Muscles of the orbit

  • Extraocular
    • 4 rectus
      • Superior rectus
        • oculomotor nerve
      • Inferior rectus
        • oculomotor nerve
      • Medial rectus
        • oculomotor nerve
      • Lateral rectus
        • abducen nerve
    • 2 oblique
      • Superior oblique
      • Inferior oblique
    • Levator Palpebrae Superioris (LPS)
    • Smooth muscles
      • Superior tarsal
      • Inferior tarsal
      • Orbitalis
  • Intraocular
    • Sphincter pupillae
    • Dilator pupillae
    • Ciliaris

Bony orbit of the eye

  • Superior
    • frontal bone
  • Inferior
    • maxilla
  • Medial
    • Lacrimal bone
  • Lateral
    • zygomatic bone

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Movements of the eyeball

  • controlled by the 4 rectus and 2 oblique extraocular muscles
    • function
      • allow us to follow moving object
      • maintain foveal fixation
      • binocular vision
      • maintain shape of eyeball
    • when the movements of 2 eyes are not coordinated
      • images of same area of visual fied not focused on same areas of retina
        • producing DIPLOPIA
      • usually due to weakness/paralysis of extraocular muscles
        • confenital/acquired, one eye rotated medially/laterally
          • producing strabismus/squint

image

A – right medial displacement

  • muscle paralysed: lateral rectus

B – right lateral displacement

  • muscle paralysed: medial rectus

C – superior displacement

D – inferior displacement

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Monoular movements

  • Movement of 1 eye to 1 direction = Duction
    • medially: adduction
    • laterraly: abduction
    • superiorly: supraduction
    • inferiorly: infraduction
  • Vertical meridian-torsional movement
    • nasal rotation, medially along antero-posterior axis
      • intortion
    • temporal rotation, laterally along antero-posterior axis
      • extortion

image

  • Agonist
    • primary muscle that moves an eye in a given direction
    • eg. abduction by lateral rectus
  • Synergist
    • muscle in the same eye that moves the eye in the same direction as the agonist (support the same movement)
    • eg. abduction supported by superior & inferior oblique
  • Antagonist
    • muscle in the same eye that moves the eye in the opposite direction of the agonist is the antagonist
    • eg. medial rectus is an antagonist to abduction by lateral rectus

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Binocular movements

  • Movements are either conjugate (versions) or disconjugate
    • movement of 2 eyes in the same direction: version
  • Levoversion
    • eyes moving in the same direction
    • eg. right eye adduction, left eye abducting
  • Convergence
    • both eyes look towards the nose
  • Yoke muscles
    • primary muscles in each eye that perform the version
    • eg. when looking to the left, left eye perform abduction by lateral rectus muscle. right eye perform adduction by medial rectus muscle. Yoke muscles: lateral and medial rectus muscle.

Visual axis

image

  • Horizontal plane
    • Primary plane of action
    • Movements are 90 degrees to plane
      • Elevation
      • Depression
  • Vertical plane
    • 2ndary plane of action
    • Movements are 90 degrees to plane
      • Adduction
      • Abduction
  • Antero-posterior plane
    • 3rd plane of action
    • Movements are 90 degress to plane
      • Intortion
      • Extortion

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Extraocular muscles

Rectus muscles

  • Common origin
    • All arise from posterior part of orbit
      • from a common tendinous ring
      • that surrounds optic canal, superior orbital fissure
  • Insertion into sclera
    • inserted into sclera
      • 6mm behind sclero-corneal junction

image

  • If not following eyeball plane
    • what are the muscles not in allignment?

Levator Palpebrae Superioris (LPS) muscle

image

  • Action: elevates upper eyelid
  • Arise from
    • Lesser wing of sphenoid
  • Partially supplied by
    • Somatic nerve (3rd cranial nerve)
    • Autonomic (sympathetic – involuntary smooth muscles)
      • Superior tarsal
        • deep part of LPS
      • Inferior tarsal
        • connects inferior tarsal plate to fascia around inferior rectus
        • both these, innervated by sympathetic nerves = responsible for widely opening the eyelids in response to fear
      • Orbitalis
        • small smooth muscles bridging inferior orbital fissure
        • may compress inferior ophthalmic nerves –making eyeballs prominent
  • Aponeurosis divides LPS into 3 layer
    • Superficial – skin of upper eyelid
    • Middle – tarsal plate of upper eyelid
    • Deep – superior conjunctival fornix

Superior Oblique muscle

image

  • Arise from
    • body of sphenoid bone
      • medial to optic canal
      • goes medially, then ends in a tendon
      • attached to a pulley: trochlea
      • then suddenly runs backward & laterally (change in direction)
      • inserted to upper & lateral quadrant of eyeball

Inferior oblique muscle

  • Arise from
    • medial part of floor
      • winds round the eyeball laterally & backwards
      • inserted to lateral part of sclera

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Movements of the extraocular muscles

Superior rectus 1 – elevation
2 – intortion
3 – adduction
Inferior rectus 1 – depression
2 – adduction
3 – extortion
Medial rectus 1- adduction
Lateral rectus 1- abduction
Superior oblique 1 – intortion
2 – depression
3 – abduction
Inferior oblique 1 – extortion
2 – elevation
3 – abduction

*know the primary, secondary and tertiery movements (refer table)

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Extraocular muscle disorders

Complete 3rd nerve palsy

  • Muscle affected
    • medial rectus
  • Clinical Presentation
    • lateral strabismus
      • Medial rectus affected
    • diplopia
      • eye cannot be moved upwards, medially & downwards
    • ptosis
      • LPS affected
    • *dilated pupil (light reflex sluggish/absent)
      • sphincter pupillae affected
    • loss of accommodation
  • Cause
    • diabetes
    • atherosclerosis
    • tumours
    • haemorrhage

4th nerve palsy

  • Muscle affected
    • Superior oblique
  • Diplopia
    • difficulty in turning the eyes downwards & laterally while walking down stairs

6th nerve palsy

  • Muscle affected
    • lateral rectus
  • Medial strabismus
    • cannot turn eye laterally
  • Cause
    • facial infection –> cavernous sinus –> infect 6th nerve supplying lateral rectus –> paralysis –> medial strabismus

Extra:

  • Sphincter pupillae
    • 3rd nerve
  • Dilator pupillae
    • sympathetic nerve

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Light reflex

  • Useful for gauging brain stem function
  • Normally pupils contrict equally
  • Direct light reflex
    • Each pupil constricts with light shone into that eye
  • Consensual pupillary reflex
    • Each pupil constricts with light shone into the OTHER eye

image http://library.med.utah.edu/kw/hyperbrain/anim/reflex.html

Pupillary light reflex pathway

  • Light stimulates retinal photoreceptors
  • Afferent fibres transmitted via optic nerve
  • Undergo hemi-decussation at optic chiasma
  • Exit from optic tract before lateral geniculate body
    • lateral geniculate body = vision
  • Enters midbrain
    • via brachium of superior colliculus
  • Synapse at pretectal nucleus
  • Fibres go to same-sided & contralateral Edinger –Westphal nuclei
    • 3rd nerve
  • Efferent fibres travel via 3rd nerve
  • Synapse at ciliary ganglion
    • Post-ganglionic fibres –> short ciliary nerve
  • Innervate left & right sphincter muscles
    • constriction of pupil
    • Same sided: direct
    • Contralateral: consensual

image

  • Pretectal nucleus
    • main station for light reflex
  • Edinger-Westphal nucleus
    • parasympathetic nucleus of oculomotor nerve
  • Sphincter muscles
    • efferent parasympathetic fibres
  • Nerve
    • before Edinger-Westphal – Optic nerve (afferent)
    • after Edinger-Westphal – Oculomotor nerve (efferent)

Anatomical bases for consensual light reflex

  • From pretectal, fibres sent to both Edinger-Westphal nucleus –> pupillary constriction in both eyes

Lack of / abnormal pupillary reflex

  • Optic nerve damage
    • sensory component
    • ipsilateral direct reflex loss
    • contralateral direct reflex intact
  • Oculomotor nerve damage
    • motor component
    • damaged side both direct & consensual reflex loss
    • opposite side both direct & consensual reflex intact
  • Argyll Robertson pupil
    • pupil does not react to light, but will react to near accommodation

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Pathway of sympathetic dilatation of pupil

  • Less well-dfined
  • Pathway:
    • Originates in hypothalamus
    • Descends uncrossed to T1
      • inferior cervical ganglion
      • medial cervical ganglion (stellate)
      • superior cervical ganglion
    • Exit from spinal cord via perivertebral sympathetic chain
    • Synapse in the superior cervical ganglion
    • Join ophthalmic division of trigeminal nerve
      • via carotid plexus
    • Innervate dilator pupillae of the iris
      • via the long ciliary nerve
      • (short ciliary nerve innervate sphincter pupillae)
  • If paralysis
    • miosis (constriction of pupil)
      • seen in horner’s syndrome

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Accommodation reflex

  • Reflex action of the eye in response to focusing on a near object
  • Coordinated changes in
    • convergence
      • medial rectus
      • Optic nerve (afferent limb) –> optic tract –> visual cortex –> long association fibre –> frontal eye field –> corticonuclear fibres (pyramidal tract) –> oculomotor nerve (efferent limb) –> contraction of medial rectus of both eyes
    • lens shape
      • ciliary muscles
      • ciliary muscle contracts, suspensory ligaments relaxed –> making lens more convex – shortening its focal length
    • pupil size
      • sphincter pupillae
      • Edinger-Westphal nucleus –> oculomotor nerve (efferent limb) –> ciliary ganglion –> short ciliary nerve –> ciliary muscle constraction –> sphincter pupillae muscle contraction
      • pupil contricts to prevent diverging light rays from hitting the periphery of the retina & producing blurred image
  • Components
    • Afferent: Optic nerve
    • Cell station: Multisynaptic
    • Efferent: Oculomotor
  • Oculomotor nerve supplies these
    • smooth muscles
      • ciliaris
      • sphincter pupillae
    • medial rectus

image

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Other reflexes

Corneal reflex

  • Important to detect 7th nerve palsy
    • Differential diagnosis: acoustic neuroma
  • Components
    • Afferent – V1
    • Cell station – medial longitudinal fasciculus
    • Efferent – Facial nerve (orbicularis)
  • Pathway
    • light touch of cornea/conjunctiva –> ophthalmic (trigeminal) nerve (Afferent limb) –> sensory nucleus of 5th CN (pons) –> medial longitudinal fasciculus –> motor nucleus of 7th CN –> facial nerve (efferent limb) –> orbicularis oculi (closure of eyelids)
  • Which reflex is done to test the 7th CN?
    • corneal reflex and conjunctival reflex
    • bilateral blinking seen
  • Tumour over the 8th CN, can compress the intercostal acoustic meatus (containing 7th CN) –> corneal reflex loss

Visual body reflex

  • Optic nerve –> optic tract –> superior colliculus –> tectospinal & tectobulbar tract –> automatic movements of the head, neck & body towards the source of visual stimulus

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Functional components

image

Horner’s Syndrome

  • Cause
    • pancoast tumour compressing the sympathetic nerves at T1 level
  • Clinical features
    • Ptosis (drooping of eyelid)
    • Enophthalmus (reduced prominence of eyeball)
    • Anhydrosis
    • Miosis (Constriction of pupil)
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Terri is obsessed with making medical school as painless as possible. She studies and compiles medical school notes in a concise, easy-to-understand format. She also enjoys reading contributions by others. She is an investor in sustainability projects. Her ideal weekend is wine tasting and experimenting on bread-making. She has yet to master the art of Sourdough baking.

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