Ординатура / Офтальмология / Английские материалы / Pediatric Opthalmology_Mukherjee_2005

Inferior rectus (I.R.)

Inferior rectus originates from the circle of Zinn. It travels slightly down and anteriorly under the globe and inserts in the sclera slightly temporal to its origin, 6 mm from the limbus in a convex fashion with convexity anteriorly. Its arc of the contact with the globe is 6.5 mm. Its length is 40 mm. It has a tendon 5.0 mm long, the width of the tendon is 10.0 mm at insertion. The inferior oblique passes under it at the anterior part. The inferior rectus is intimate with inferior oblique and the lower lid muscle. It merges with the ligament of Lockwood to give support to the globe from below.

Its direction from its origin to insertion is similar to that of superior rectus. Its long axis forms an angle of 23° with the visual axis when the eyeball is in primary position.

When the eyeball is abducted 23°, the axis of the muscle and visual axis coincide at this position, it is a depressor of the globe that increases with abduction and decreases in adduction.

The primary action of the inferior rectus is depression. Its secondary action is extorsion, which is best when the eye is adducted by 67°. The tertiary action of inferior rectus is adduction.

Thus the inferior rectus is a depressior, extortor and adductor.

The oblique muscle

The two obliques, the superior and inferior obliques are unique in their arrangement in relation to the globe and visual axis. The effective origins of both the muscles are in front of the globe from the medial side of the rim of the orbit, both get inserted behind the equator farther away from the farthest insertion of any rectus i.e. 14 mm and 17 mm for SO and IO respectively. Their main function is to keep the upper pole of the limbus at 12 O’clock position irrespective of the tilt of the head. They have a forward pulling effect on the globe, which is counteracted by backward pull of the recti. Hence in case of paralysis of third nerve with sixth nerve the eyeball is slightly proptosed. Both of them are abductors.

The superior oblique

The superior oblique has the distinction of having longest muscle belly, and tendon. The muscle is divided into a long and a short part. The shorter part is the effective part of the muscle. It is also a muscle that is innervated by single cranial nerve, a distinction shared by lateral rectus that too has individual nerve supply. The muscle is supplied by contra lateral nucleus of fourth nerve. Due to its peculiar position, the muscle is most susceptible to trauma. Congenital anomaly of superior oblique is very common.

The superior oblique takes origin from the periosteum of the body of sphenoid little superior and medial to the optic foramen.

The muscular part 40 mm in length runs anteriorly and nasally and upwards up to the trochlea at the superio medial aspect of the rim of the orbit where it passes over the pully and is converted almost fully to its tendon that is 20 mm in length. The tendon runs backwards, downwards and laterally below the superior rectus. The tendon is inserted in the sclera in a wide insertion behind the equator in the posterio lateral quadrant. The muscular part is called direct part while the tendon is called the reflected part. The attachment of the muscle to the trochlea is called its effective origin. The temporal side of the muscle is inserted nearer

the limbus than the medial end. The temporal end is inserted about 14 mm from the limbus, the medial end is inserted about 19 mm from the limbus. The insertion is rounded with convexity away from the limbus. At the insertion, the posterior border is very near the vertex vein.

The effective muscle plane is formed by the reflected part of the muscle, which forms an angle of 54° with the visual axis in primary position.

When the muscle contracts from this position, the eyeball intorts. The primary action of superior oblique is intorsion. When the globe is adducted 54°, the muscle plane coincides with visual axis and when the muscle contracts the eyeball is depressed, the secondary action is depression while the tertiary action is abduction.

The muscle is supplied by fourth nerve that enters the orbit outside the circle of Zinn and enters the muscle belly from the orbital surfaces. All other extra ocular muscles get their nerve supply from the ocular surface. As the muscle lies outside the muscle cone, its motor nerve enters the orbit outside the circle of Zinn and the motor supply is from outside, the muscle is least affected by retro bulbar anaesthesia but well anathesized by peribulbar injection.

Test for fourth nerve in presence of third nerve palsy. Involvement of superior oblique in presence of third nerve palsy is an important neuro ophthalmic test. In complete paralysis of third nerve, the eye can not be adducted, the vertical action of SO which is best in adduction can not be tested. The eye is already abducted which is best position for intorsion so if the patient attempts to look down the eyeball intorts.

Blood supply. The muscle gets it blood supply from the superior muscular branch of the ophthalmic artery.

The inferior oblique

The inferior oblique too has few peculiarities. It is the shortest extra ocular muscle— 37 mm. It has almost no tendon (only 1 mm). It is only extra ocular muscle that have true anterior origin from posterior lacrimal crest of the inferio medial part of rim of the orbit. Its insertion is very close to the fovea.

After origin the muscle passes backwards and laterally between the floor of the orbit and inferior rectus with in the Lockwoods ligament. It passes under the globe and gets inserted 17 mm from the limbus, 1 mm anterior and below the fovea.

The anterior end of the insertion lies in the same plane as lower end of insertion of the lateral rectus. The insertion of the muscle is covered by the lateral rectus.

The action of the inferior oblique is as complex as superior oblique. Its primary action is extortion, which is best in abduction. The secondary action is elevation and tertiary action is abduction.

Its motor supply is through the inferior division of the third nerve.

Its blood supply is through the inferior orbital artery and medial muscular branch of ophthalmic artery.

The muscle is most likely to be injured in fracture floor of the orbit.

Spiral of Tillaux

The attachment of the extra ocular muscles is not the same for all the recti, the distance vary from muscle to muscle. The medial rectus is inserted nearest to the limbus followed by interior and lateral rectus. The superior rectus is farthest from the limbus, the curved imaginary line joining the insertion of the recti form a spiral called spiral of Tillaux. Its knowledge helps in finding the attachment of the muscles during squint surgery.

Summary of action of extra ocular muscles

All recti are Adductors except lateral. The Superiors are Intortors (SR and SO). The Inferiors are Extortors (IR and IO). The mnemonic for cyclo vertical muscles is SIN RAD.

Thus superiors are Intortors. The recti are adductor, this leaves the obliques as abductors and inferiors as extortors.

Development of extra ocular muscles12,13,14

All voluntary muscles in the body develop from para-axial mesoderm. Ocular muscles are no exception. Presence of extra ocular muscles is visible as early as four weeks of gestation. The para axial mesoderm first forms a common pre muscle mass that condensates into two groups i.e. the pre mandibular and the maxillo mandibular condensation. The first is supplied by oculo motor. From the first condensation develop all the muscles supplied by the third nerve. The remaining two develop from maxillo mandibular condensation which again divides into two groups called second head somite and third head somite. From the former develops the superior oblique. The lateral rectus develops from the latter.

The three muscle masses are destined to be supplied by three different cranial nerves. The oculomotor reaches the pre mandibular mass while the trocheal supplies the superior oblique developing from the second head somite and the abducent supplies the lateral rectus developing from the third head somite.

The insertion of the muscles develop separately from the scleral condensation near the limbus. The muscle masses differentiate from behind forward hence the origin of the muscle is more constant than the insertion. The levator is the last to differentiate from the superior

rectus. As the two muscles remain undifferentiated for long time and have a common motor supply i.e. upper division of third nerve that terminates in the levator after piercing the superior rectus. The congenital anomalies of levator are mostly associated with congenital anomalies of superior rectus. The medial rectus is the first muscle to differentiate.

The third nerve reaches its designated muscle mass roughly at 7-8 mm stage while the abducent reaches the lateral rectus at 8-9 mm and the trochlear reaches the superior oblique at 10-12 mm stage.

Development of oculomotor nerves12

The nerves concerned with movement of the eye develop from the neuroblast nearest to the cranial part of the neural tube. The nucleus of the third nerve is first to appear and the abducent the last. The fourth nerve reaches its target muscle last, its nucleus develops in between development of III and VI nucleus. The nerves start developing from the cranial end and terminate in the ocular end.

Anatomy of the nerves involved in ocular movement

Three cranial nerves i.e. the oculo motor, trochlear and abducent nerves are directly involved in ocular movements. Other neural pathway involved indirectly with ocular movements are8 :

•Fronto mesencephalic path

•Parieto-occipital temporal mesencephalic path

•Occipito mesencephalic path

•Para median pontine reticular formation

•Labyrinthine pontine path.

The oculomotor nerve

This is the third cranial nerve, it is the only nerve that supplies more than one extra ocular muscles, and intra ocular muscles. It has largest nucleus situated in the mid brain at the level of superior colliculum. It is divided into paired and unpaired nuclei. The paired nuclei are again divided into three sub-groups and supply the ipsilateral, medial and inferior rectus and inferior oblique individually. The unpaired nuclei are motor cell, pool of superior rectus, Perlias nucleus, caudal central nucleus for levator. The Perlias nucleus is supposed to be the center of convergence. The pre-ganglionic parasympathetic fibres originate from the Edinger Westphal nucleus. This nucleus also contains fibres for accommodation and light reflex.

The nucleus of the four nerves is below the nucleus for inferior rectus.

The superior rectus and superior oblique muscle have their motor pool in the contra lateral side.

Connections of the third nerve nucleus15

The third nerve nucleus is connected with following tract and nuclei :

1.Cortico nuclear tract of both sides.

2.Visual cortex through tecto bulbar tract.

3.Pretectal nuclei of both sides.

4.Nuclei of fourth, sixth and eighth nerve through medial longitudinal fibres.

The nuclear mass is supplied by posterior cerebral artery, its branches.

For ease of description, course of all nerves that supply extra ocular muscles have been divided into :

1.Fascicular,

2.Basilar,

3.Intra cavernous, and

4.Orbital parts.

This helps to understand the relation of various structures that come on the way of the nerves. The lesions of these structures produce definite clinical features that help to pin-point the exact location of the lesion and many a times the nature of the lesion.

1.The fascicular part. The fibres of the third nerve travel anteriorly in the substance of the mid brain to pass through the red nucleus and the medial aspects of the cerebral peduncle. The fascicles join to form the two trunk of the third nerve on either side of the mid line and emerge in the inter peduncular space (fossa) in front of the mid brain. Involvement of fascicles at this level produces a symmetric lesions of the muscles supplied by individual nuclei.

2.The basilar part. As the oculomotor nerve travels forward in the inter peduncular space. The nerve passes through the sub arachnoid space, and passes under the posterior cerebral artery and above the superior cerebraller artery. The nerve passes parallel to the posterior communicating artery. The fourth nerve also passes in between the posterior cerebral and superior cerebellar artery, lateral to the third nerve. It is clear that in the inter peduncular space the nerve is not related to any nerve, tract or cranial nerve hence lesion in this area do not have any systemic neurological deficits only defect produced is in the ocular muscles and the lesions are mostly vascular. It is the commonest site for isolated third nerve involvement.

3.Inter cavernous part. At the anterior aspect of the inter peduncular space, the nerve passes between the posterior clinoid and the free edge of the tentorium to pierce the dura to enter the cavernous sinus. Initially it lies near the roof of cavernous sinus and then in the lateral wall of the sinus. The nerve throughout its course in the lateral wall of the cavernous sinus lies above the fourth nerve, ophthalmic and maxillary nerves. The fourth nerve crosses the third nerve to be superior and medial to the oculomotor nerve before the third nerve leaves the cavernous sinus to enter the orbit, however, the ophthalmic and maxillary division of the fifth nerve remain inferior to the third nerve through out its inter cavernous course. At the anterior end of the cavernous sinus, the third divides in to two division i.e. a smaller superior division that supplies the levator and the superior rectus and a larger inferior division that supplies the medial rectus, inferior rectus and inferior oblique. The branch to inferior oblique also carries the parasympathetic fibres to the ciliary ganglion.

4.Intra orbital part. The two divisions enter the orbit through the superior orbital fissure inside the circle of Zinn.

The superior division passes over the optic nerve for some distance and pierces the superior rectus from its ocular surface. Some fibres travel superiorly to supply the levator palpebral from below.

The inferior division enters the inferior rectus, medial rectus and inferior oblique from the ocular surfaces.

The trochlear nerve15,16,17

Trochlear nerve is an unique cranial nerve. The special features of the nerve are :

1.It is the nerve with longest intra cranial course.

2.It is the thinnest cranial nerve.

3.It is the only cranial nerve that emerges from the brain on the dorsal side.

4.It is the only cranial nerve that decussates completely.

5.Its nucleus supplies single extra ocular muscle i.e. superior oblique on the contral lateral side.

6.Its fascicles are so small and so close to the nucleus that its not possible to differentiate between a nuclear and a fascicular lesion.

7.It enters the orbit through the superior orbital fissure outside the circle of Zinn.

8.It innervates the superior oblique from the orbital surface.

The nucleus of the fourth nerve

The fourth nerve has a single nucleus on the either side of the mid line just caudal to the third nerve nucleus in the grey matter of aqueduct at the level of inferior colliculus. The nucleus is so close to the third nucleus that it seems to be continuous with it. The nucleus of the fourth nerve innervates contra lateral superior oblique.

Fascicular part

The fibres of the fourth nerve bend round the aqueduct and decussate completely in the anterior medullary velum and leave the brain stem on the dorsal surface. Thus fourth nerve is the only cranial nerve that decussates fully and emerges on the dorsal side of the brain stem.

The fascicles are so small and so near the fourth nerve nucleus that it is impossible to differentiate between nuclear and fascicular lesions. The lesions in the mid brain are best called fasciculo nuclear lesion.

A small lesion dorsal to the aqueduct in the mid line produces bilateral fourth nerve palsy.

The basilar part

The fourth nerve curves round the brain stem from behind to have an anterior route to reach the superior oblique. In the basilar part its course is similar to that of third nerve. It passes between the posterior cerebral and superior cerebellar arteries lateral to third nerve away from the posterior communicating artery. Hence it is not involved in aneurysm of posterior communicating artery which is a common cause of isolated third nerve palsy.

In the basilar part it does not come in relation to any other cranial nerve or nerve tract hence lesion at this level produce isolated fourth nerve palsy. The commonest cause of involve-

ment of fourth nerve in the basilar part is head injury by which the nerve is pressed against the free edge of tentorium. The same traumatic force when directed posteriorly may cause bilateral fourth nerve palsy in the anterior medullary velum causing bilateral fourth nerve palsy.

Cavernous part

After leaving the basilar part the fourth nerve pierces the dura to enter the cavernous sinus lateral and inferior to the third nerve in the wall of the cavernous sinus. The fourth nerve changes its course upwards in the anterior part of the cavernous sinus and becomes superior to the third nerve to enter the orbit by the superior orbital fissure at a higher level than third and sixth nerve outside the circle of Zinn.

The orbital part

The fourth nerve supplies the belly of the superior oblique from its orbital surface. The third and sixth nerve enter the other extra ocular muscles from their bulbar surfaces.

The abducent nerve (abducens nerve)16,18

The nucleus. The abducent or the sixth cranial nerve takes origin from its nucleus that is situated in the mid pons under the floor of the fourth ventricle.

The fascicular part. The fascicles of the seventh nerve encircle the nucleus of the sixth nerve at this level making it impossible for an isolated sixth nerve palsy due to a nuclear lesion. The medial longitudinal fibres pass medial to the sixth nerve nucleus. There after the fibres of the sixth nerve pass through the substance of the pons with superior-olivary- nucleus on the lateral and pyramidical tract on the medial side.

Basilar part. Each abducent nerve comes out of the brain stem at the lower border of the pons at the ponto medullary junction one centimeter from the mid-line. The nerve goes up on the anterior face of the pons for a short distance only to be crossed by anterior inferior cerebellar artery.

The sixth nerve pierces the dura 2 cm below the posterior clinoid process and passes above the inferior petrosal sinus beneath the Gruber’s (petro clinoid) ligament to enter the cavernous sinus.

The cavernous part

In the cavernous sinus unlike other cranial nerves that lie in the lateral wall of the sinus the sixth nerve lies in the substance of the cavernous sinus between the internal carotid and the wall of the sinus.

The orbital part

The sixth nerve enters the orbit through the superior orbital fissure within the circle of Zinn. It supplies only the ipsilateral lateral rectus from its orbital side.

Clinical features of lesions at various levels of cranial nerves :

The lesions can be divided into :

1.Nuclear

2.Infra nuclear

(a) Fascicular (b) Basilar (c) Cavernous (d) Orbital

Characteristics of oculomotor nerve palsy20,21

Nuclear. Nuclear lesions are rarer than lesions at other locations. A nuclear lesion should have following essential features :

1.Bilateral superior rectus palsy with bilateral ptosis.

2.Bilateral ptosis

3.Unilateral third nerve with contra lateral superior rectus and levator involvement.

4.Bilateral total third nerve palsy

5.Bilateral external ophthalmoplegia

6.Bilateral internal ophthalmoplegia.

The cause of such selective involvement lies in the fact that :

1.Each superior rectus is innervated by contra lateral third nerve nucleus.

2.Both leavtors are supplied by one central caudal nucleus.

3.Large lesion may involve all the nuclei and cause bilateral third nerve palsy.

Webino. Wall eyed bilateral inter nuclear ophthalmoplegia is associated with bilateral exotropia with absent convergence due to involvement of both medial rectus sub nuclei and medial longitudinal fascicular (not seen in children).

Inter nuclear ophthalmoplegia22,23 is caused due to lesion of medial longitudinal fasciculus (MLF) that block the impulse from contra lateral para median pontine reticular formation (P.P.R.F.) to ipsilateral third nuclei. The lesion is in between the nucleus of third and fourth nerve. Its clinical features consist of - diminished adduction, adduction nystagmus, may have absent convergence (not seen in children).

One and half syndrome22,23

There is a combination of horizontal gaze palsy on one side with inter nuclear ophthalmoplegia (INO) due to lesion of P.P.R.F. and M.L.F. in the pons. Horizontal gaze palsy constitute the One of the syndrome and INO is the remaining half, both together constituting one and half syndrome (Not seen in children).

Fascicular lesion24

The fascicular lesions of third nerve are associated with involvement of either red nucleus, cortico spinal tract or superior cerebellar peduncle depending upon the position of the lesion. The various syndrome are :

1.Weber’s syndrome. Third nerve palsy on one side and contra lateral hemiplegia due to lesion involving third nerve fascicle.

2.Claude’s syndrome. Ipsilateral third nerve palsy with contra lateral ataxia/tremor due to involvement of third nerve fascicle and the red nucleus.

3.Benedict’s syndrome. Third nerve palsy with contra lateral hemiplegia and tremors due to lesion involving third nerve fascicle, red nucleus and cortico spinal tract.

4.Nathnagel’s syndrome. Ipsilateral third nerve palsy, cerebellar ataxia on the same side due to involvement of third nerve fascicle and superior cerebellar peduncle.

Basilar lesions

The basilar parts of the third nerve is in close relation with three vessels without any other cranial nerve or tract in the vicinity. The basilar lesions are mostly vascular and isolated. The third nerve trunk passes between the posterior cerebral and superior cerebellar artery near the trunk of the basilar artery, hence the nerve may be compressed by aneurysm of these vessels or that of posterior communicating artery, which runs parallel to the third nerve trunk in the inter peduncular (basilar) part. These lesions generally are painful and involve the pupil.

The basilar part of the third nerve is involved in uncal herniation at this level, which can be brought about due to an extra dural hematoma that presses the nerve trunk over the tentorial edge. It manifests as total third nerve palsy with fixed dilated pupil (Hutchinson’s pupil).

Traumatic lesion of third nerve

Traumatic paralysis of third nerve is less common than that of fourth nerve. It is mostly seen in closed head injury. The common sites for involvement of third nerve in closed head injury is extends between its exit from the ponto medullary junction to its entry in the cavernous sinus. The nerve may be avulsed near the brain stem. It may be forced down over the tentorial edge, there may be hemorrhage in the nerve, or there may be contusion of the nerve during its passage in the inter peduncular space. Such lesions are generally associated with fracture base of skull, loss of consciousness, total third nerve palsy and Hutchinson’s pupil.

Cavernous sinus lesions

In the cavernous sinus, the third nerve lies in the lateral wall of the sinus with fourth nerve and branches of the fifth nerve. The fourth nerve initially lies inferior to the third nerve but before it leaves the sinus it takes an upward course. The sixth nerve lies in the substance of the sinus. The oculosympathetic accompanies the internal carotid. Hence the lesion of the cavernous sinus does not produce isolated lesion of any single nerve, it generally involves all the nerves along with sympathetic. The lesions are generally partial. Involvement of fifth nerve is the cause of pain along with ophthalmoplegia. In some instances there may be loss of sensation on the distribution of fifth nerve. Generally pupil is of normal size. This is explained on the basis of two diagonally opposite mechanism acting simultaneously. The paralysis of third nerve should cause dilatation of pupil, which is counteracted by superimposed miosis of associated Horner’s syndrome. Aberrant regeneration of third nerve is common.

Orbital lesions

The third nerve divides into two parts just before entering the orbit, the upper supplies the levator, and superior rectus, the inferior supplies the inferior rectus and inferior oblique and the medial rectus. It also contains parasympathetic fibres. The orbital lesions are generally partial, associated with involvement of fourth and sixth nerve. Sixth nerve under action is

more common than fourth nerve under action. It is generally associated with mild proptosis. Pupil may be involved in lesions of lower division.

Painful ophthalmoplegia. Painful ophthalmoplegia may be confined to third nerve palsy or with multiple nerve palsies.

The causes of painful third nerve palsy are :

1.Posterior communicating artery aneurysm.

2.Basilar artery aneurysm

3.Aneurysm of intra cavernous carotid

4.Diabetic oculomotor palsy

5.Ophthalmoplegic migraine

Painful ophthalmoplegia involving more than one nerve can be due to any of the following :

1.Cavernous sinus thrombosis

2.Superior orbital fissure syndrome

3.Gradenigo’s syndrome

4.Pseudo tumours of the orbit

5.Metastatic tumours

6.Lymphoma

7.Naso pharyngeal carcinoma

8.Sinusitis

9.Para sellar growths

Tolosa Hunt syndrome25

This is a chronic non specific granulomatous involvement of anterior cavernous sinus and/or superior orbital fissure in isolation or combination. This is also called superior orbital fissure syndrome or anterior cavernous sinus syndrome. A localised lesion may involve the apex of the orbit and is called as orbital apex syndrome. One of the important features of which is diminished vision due to involvement of optic nerve.

The Tolosa Hunt syndrome may start as an acute episode of unilateral boring pain with diplopia and mild proptosis that may become chronic. Spontaneous recovery is known. Recurrence is common even with treatment.

The muscle involvement is varied, may begin with involvement of one muscle only to be followed by involvement of other muscles. The ocular palsy is partial. Pupillary involvement is variable. It may be normal, dilated or constricted depending upon the part of the third nerve involved and its effect being counteracted by sympathetic system.

As the first division of the fifth nerve is affected, there may be hypothesia or anaesthesia on the distribution of the fifth nerve.

ESR is always raised. The condition shows improvement following administration of systemic steroids that has to be used for months. Most probable cause is an auto immune disturbance resulting in formation of non specific granuloma of the orbit or cavernous sinus. In adults it is generally unilateral but in children it may be bilateral.