TSM47: the orbit25/10/08

bones and vessels of the orbit
  • The bony orbit is made up of seven different bones:
  • The roof is formed by the frontal bone
  • The lateral and inferior walls are formed by the zygomatic bone and maxilla
  • From lateral the medial, the posterior wall is formed by sphenoid, palatine, ethmoid, lacrimal
  • There are a number of openings in the orbital walls that allow passage of various vessels:
  • Optic canal –rounded hole transmitting the optic nerve and ophthalmic artery
  • Supra-orbital fissure–infero-medial slope leading to the middle cranial fossatransmitting:
  • All oculomotor cranial nerves– oculomotor, trochlear and abducens(CNIII, IV, VI)
  • Branches of the ophthalmic division of the trigeminal (CNV1 – see below)
  • Superior ophthalmic vein
  • Infra-orbital fissure –infero-lateral slope leading to pterygopalatine and temporal fossae:
  • Maxillary nerve (CNV2)
  • Inferior ophthalmic vein

Learning Outcomes

Describe the basic anatomy of lacrimation

  • Lacrimation is the production of tears
  • The lacrimal gland is situated in the lacrimal fossa in the antero-lateral roof of orbit
  • Secretory ducts project onto the superior temporal conjunctiva
  • Tears flow into the medial angle of the eye and pass through the lacrimal puncta
  • Medial to the lacrimal caruncle[TG1] the two lacrimal ducts join at the lacrimal sac
  • The orbicularis oculimuscle compresses the lacrimal sac to encourage drainage
  • The nasolacrimal duct conducts the tears into the nasal cavity below the inferior concha
  • Sensory innervation to the lacrimal gland is via the lacrimal nerve (branch of V1 – see below)
  • Secretomotor innervation to the lacrimal gland is via parasympatheticbranches
  • Fibres originate from the facial nerve and follow complicated pathway
  • Greater petrosal nerve to pterygopalatine ganglion; follows zygomatic branch of V2

Describe the types of eye movements

  • Eye movements are described relative to the visual axis:
  • Elevation and depression– movement in the vertical plane
  • Abduction and adduction–directing the pupil laterally and medially, respectively
  • Intorsion and extorsion–internal and external rotation
  • The pupil can also dilate and constrict:
  • Miosis–constriction viaparasympathetic innervation of circumferentialsphincter pupillae
  • Mydriasis–dilation viasympathetic innervation of radialdilator pupillae

Describe the basic anatomy of extra-ocular muscles

  • The orbital axis is oriented slightly lateral to the visual axis
  • The muscles of the eye act along the orbital axis and as suchoften have multiple effects
  • Most movements of the eye involve simultaneous contributions from multiple muscles
  • The sevenextra-ocular muscles control movements of the eye and upper eyelid
  • Elevation of the upper eyelid is achieved by a combination of striated(main) and smooth muscles:
  • Levator palpebrae superioris – most superior muscle of orbit,superiorCNIII innervation
  • Superior tarsal (smooth) – assists,superior cervical ganglion sympathetic innervation
  • Simple abduction and adduction of the eye are achieved by two dedicated extra-ocular muscles:
  • Medial rectus –adducts the eye, inferior CNIII innervation
  • Lateral rectus – abducts the eye, CNVI innervation
  • Elevation and depression with adduction are achieved by the remaining rectus muscles:
  • Superior rectus –elevates and adducts the eye, superior CNIII innervation
  • Inferior rectus– depresses and adducts the eye, inferior CNIII innervation
  • Elevation and depression with abduction are achieved by the two oblique muscles:
  • Superior oblique –depresses and abducts the eye, CNIV innervation
  • Inferior oblique –elevates and abducts the eye, inferior CNIII innervation

Describe the nerve supply to the eye

  • The ophthalmic division of the trigeminal nerve (CNV1)transmits three branches into the orbit:
  • Nasociliary nerve– supplies ciliary ganglion, mucous membrane of the nose, conjunctiva
  • Frontal nerve –sensory supply to supra-orbital regions of the face
  • Lacrimal nerve– sensory tolacrimal gland,superficial temporal conjunctiva
  • The oculomotor nerve (CNIII) gives pre-ganglionic parasympathetic branches to the ciliary ganglion
  • Short post-ganglionic fibres to the sphincter pupillae (of pupil) and ciliary muscles (oflens)
  • Thoracic spinal nerves (T1) give pre-ganglionic sympathetic branches to the upper cervical ganglion
  • Post-ganglionic fibrestravel with the ophthalmic artery to the dilator pupillae muscle
  • To summarise the motor nerve supply to the extra-ocular muscles as detailed above:
  • Superior oblique is supplied by the trochlear nerve (CNIV)
  • Lateral rectus is supplied by the abducens (CNVI)
  • All others are supplied by the oculomotor nerve (CNIII)

Describe the pupillary light reflex

  • The pupillary light reflex causes the pupil to constrict when the retinareceives a bright light stimulus
  • This is primarily a subcortical reflex and so its absence clinically implies brainstem damage
  • Afferent fibres decussate at the optic chiasm and proceed bilaterally
  • Impulses bypass the lateral geniculate nucleiand synapse in brainstem pretectal nuclei
  • Relays to the Edinger-Westphal nucleus stimulate sympathetic innervation of dilator pupillae
  • Bright light stimulation of one eye therefore results inbilateral pupillary dilation
  • Dilation of the stimulated eye is the directresponse; dilation of the other eye is consensual

[TG1]Small reddish fleshy mass in medial angle of eye