Cranial Nerve Palsy

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  • Extraocular muscles
    • Origin and insertion
      • The annulus of Zinn
        • The rectus muscles originate from a common tendinous ring (the annulus of Zinn) it  is attached at the apex of the orbit (MCQ)
        • It encircles the optic foramina and medial part of the superior orbital fissure
      • All the four recti run forward around the eyeball and are inserted into the sclera, by flat tendons (about 10-mm broad) at different distances from the limbus
      • The superior oblique muscle
        • arises from the bone above and medial to the optic foramina.
        • It runs forward and turns around a pulley — ‘the trochlea’(MCQ)
        • It is inserted in the upper and outer part of the sclera behind the equator
      • The inferior oblique muscle
        • inserted into the lower and outer part of the sclera behind the equator
    • Nerve supply
      • third cranial nerve (oculomotor) supplies the superior, medial and inferior recti and inferior oblique muscles.
      • fourth cranial nerve (trochlear) supplies the superior oblique
      • sixth nerve (abducent) supplies the lateral rectus muscle.
    • Actions
      • The extraocular muscles rotate the eyeball around vertical, horizontal and antero-posterior axes.
      • Medial and lateral rectus muscles are almost parallel to the optical axis of the eyeball; so they have got only the main action.
      • While superior and inferior rectus muscles make an angle of 23o and reflected tendons of the superior and inferior oblique muscles of 51o with the optical axis in the primary position; so they have subsidiary actions in addition to the main action.

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  • Types of ocular movements
    • Uniocular movements are called ‘ductions’
      • Adduction
        • It is inward movement (medialrotation) along the vertical axis.
      • Abduction.
        • It is outward movement (lateral rotation) along the vertical axis.
      • Supraduction.
        • It is upward movement (elevation) along the horizontal axis.
      • Infraduction.
        • It is downward movement (depression) along the horizontal axis.
      • Incycloduction (intorsion).
        • It is a rotatory movement along the anteroposterior axis in which superior pole of the cornea (12 O’clock point) moves medially.
      • Excycloduction (extorsion).
        • It is a rotatory movement along the anteroposterior axis in which superior pole of the cornea (12 O’clock point) moves laterally.
    • Binocular movements. These are of two types: versions and vergences.
      • Versions
        • also known as conjugate movements
        • are synchronous (simultaneous) symmetric movements of both eyes in the same direction.
        • Dextroversion.
          • It is the movement of both eyes to the right.
          • It results due to simultaneous contraction of right lateral rectus and left medial rectus.
        • Levoversion.
          • It refers to movement of both eyes to the left.
          • It is produced by simultaneous contraction of left lateral rectus and right medial rectus.
        • Supraversion.
          • It is upward movement of both eyes in primary position.
          • It results due to simultaneous contraction of bilateral superior recti and inferior obliques.
        • Infraversion.
          • It is downward movement of both eyes in primary position.
          • It results due to simultaneous contraction of bilateral inferior recti and superior obliques.
        • Dextrocycloversion.
          • It is rotational movement around the anteroposterior axis
          • superior pole of cornea of both the eyes tilts towards the right.
        • Levocycloversion.
          • It is just the reverse of dextrocycloversion.
          • In it superior pole of cornea of both the eyes tilts towards the left.
      • Vergences, also called disjugate movements, are synchronous and symmetric movements of both eyes in opposite directions
        • Convergence
          • .It is simultaneous inward movement of both eyes
          • results from contraction of the medial recti.
        • Divergence.
          • It is simultaneous outward movementof both eyes
          • produced by contraction of the lateral recti.
  • Synergists, antagonists and yoke muscles
    • Synergists.
      • It refers to the muscles having the same primary action in the same eye
      • superior rectus and inferior oblique of the same eye act as synergistic elevators.
    • Antagonists.
      • These are the muscles having opposite actions in the same eye.
      • antagonists to each other in the same eye
        • medial and lateral recti
        • superior and inferior recti
        • superior and inferior obliques
    • Yoke muscles (contralateral synergists)
      • It refers to the pair of muscles (one from each eye) which contract simultaneously during version movements.
      • right lateral rectus and left medial rectus act as yoke muscles for dextroversion movements.
      • Other pairs of yoke muscles are:
        • right MR and left LR
        • right LR and left MR
        • right SR and left IO
        • right IR and left SO
        • right SO and left IR
        • right IO and left SR.
    • Contralateral antagonists.
      • These are a pair of muscles (one from each eye) having opposite action
      • for example,
        • right LR and left LR
        • right MR and left MR
  • Laws governing ocular movements
    • Hering’s law of equal innervation (MCQ)
      • an equal and simultaneous innervation flows from the brain to a pair of muscles which contract simultaneously (yoke muscles) in different binocular movements
      • During dextroversion
        • right lateral rectus and left medial rectus muscles receive an equal and simultaneous flow of innervation.
      • During convergence
        • both medial recti get equal innervation.
      • During dextroelevation
        • right superior rectus and left inferior oblique receive equal and simultaneous innervation.
    • Sherrington’s law of reciprocal innervation.
      • during ocular motility, increased flow of innervation to the contracting muscle is accompanied by decreased flow of innervation to the relaxing antagonist muscle.
      • For example, during dextroversion,
        • an increased innervation flow to the right LR and left MR is accompanied by decreased flow to the right MR and left LR muscles.
  • Diagnostic positions of gaze
    • There are nine diagnostic positions of gaze
    • These include one primary, four secondary and four tertiary positions.
    • Primary position of gaze.
      • It is the position assumed by the eyes when fixating a distant object (straight ahead) with the erect position of head
    • Secondary positions of gaze.
      • These are the positions assumed by the eyes while looking straight up, straight down, to the right and to the left
    • Tertiary positions of gaze.
      • These describe the positions assumed by the eyes when combination of vertical and horizontal movements occur.
      • These include position of eyes in
        • Dextroelevation
        • Dextrodepression
        • Levoelevation
        • levodepression
    • Cardinal positions of gaze.
      • These are the positions which allow examination of each of the 12 extraocular muscles in their main field of action.
      • There are six cardinal positions of gaze,
        • Dextroversion
        • Levoversion
        • Dextroelevation
        • Levoelevation
        • dextrodepression
        • levodepression
  • PARALYTIC STRABISMUS
    • It refers to ocular deviation resulting from complete or incomplete paralysis of one or more extraocular muscles.
    • Symptoms
      • Diplopia.
        • It is the main symptom of paralytic squint.
        • It is more marked towards the action of paralysed muscle.
        • It may be crossed (in divergent squint) or uncrossed (in convergent squint).
        • It may be horizontal, vertical or oblique depending on the muscle paralysed. Diplopia occurs due to formation of image on dissimilar points of the two retinae
      • Confusion.
      • Nausea and vertigo.
      • Ocular deviation.: It is of sudden onset.
    • Signs
      • Primary deviation. (MCQ)
        • It is deviation of the affected eye
        • it is away from the action of paralysed muscle, e.g., if lateral rectus is paralysed the eye is converged.
      • Secondary deviation (MCQ)
        • It is deviation of the normal eye seen under cover, when the patient is made to fix with the squinting eye.
        • It is greater than the primary deviation.
        • This is based on Hering’s law of equal innervation of yoke muscles.
        • This is due to the fact that the strong impulse of innervation required to enable the eye with paralysed muscle to fix is also transmitted to the yoke muscle of the sound eye resulting in a greater amount of deviation.
      • Restriction of ocular movement.
        • It occurs in the direction of the action of paralysed muscles
      • Compensatory head posture.
        • It is adopted to avoid diplopia and confusion.
        • Head is turned towards the direction of action of the paralysed muscle,
        • if the right lateral rectus is paralysed, patient will keep the head turned towards right
      • False projection or orientation.
        • It is due to increased innervational impulse conveyed to the paralysed muscle.
        • It can be demonstrated by asking the patient to close the sound eye and then to fix an object placed on the side of paralysed muscle.
        • Patient will locate it further away in the same direction.
        • For example, a patient with paralysis of right lateral rectus will point towards right more than the object actually is.
    • Pathological sequelae of an extraocular muscle palsy
      • These occur more in paralysis due to lesions of the nerves than the lesions of muscles.
      • These include:
        • Overaction of the contralateral synergistic muscle.
        • Contracture of the direct antagonist muscle.
        • Secondary inhibitional palsy of the contralateral antagonist muscle.
      • For example, in paralysis of the right lateral rectus muscle there occurs
        • Overaction of the left medial rectus,
        • Contracture of the right medial rectus and
        • Inhibitional palsy of the left lateral rectus muscle.
  • Clinical varieties of ocular palsies
    • Isolated muscle paralysis.
      • Lateral rectus and superior oblique are the most common muscles to be paralysed singly, as they have separate nerve supply
    • Paralysis of the third cranial nerve (MCQ)
      • Clinical features of third nerve palsy include:
        • Ptosis
          • Occurs due to paralysis of the LPS muscle.
        • Deviation.
          • Eyeball is turned down, out and slightly intorted due to actions of the lateral rectus and superior oblique muscles.
        • Ocular movements are restricted
          • occurs in all the directions except outward.
        • Pupil is fixed and dilated
          • occurs due to paralysis of the sphincter pupillae muscle.
        • Accommodation is completely lost
          • occurs due to paralysis of the ciliary muscle.
        • Crossed diplopia is elicited on raising the eyelid.  
        • Head posture may be changed if pupillary area remains uncovered.
    • Total ophthalmoplegia.
      • In this condition all extraocular muscles including LPS and intraocular muscles, viz., sphincter pupillae, and ciliary muscle are paralysed.
      • It results from combined paralysis of third, fourth and sixth cranial nerves.
      • It is a common feature of orbital apex syndrome and cavernous sinus thrombosis.
    • External ophthalmoplegia.
      • In this condition, all extraocular muscles are paralysed, sparing the intraocular muscles.
      • It results from lesions at the level of motor nuclei sparing the Edinger-Westphal nucleus.
    • Internuclear ophthalmoplegia.
      • In this condition there is lesion of the medial longitudinal bundle.
      • it is characterised by:
        • defective action of medial rectus on the side of lesion
        • horizontal nystagmus of the opposite eye
        • normal  convergence.
  • Investigations of a case of paralytic squint
    • Diplopia charting
      • In it patient is asked to wear red and green diplopia charting glasses.
      • Red glass being in front of the right eye and green in front of the left.
      • Then in a semi-dark room, he is shown a fine linear light from a distance of 4 ft. and asked to comment on the images in primary position and in other positions of gaze.
      • Patient tells about the position and the separation of the two images in different fields.
    • Hess screen test. Hess screen/Lees screen test tells about the paralysed muscles and the pathological sequelae of the paralysis, viz., overaction, contracture and secondary inhibitional palsy.
    • Field of binocular fixation.
    • Forced duction test (FDT).
      • It is performed to differentiate between the incomitant squint due to paralysis of extraocular muscle and that due to mechanical restriction of the ocular movements. FDT is positive (resistance encountered during passive rotation) in cases of incomitant squint due to mechanical restriction
      • FDT is negative in cases of extraocular muscle palsy.
  • Management
    • Surgical treatment. It should be carried out in case the recovery does not occur in 6 months.

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