Anatomy of Orbit in a Nutshell

Anatomy of Orbit

Introduction: Using Anatomy Revealed

Common Tendinous ring:

• Supplies origin for many intrinsic muscles of the eye
• Surrounds the optic nerve at the back of the orbit.

Approaching superiorly through anterior cranial cavity:

• Periorbita is periosteum inside the orbit
• Orbit is packed with fat (deep to periorbita from this direction)
• With large blood vessel: superior ophthalmic vein
  • Vessel draining orbit are large
  • Not to be confused with arteries
• Optic nerve is a very fine plexus
• Nasociliary nerve: passes over the optic nerve laterallyà medially
• Opthalmic artery will have same path across optic nerve.
• Deep to intrinsic muscles, will see infraorbital groove for infraorbital nerves / vessels.
• Inferior orbital fissure: connects orbit to infratemporal fossa
• Nasolacrimal fossa: tears flow into canal relating to this foss, thus excessive tears end up in nasal cavity.

Surface features of the eye and orbit

• Palpebral fissure – space between the eyelids
• Medial canthus– medial angle where upper and lower lids unite
• Lateral canthus– lateral angle where upper and lower lids unite
• Lacrimal caruncle–
  • Fleshy, yellowish mass in the medial canthus that contains modified sweat and oil (sebaceous) glands
  • Help channel tears across eye and into nasolacrimal apparatus
• Iris– circular, pigmented muscular structure posterior to the cornea that regulates the amount of light entering the eye
• Pupil– opening in the iris through which light enters the eye. Is black because inside of the eye is heavily pigmented (retinal pigmented epithelium)

Walls of the Orbit:

• Roof: Frontal bone (and lesser wing of sphenoid)
• Floor: Maxilla (also zygomatic and palantine) Is very thin
• Medial: Ethmoid bone (also, frontal, lacrimal, sphenoid) Is very thin (thinner than floor)
• Lateral: Zygomatic bone (also greater wing of sphenoid)

Important fissures

• Inferior orbital fissure:
• Superior orbital fissure: these two will connect with each other at the back of the orbit
• Optic canal– where vessels pass to and from structures in the orbit
• Ethmoid foramina–
  • Anterior / posterior
  • openings for nerves / vessels passing between orbit and nasal cavity
  • Also leads to numerous paranasal sinuses

Orbital Blowout Fracture:

• Fracture of bones of orbit (secondary to blunt blow to the eye- fist, elbow, baseball)
• Forces eye to the back of the orbit
• Fractures usually the floor or the medial wall
• Have pain / tenderness around the eye and often double vision
  • As eye is forced back into the eye, it may get trapped
  • Muscles that move the eye on that side are trapped, thus can’t focus, leading to diplopia (double vision)
• Entrapment of inferior oblique or rectus muscles can limit upward or downward gaze
• May have bleeding into sinuses- paranasal sinuses such as maxillary sinus, sphenoidal sinus

Important points on shape:

• Orbits have pyramid shape
• Medial walls are parallel to each other
• Lateral walls are at 90 degrees to each other
• Orbital axis:
  • Is not the same as visual axis
  • Through the middle of the eye (obliquely) and orbit
• Axis of the eyeball is in “primary gaze” where eyes are focused at infinity straight ahead.
• Have an angle  between axis of the orbit and axis of the eye in primary gaze
  • Is approximately 23 degrees
  • Is important clinically to test cranial nerves innervating intrinsic eye muscles

Important channels:

• Optic canal: contains
  • CN II (optic nerve)
  • Ophthalmic artery (branch of Internal Carotid)
• Superior orbital fissure
  • CN III, IV, VI (six), and V1 (ophthalmic division of trigeminal)
  • Ophthalmic nerve
  • Ophthalmic veins
  • Common Tendinous ring

• • • Encircles the optic canal
    • Structures in superior orbital fissure are divided into whether they pass inside / outside this ring:
      1. Inside: CN II, CN III, CN VI, ophthalmic artery
      2. Outside: V1, CN IV, ophthalmic veins

• Inferior orbital fissure: contains infraorbital nerve (branch of CN V2), artery, vein

Orbit innervation:

• Ophthalmic nerve (V1) branches:
  • Lacrimal nerve – sensory to upper eye lid (esp. lateral portion)
  • Frontal nerve – to skin over frontal bone

1. 1. • Supraorbital
      • Supratrochlear

1. • Nasociliar nerve– crosses optic nerve and gives branches

• • • Ethmoidal nerves (anterior / posterior)
      1. Anterior Ethmoidal – supplies nasal cavity and anterior / middle ethmoid air cells (small paranasal sinuses)
      2. Posterior Ethmoidal – supplies sphenoidal sinus and posterior ethmoid air cells.
    • Long ciliary nerves – supplies eye

• • • Ends as the infratrochlear nervee→ supplies upper lid

• • • Also gives off nerves that travel to the eyeball itself

Ciliary Ganglion: is a parasympathetic Ganglion in the orbit

• Has sensory fibers, sympathetic fibers, parasympathetic fibers
• Sensory fibers (from V1) arise out of the trigeminal ganglia has branches
  • Long ciliary nerve– contains sensory fibers that go around back of eye and go into the cornea. These are thus sensory to the eye.
  • Other fibers travel to ciliary ganglion via nasociliary root of ciliary ganglion.
  • Short ciliary nerves come off of ciliary ganglia and supply cornea / sclera

Sympathetics:

• Arise out of thoracic spinal cord
• Travel on internal carotid as internal carotid plexus
• Travel along with V1 to the back of the eyeà course around eye ball and innervate the papillary dilator (dilates the pupil).
• Sympathetic stimulation dilates the pupil.
• Other sympathetic fibers follow along blood vessels from internal carotidà leave to ciliary ganglion via sympathetic root of ciliary galnglion

Parasympathetic fibers:

• Preganglionic fibers arise from the Westphal nucleus
• Travel in CN III through superior orbital fissure (enters orbit)
• A group will branch off à leave CN IIIe→ forms parasympathetic root of ciliary ganglionà contact the ciliary ganglion where they synpasee→
• Postganglionic fibers will go to pupil and innervate ciliary muscle (important for near vision, controls shape of lens) and the papillary dilator (will constrict pupil)
• Short ciliary have sensory, sympathetic, parasympathetic
• Long ciliary nerves lack parasympathetic

Vasculature of the orbit

• Primary artery of the orbit is the Ophthalmic artery (branch of internal carotid artery after it enters the skull)
  • Passes inferior to the optic nerve in the optic canal
  • Travels with the nasociliary nerve
  • Ophthalmic Gives off branches

1. 1. • Lacrimal arterye→ lacrimal gland, then goes onto face around orbit
      • Supraorbital

• • • Anterior / posterior ethmoid arteries

• • • Central artery of the retina:
      1. Enters CN II
      2. Provides only blood supply to inner retina to supply retinal ganglion cells (are connection to brain via CN II)
  • Terminates on the face as
    • Dorsal nasal – top of nose
    • Supratrochlear

1. • Anastomosis between middle meningeal artery and lacrimal artery allows collateral circulation
2. Veins of the orbit:
   • Superior ophthalmic veins
   • Inferior ophthalmic veins
   • Both ophthalmic veins connect with facial vein
   • There are no valves in these veins
   • 2 sites for drainage of ophthalmic veins:

1. 1. • To the pterygoid venous plexus in the infratemporal fossa
      • To the cavernous sinus in the cranial cavity:

1. • Clinical:

1. 1. • Cavernous sinus is on either side of the pituitary gland
      • Have internal carotid artery passing through cavernous sinus

• • • Danger is if have an aneurysm of the internal carotid artery (or tumor of pituitary) within this portion in the cavernous sinus

• • • If expansion occurse→ it will impact many nerves in the area
      • CN VI Abducens is first to be disrupted (innervates lateral rectus). If this occurs, patient has double vision.
      • In lateral wall of this sinus:
        1. Cranial nerve III
        2. Cranial nerve IV
        3. V1, V2 (V3 has left through foramen ovale already)

Horner’s Syndrome:

• Interruption of sympathetic innervation to head and neck
• Usually occurs in the neck
• Due to metastasis of bronchial tumor that spreads to sympathetic trunk in the neck
• May be due to radial neck dissection
• Sympathetics control papillary dilation of the eye (would be lacking in affected side)
• Clinical signs:
  • Ptosis: drooping of eyelids. Results from paralysis of tarsal muscles (smooth muscles) especially the upper lid and paralysis of the superior tarsal muscle.
  • Pupillary constriction: results from paralysis of papillary dilator muscle
  • Apparent Enophthalmos: sinking of eye into orbit. Results from paralysis of orbitalis muscle in floor of the orbit (a vestigal muscle in humans that spans the inferior orbital fissure)
  • Vasodilation and anhydrosis: lack of sweating on face / neck. Results from lack of sympathetic (vasoconstriction) innervation to blood vessels and sweat glands.
• Congenital Horner’s Syndrome:
  • Typically seen around 2 years of age
  • Heterochromia (2 different eye colors)
  • Absence of horizontal eye fold / crease
  • Iris pigmentation (under sympathetic control during development) is completed by age of two.