Eye Orbit Anatomy

The Orbit

• A pyramidal-shaped bony cavity is located on each side of the anterior skull
• Each orbit houses and protects the eyeball, associated muscles, nerves, vessels, and most of the lacrimal apparatus
• The orbit is lined by the periorbita, which forms the fascial sheath of the eyeball
• The periorbita connects to the optic canal, superior and inferior orbital fissure, and orbital margins
• The orbit consists of four walls and an apex

Orbit Walls

• The superior wall (roof) comprises the orbital part of the frontal bone and the lesser wing of the sphenoid bone
• The medial wall (paper-thin) consists of the ethmoid, lacrimal, frontal, and sphenoid bones
• The inferior wall (floor) featuring the inferior orbital fissure, includes the maxilla, zygomatic bone, and palatine bone
• The lateral wall, which includes the superior orbital fissure, consists of the frontal process of the zygomatic bone and the greater wing of the sphenoid bone
• The apex includes the optic canal, located in the lesser wing of the sphenoid bone

Eyelids and Lacrimal Apparatus

• Eyelids are mobile, flexible structures that cover the globe anteriorly and contain musculocutaneous and orbital components posteriorly

Eyelid Functions

• Shields the eye from injury and excessive light
• Keeps the cornea moist via spreading lacrimal fluid as they open and close

The Orbicularis Oculi Muscle

• A striated muscle sheet beneath the skin that closes the eye and wrinkles the forehead vertically
• The lacrimal part draws the eyelids and lacrimal puncta medially, lying deep to the palpebral part
• The palpebral part gently closes the eye to keep the cornea from drying
• The orbital part strongly closes the lids to protect against glare and dust
• The muscle is innervated by the zygomatic and temporal branches of the facial nerve (CNVII)

Eyelid Coverings

• Thin skin (externally)
• Palpebral conjunctiva (internally) reflects onto the eyeball and is continuous with the bulbar conjunctiva

Conjunctival Fornices

• Deep recesses formed where the palpebral conjunctiva reflects onto the eyeball
• Includes superior and inferior aspects

Conjunctival Sac

• The space bound by the conjunctival membrane, between the palpebral and bulbar conjunctiva
• Fornices are part of this sac

Tarsal Plates

• Dense connective tissue bands forming the structural "skeleton" of the eyelid

Tarsal Glands

• Generate a lipid secretion that lubricates the edges of the eyelids
• Prevents the eyelids from sticking together when closed

Ciliary Glands

• Large sebaceous glands associated with the eyelashes

Canthi

• The angle or corner of the eye where the eyelids meet

Palpebral Ligaments

• The medial palpebral ligament attaches the tarsal plates to the medial margin of the orbit, serving as the origin and insertion point for the orbicularis oculi muscle
• The lateral palpebral ligament attaches the tarsal plates to the lateral margin of the orbit but does not facilitate direct muscle attachment

Orbital Septum

• A weak membrane from the tarsal plates to the orbital margins, continuous with the periosteum

Lacrimal Apparatus Components

• Lacrimal glands secrete lacrimal fluid via parasympathetic fibers from CN VII
• The glands are in the fossa for the lacrimal gland in the superolateral orbit and are divided into superior (orbital) and inferior (palpebral) parts by the levator palpebrae superioris muscle tendon

Lacrimal Ducts

• Allows lacrimal fluid from glands to enter conjunctival sac
• Approximately 12 ducts open into the superior conjunctival fornix

Lacrimal Lake

• A triangular space where tears collect at the medial angle of the eye
• The lacrimal caruncle is a small mound of modified skin inside the lacrimal lake

Lacrimal Papilla

• A small elevation at the medial end of each eyelid

Lacrimal Punctum

• The opening on each lacrimal papilla

Lacrimal Sac

• The dilated superior section of the nasolacrimal duct

Lacrimal Canaliculi

• Small canals located between the lacrimal punctum and the lacrimal sac that transport lacrimal fluid from the lacrimal lake to the nasolacrimal duct

Nasolacrimal Duct

• Conveys lacrimal fluid to the nasal cavity

Lacrimal Fluid Flow

• Lacrimal gland → lacrimal ducts → conjunctival sac → lacrimal lake → lacrimal puncta → lacrimal canaliculi → lacrimal sac → nasolacrimal duct → nasal cavity
• Lacrimal fluid passes over the eyeball from superolateral to inferomedial

Lacrimal Apparatus Innervation

• Parasympathetic innervation involves presynaptic fibers from the greater petrosal nerve to the nerve of the pterygoid canal, synapsing at the pterygopalatine ganglion, and postsynaptic fibers from the zygomatic nerve to the lacrimal nerve
• Sympathetic solely innervates blood vessels, where postsynaptic fibers arise from the superior cervical ganglion, travel through the internal carotid plexus, deep petrosal nerve to the nerve of pterygoid canal, through the pterygopalatine ganglion, finally through the zygomatic nerve, to the lacrimal nerve

Orbital Contents

• Includes: the eyeball and optic nerve, ocular muscles, fascia, nerves, vessels, fat, the lacrimal gland and lacrimal sac
• Alignment is determined by the extraocular muscles plus surrounding tissue

Ocular Muscles

• The direction of pupil movement defines muscle operation

Ocular Movement Terms

• Elevation: Pupil moves upward
• Depression: Pupil moves downward
• Abduction: Pupil moves laterally
• Adduction: Pupil moves medially
• Intorsion: Pupil rotates medially or nasally
• Extorsion: Pupil rotates laterally or temporally

Levator Palpebrae Superioris

• Elevates the superior eyelid and opposes the orbicularis oculi muscle
• Distally broadens into an aponeurosis attaching to the tarsal plate
• Works together with the superior rectus and levator palpebrae superioris muscles

Recti Muscles and Main Actions

• Superior rectus elevates, adducts, and medially rotates the eyeball
• Inferior rectus depresses, adducts, and laterally rotates the eyeball
• Medial rectus adducts the eyeball
• Lateral rectus abducts the eyeball
• All originate from a common tendinous ring around the optic nerve and superior orbital fissure and they attach to the sclera on the eyeball's anterior half

Oblique Muscles

• Both the superior and inferior oblique muscles tested while eye is adducted

Superior Oblique Function

• Turns the pupil downward; helps abduct and medially rotate the eyeball
• Tested by having the patient look downward while the pupil is oriented nasally

Inferior Oblique Function

• Turns the pupil upward; assists abduction and laterally rotates the eyeball
• Testing achieved via instructing a patient to look upward with with the eye adducted

Oblique Muscles Origin

• The superior oblique originates from the sphenoid bone and its tendon passes through a trochlea on the superomedial orbit
• The inferior oblique arises from the anterior part of the orbit floor, both muscles insert on the sclera of the eye

Eye Movements

• Six extraocular muscles, controlled by cranial nerves III, IV, and VI, facilitates visual fixation
• Maintaining fixation requires exquisite coordination, involving nuclei of cranial nerves III, IV, and VI and higher centers in the cortex and brainstem

Muscle Actions During Gaze

• Medial rectus adducts the eye
• Inferior rectus facilitates downward gaze
• Superior rectus and the inferior oblique combine for upward gaze
• The levator palpebrae superioris and superior rectus work together to lift the eyelid

Fascial Sheath and Ligaments

• Tenon's capsule (bulbar sheath) encloses the eyeball from the optic nerve to the corneoscleral junction, being pierced and reflected by muscle tendons
• Check ligaments (medial and lateral) extend from the tubular sheaths of the medial and lateral recti muscles to the lacrimal and zygomatic bones, limiting adduction and abduction
• The suspensory ligament, a blend of check ligaments and fascia from the inferior rectus/oblique muscles, supports the eyeball inferiorly

Cranial Nerve III (Oculomotor) Actions

• Plays a large role in eye movements
• Somatic motor component: innervates the superior, medial, and inferior rectus and inferior oblique muscles, along with the levator palpebrae superioris
• Visceral motor component innervates the constrictor pupillae and ciliary muscles

Formation of the Oculomotor Nerve

• At the junction of the midbrain and pons, somatic motor fibers merge with parasympathetic fibers from the Edinger-Westphal nucleus
• The nerve courses anteriorly, piercing the dura and entering the cavernous sinus
• The nerve then passes: along the lateral wall of the cavernous sinus, superior to the trochlear nerve, through the superior orbital fissure, then through the common tendinous ring
  • Superior division: supplies the superior rectus and levator palpebrae superioris
  • Inferior division: supplies the medial and inferior rectus, and the inferior oblique

Parasympathetic Fibers in the Oculomotor Nerve

• Presynaptic parasympathetic fibers travel through the inferior division of the oculomotor nerve to synapse in the ciliary ganglion
• Postsynaptic fibers exits the ciliary ganglion as short ciliary nerves to the eyeball to control constrictor pupillae and ciliary muscles to alter the pupil size and lens shape

Cranial Nerve IV (Trochlear) Actions

• Supplies the superior oblique muscle only
• Enters the cavernous sinus and the orbit through the superior orbital fissure, above the common tendinous ring
• Stimulation results in inward rotation and downward-lateral movement of the eye

Cranial Nerve VI (Abducens) Actions

• Supplies the lateral rectus muscle
• Enters the cavernous sinus lateral to the internal carotid artery and the orbit through the medial end of the superior orbital fissure, where the common tendinous ring encircles it
• Causes contraction of the lateral rectus muscle for abduction

Lacrimal Nerve Route and Actions

• A branch of the ophthalmic nerve (CN V1), supplies the lacrimal gland, conjunctiva, and the skin of the superior eyelid

Frontal Nerve Route and Actions

• A branch of the ophthalmic nerve (CN V1) that branches into the supraorbital (medial and lateral branches) and supratrochlear nerves, supplying the upper eyelid, forehead, and scalp

Nasociliary Nerve Route and Actions

• A branch of the ophthalmic nerve (CN V1) that divides into anterior and posterior ethmoidal nerves, which supply the mucous membrane and dura, and the infratrochlear nerve, which supplies the eyelids, conjunctiva and the skin of the nose and lacrimal sac

Short Ciliary Nerves

• Branches of the ciliary ganglion entering the eye
• Carry postsynaptic parasympathetic fibers originating in the ciliary ganglion
• Carry postsynaptic sympathetic fibers that passed through the ciliary ganglion
• Carry afferent fibers from the nasociliary nerve that passed through the ciliary ganglion

Long Ciliary Nerves Actions

• Branches of the nasociliary nerve that carry postsynaptic sympathetic fibers to the dilator pupillae
• Also carry afferent fibers from the iris and cornea

Nerves Entering the Orbit

• CN III, CN IV, CN V1 (Ophthalmic) and CN VI enters through the superior orbital fissure
• The lacrimal nerve, frontal nerve and nasociliary (branches of V1) are also included in this nerve group

Vasculature of the Orbit

• Ophthalmic artery - a branch of the internal carotid artery

Ophthalmic Arteries

• Central artery of the retina
• Supraorbital artery
• Supratrochlear artery
• Lacrimal artery
• Dorsal nasal artery
• Short posterior ciliary arteries
• Long posterior ciliary arteries
• Posterior ethmoidal arteries
• Anterior ethmoidal arteries

Ophthalmic Veins

• Superior ophthalmic vein
• Inferior ophthalmic vein
• These veins communicate with the angular vein anteriorly
• The superior and inferior ophthalmic veins then unite and pass through the superior orbital fissure, draining into the cavernous sinus
• The inferior ophthalmic vein may also drain into the pterygoid plexus

Fractures

• Indirect traumatic injury displacing orbital walls is known as a "blowout" fracture
• Medial wall fractures may involve the ethmoidal and sphenoidal sinuses
• Inferior wall fractures may involve the maxillary sinus
• Resultant intraorbital bleeding causes exophthalmos
• A sharp object may penetrate the superior wall and enter the frontal lobe

Orbital Tumors

• Malignant tumors may erode bony walls of the orbit, compressing the optic nerve and orbital contents
• Tumors in the middle cranial fossa can enter through the superior orbital fissure
• Those in the temporal/infratemporal fossa enter via the inferior orbital fissure
• Approximately 2.5 cm (1 inch) of the eyeball is exposed when the pupil is turned medially

Injury to the Nerves Supplying the Eyelid

• A lesion of the oculomotor nerve (CN III) causes paralysis and upper eyelid droop (ptosis)
• Damage to the facial nerve (CN VII) causes orbicularis oculi paralysis, preventing eyelid closure
• Resultant loss of tonus is then the lower eyelid causes eversion from the surface of the eye and tear production occurs

Inflammation of the Palpebral Glands

• Any gland in the eyelid may become inflamed and swollen from infection or obstruction

Sty (Hordeolum)

• A painful red, suppurative swelling
• Develops on the eyelids if the ducts of the ciliary glands become obstructed

Chalazia and Tarsal Chalazion

• Cysts of the sebaceous glands
• The tarsal glands causes inflammation, protruding and rubbing against the eyeball

Horner Syndrome

• Interruption of a cervical sympathetic trunk results in paralysis of the superior tarsal muscle via sympathetic fibers, causing ptosis
• Includes a constricted pupil; sinking, redness, and dryness of the eye; and increased temperature of the affected side

Paralysis of the Extraocular Muscles

• May cause diplopia (double vision)
• Paralysis of the lateral rectus results in an inability to abduct the pupil on the affected side, causing the pupil to be fully adducted by the medial rectus muscle

Pupillary Light Reflex

• Light entering the eye causes signals that leads to pupillary constriction via the parasympathetic pathway
• Light shone in either eye causes constriction of the pupil in the same eye, called the direct light reflex, and constriction in the other eye, called the consensual light reflex
• Damage to CN III results in a loss of the direct light reflex, but the consensual light reflex functions

Accommodation Reflex Adaptations for focus

• Increased Lens Curvature: The suspensory ligament relaxes due to ciliary muscle contraction
• Pupillary Constriction: The Edinger-Westphal signals the sphincter, narrowing the pupil to sharpen focus
• Convergence of the Eyes: The oculomotor nucleus signals extraocular muscles

Oculomotor Nerve (CN III) Lesions

• Vascular issues, inflammation, herniation, and sinus pathologies can cause oculomotor damage
• Issues can manifest as Strabismus or ptosis due to inactivation of levator palpebrae superioris and unopposed action of orbicularis oculi

Trochlear Nerve (CN IV) Lesions

• Can be caused by: compression, inflammation, aneurysms
• Patients with Trochlear Nerve Palsy exhibit visual difficulty when going downstairs and will compensate with torticollis

Abducens Nerve (CN VI) Lesions

• Can be caused by: compression, inflammation, aneurysms, medial ear infections, base of the skull fractures, and tumors
• Paralysis leads to strabismus due to an inability to abduct the affected eye past the midline of gaze