fossa and runs as the infraorbital nerve along the floor of the orbit. It occupies the infraorbital canal and exits the orbit through the infraorbital foramen.
This branch supplies the lower lid, side of the nose, upper lip, the teeth, maxillary sinus, roof of the mouth and soft palate. Lesions in any of these areas can cause referred pain in the eye.
Mandibular Division
Exits through the foramen ovale and supplies muscle of mastication and skin of the mandible, buccal mucosa, lower lip, tongue, external ear and tympanum.
FACIAL NERVE
The motor area for the facial nerve is situated in the precentral gyrus of the frontal cerebral cortex. The fascicles from here pass through the genu of the internal capsule and cerebral peduncles to the pons. The facial nucleus that supplies the frontalis, orbicularis and corrugator receive fibers from both right and left side but the lower face receives only crossed fibers. Hence supranuclear lesions will involve only the contralateral lower face.
The nucleus of the facial nerve contains 7000 nuclei and is located in the lower pontine tegmentum in the ventrolateral angle. It has four groups of nuclei
a.auricular and occipital areas receive fibers from the dorso medial nucleus
b.frontalis, corrugator and orbicularis oculi receive fibers from intermediate
nucleus
c. platysma from ventromedial
d. buccinator and buccolabial from the lateral group of nuclei.
The axons exit from the nucleus dorsally and the fibers loop around the nucleus of the sixth nerve to form the facial colliculus. It then emerges from the lateral side of the pons. The parasympathetic fibers for the sublingual, submandibular and lacrimal glands arising from the superior salivatory nucleus forms the nervus intermedius. These two nerves along with the VIII cranial nerve exit from the ventrolateral aspect of the pons. This occurs at the cerebello pontine angle. Near the internal auditory meatus the nervus intermedius joins
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the facial nerve. The eighth cranial nerve is closely situated and will be affected in any lesions affecting the facial nerve at this level. The secretion of saliva and tears also will be affected. A tumour at this position can also affect the V, 9th, 10th and 11th nerve. Vascular compression here will cause hyperkinesia of the facial muscles.
The facial nerve enters the temporal bone through the internal auditory meatus and passes through the fallopian (facial) canal, which is 28-30 mm long. There are three segments here the labrynthine, tympanic and mastoid segments. The geniculate ganglion belongs to the labrynthine part. Here the facial nerve gives off its first branch the greater superficial petrosal nerve. This branch travels along the floor of the middle cranial fossa to synapse in the spheno palatine ganglion. The postganglionic fibers travel with the zygomatico temporal branch of the fifth cranial nerve (maxillary) and join the lacrimal nerve (nasociliary) to supply secretory fibers to the lacrimal gland (Fig. 10.15).
Fig. 10.15: Nuclei of cranial nerves
At the geniculate ganglion the facial nerve turns posteriorly to form a genu to enter the tympanic portion of the facial canal. It then emerges from the middle ear below the incus between the auditory and horizontal semicircular canals. Here the facial nerve turns downwards. The terminal branch of the nervus intermedius called the chorda tympani arises here. This nerve supplies the submandibular and sublingual salivary glands and sensory fibers for the external auditory meatus and taste from the anterior 2/3rds of the tongue. The nerve then exits through the stylo mastoid foramen behind the mandibular angle. It gives motor branches to the posterior belly of the digastric, stylo hyoid and posterior auricular muscles. The main trunk enters the parotid gland and divides in to upper and lower branches, which again divide into temporal, zygomatic, buccal, mandibular and cervical branches.
Pituitary Gland and the Hypophysis Cerebri |
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11 Pituitary Gland and
the Hypophysis Cerebri
INTRODUCTION
The pituitary gland is a small oval structure 8 to 12 mm in size. It is wellprotected in the sella tursica (fossa for the pituitary gland) present in the body of the sphenoid. If a slanting line drawn from the root of the nose to the posterior margin of the foramen magnum the pituitary gland can be located in the middle of this line. The sella is situated in between the tuberculum sellae and optic groove in front and the dorsum sellae behind (Fig. 11.1).
Fig. 11.1: Pituitary gland
The pituitary fossa is covered by the dura which is called the diaphragma sellae. The pituitary stalk or the infundibulum pierces this. The cavernous sinus is present on either side with the dura in between. Hence tumors of the pituitary gland will affect the 3rd and 4th nerve present in the wall of the sinus but the sixth nerve is protected by the internal carotid.
The intercavernous sinuses are present in front and behind the pituitary gland while the sphenoidal air sinuses are present below. The circle of Willis is present above.
The gland is supplied by the hypophysial branches of the internal carotid and is drained by the veins into the intercavernous plexus.
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BLOOD SUPPLY TO THE EYE AND OCULAR ADNEXA
The ophthalmic artery a branch of the internal carotid supplies the eye and the other structures like scalp up to the vertex, orbit and the lateral wall of the nose. The vessel arises just after the internal carotid makes the fifth bend and pierces the roof of the cavernous sinus. The artery travels upwards for a few millimeters and then forwards to lie on the medial side of the optic nerve (intra-cranial portion). Then as the vessel passes strait ahead it comes to lie on the lateral side of the nerve.
It enters the orbit through the optic canal within the dural sheath of the nerve piercing the sheath only after it enters the orbit. Thus the vessel is attached to the nerve. In the orbit, it lies within the muscle cone between the optic nerve and the lateral rectus muscle. The ciliary ganglion is also situated lateral to the artery. It then ascends and crosses the optic nerve under the superior rectus muscle to reach the medial side. The vessel travels forwards above the medial rectus up to the maxillary process of the frontal bone. Here it divides in to the dorsal nasal and supra trachlear branches (Fig. 11.2).
Fig. 11.2: Blood supply to the eyeball
BRANCHES OF THE OPHTHALMIC ARTERY
1.Central retinal artery: This is an end artery with minimal anastomosis at the level of the circle of Zinn. This vessel is very thin about 0.28 mm in diameter. The central retinal artery, is given off by the ophthalmic artery near the optic foramen, along with the medial ciliary trunk. It travels forward very close to and below the optic nerve. It pierces the optic nerve from the inferomedial aspect about 15 mm from the optic foramen. It is covered both by the dura and arachnoid. It then runs forward and pierces the pia, 12 mm behind the optic disc, to enter the nerve. It carries with it the pia mater, pial blood vessels and sympathetic nerve plexus called the nerve of Tiedemann.