Session 5: The Orbit and the Eyeball Flashcards
What is meant by the Orbital and Optical Axes?
The orbital axis is ~45 degrees
The optical axis is the one you use when you’re looking forward
What 7 bones make up the orbit? What 3 holes are there in the orbit and what do they allow to pass through?
Bones: Lacrimal, Sphenoid, Palatine, Frontal, Maxilla, Zygomatic, Ethmoid
LEARN SEVEN PARTS FOR MY ZOMBIE EYES
- Holes in the orbit (Medial to lateral)
- Orbital/Optic Canal: CN II aka “blindspot”
- Superior Orbital Fissure: CN III, IV, V1 and VI
- Inferior Orbital Fissure: CN V2
Describe the walls of the orbit
The orbit is a pyramidal, bony cavity in the facial skeleton which contains and affords protection to the eyeball and its associated muscles together with a number of nerves and blood vessels and most of the lacrimal apparatus
[*] It has four walls – superior, medial, lateral and inferior – formed by bones of the skull. The bones are pneumatic – have air cells which form sinuses. NB: the first bone listed forms the main part of the wall.
- Superior (roof): frontal bone and lesser wing of sphenoid. The frontal bone separates the orbit from the anterior cranial fossa.
- Floor (inferior): maxilla, zygomatic and palatine. The maxilla separates the orbit from the underlying maxillary sinus.
- Medial: ethmoid, maxilla, lacrimal and sphenoid bones. The ethmoid bone separates the orbit from the ethmoid sinus. NB: the medial walls are parallel
- Lateral: zygomatic (including zygomatic suture), and greater wing of sphenoid
[*] Its apex is the optic foramen located at the opening to the optic canal – that permits the passage of vessels and nerves including the optic nerve and ophthalmic artery.
[*] The base of the orbit opens out in the face and is bounded by the eyelids. It is also known as the orbital rim.
What are the contents of the superior orbital fissure? (lateral => medial)
[*] Lacrimal nerve
[*] Frontal nerve (branch of the ophthalmic nerve)
[*] Trochlear nerve (CN IV)
[*] Superior branch of Oculomotor nerve (CN III)
[*] Nasociliary nerve (branch of the ophthalmic nerve)
[*] Inferior branch of the Oculomotor nerve (CN III)
[*] Abducent nerve (CN VI)
[*] Opthalmic veins (Superior)
[*] Sympathetic nerves
Large French Teenagers Sit Numb in Anticipation of Sweets
[Text Box: NB: the fossa for the lacrimal gland is a depression for the lacrimal gland to sit in, and the fossa for lacrimal sac to nasolacrimal duct is where tears collect in a lacrimal lake]
What are the contents of the Inferior Orbital Fissure?
The inferior orbital fissure (a horizontal fissure) transmits the maxillary nerve (a branch of CN V), the inferior ophthalmic vein and sympathetic nerves.
NB: after the maxillary nerve enters the infraorbital canal, it is frequently called the infraorbital nerve.
What are other minor openings of the orbit
Other minor openings into the orbital cavity include the nasolacrimal canal, which drains tears from the eye to the nasal cavity and is located on the medial wall of the orbit. Others are supraorbital foramen and infraorbital canal – they carry small neurovascular structures.
Describe the Optic Nerve including its anatomical course
[*] Exits the orbit via the optic canal.
[*] 1.2m axons from retinal cells.
[*] Protective coverings of pia (thin), arachnoid (spidery network) and dura (strong, fibrous) mater of meninges. The meninges fuse with the sclera of the eye.
- Continuous with the brain
- Infection of the orbit can spread, tracking backwards along the meninges to the brain to cause meningitis
Anatomical Course of the optic nerve describes the transmission of special sensory information from the retina of the eye to the primary visual cortex of the brain.
- Extracranial: the optic nerve is formed by the convergence of axons from the retinal ganglion cells These cells in turn recieve impulses from the photoreceptors of the eye (the rods and cones). After its formation, the nerve leaves the bony orbit via the optic canal (a passageway through the sphenoid bone). It enters the cranial cavity, running along the surface of the middle cranial fossa (in close proximity to the pituitary gland).
- Intracranial: within the middle cranial fossa, the optic nerves from each eye unite to form the optic chiasm. At the chiasm, fibres from the nasal (medial) half of each retina cross over, forming the optic tracts.
- Left optic tract contains fibres from the left temporal (lateral) retina, and the right nasal (medial) retina.
- Right optic tract contains fibres from the right temporal retina and the left nasal retina.
- Each optic tract traves to its corresponding cerebral hemisphere to reach the Lateral Geniculate Nucleus (LGN), a relay system located in the thalamus; the fibres synapse here. Axons from the LGN then carry visual information via a pathway known as the optic radiation to the visual cortex.
What is meant by the Optic Chiasm?
The optic chiasma is very close to the pituitary gland and the chiasma is also a point for partial crossover for the optic nerves. It allows the visual cortex to receive the same hemispheric visual field from both eyes. Superimposing and processing these monocular visual signals also the visual cortex to generate binocular and stereoscopic vision.
How is the front of the eye protected?
The front of the eye is exposed, and the eyelids and lacrimal glands are important protective mechanisms – protecting from light and injury.
[*] By closing the eyelids, small particles are prevented from landing on the anterior surface of the eyeball.
[*] The lacrimal secretions serve to keep the surfaces moist. The eyelids prevent corneal drying through controlled spread of lacrimal fluid – otherwise scarring could occur, leading to impaired vision.
[*] The gap between the eyelids is called the palpebral fissure.
[*] The left and right palpebral fissures are often not equal in size. This might be normal but could be ptosis (pronounced dropping of the upper eyelid)
The eyelids, which protect the cornea and the eyeball from injury, keep the cornea moist by covering it with lacrimal fluid.
[*] On the inner surface, the eyelids are lined with conjunctiva that is reflected onto the eyeball where it is continuous with the conjunctiva overlying the anterior surface of the eye (palpebral conjunctiva and bulbar conjunctiva).
[*] The eyelids are strengthened by tarsal plates (dense bands of connective tissue) that contain tarsal glands, the secretion of which lubricates the edge of the eyelids and prevents them from sticking together when they close.
[*] The eye blinks when the cornea becomes dry and the eyelids carry a film of fluid over the cornea.
[*] Dust and other foreign material is also swept across to the medial angle of the eye and removed.
Describe the periosteum of the orbit
The periosteum (periorbita) lining the bones of the orbit forms the fascial sheath of the eyeball.
[*] It is continuous at the optic canal and superior orbital fissure with the periosteal layer of the dura mater
[*] It is continuous over the orbital margin and through the inferior orbital fissure with the periosteum covering the external surface of the cranium,
[*] A part of the lateral wall of the orbit, its remaining walls, particularly the medial and inferior, are thin and may fracture as a result of blows and direct trauma.
What are the 2 weak points of the orbit?
- Medial wall called lamina papyracae is paper-thin, easily eroded by infection in ethmoid sinus. So sinusitis can cause sight- and life-threatening orbital cellulitis.
- Inferior wall is the most common site of orbital blow-out fracture. When a blunt object hits the globe or the strong orbital rim, force gets transmitted and the thin orbital floor cracks - but not the orbital rim. In this crack, structures at the inferior site of the eye ball (fat, inferior rectus, CN V2) get trapped and cause diplopia and numbness. The whole eyeball is trapped in this posterior displacement (enophthalmos). Often the fragile medial wall also breaks and air leaks from the nasal sinuses into the eye (orbital emphysema).
Describe fractures of the orbit
- Usually at bony sutures
- Medial wall (thin) fractures – can involve ethmoidal and sphenoidal sinuses
- Inferior wall (thin) fractures – can involve maxillary sinuses
- Outer margins are stronger
- 2 major types of orbital fractures: orbital rim and blowout (more detail later)
- Any fracture of the orbit will result in intraorbital pressure, raising the pressure in the orbit, causing exophthalmos (protrusion of the eye) and there may also be involvement of surrounding structures e.g. haemorrhage into one of the neighbouring sinuses. This is due to the expansion of the fat and connective tissue due to the increase in intraorbital pressure.
- NB: Enophthalmos cn be seen after a larbe orbital floor fracture because of the increase in orbital cavity volume. Exophthalmos can be a presenting sign of a retrobulbar haematoma.
Describe the anatomy of the upper eyelid (outer => inner)
- Skin
- Areolar tissue
- Fibres of orbicularis oculi (circular muscles coming around the eye => close the eye) supplied by the facial nerve
- Levator palpebrae superioris (lift upper eyelid up)
- Superior tarsus (dense connective tissue, strengthening ‘skeleton’ – and also present on the lower eyelid) is behind the LPS.
- Tarsal (Meibomian) glands within superior tarsus secrete oil – helps keep eye moist and keep the fluid on the eye surface
- Ciliary glands (sebaceous) – base of the eyelashes
- Palpebral conjunctiva
What is meant by the lacrimal apparatus and describe the lacrimal glands
The lacrimal apparatus consists of lacrimal glands, lacrimal ducts and lacrimal canaliculi.
[*] The lacrimal gland, which secretes the lacrimal fluid (tears), lies in a fossa on the superolateral part of the orbit. Eyes close laterally to medially, pushing fluid to collect at lacrimal lake (medial canthus)
- Watery physiological saline
- Contains the bacteriocidal lysozyme enzyme
- Moistens and lubricates the surfaces of the conjunctiva and cornea
- Provides some nutrients and dissolved oxygen to the cornea
Describe the lacrimal ducts and canaliculi. What happens to the fluid? And describe the nerve supply to the lacrimal apparatus
[*] The lacrimal ducts conducts lacrimal fluid from the gland to the conjunctival sac. The fluid passes into the lacrimal lake at the medial angle of the eye from which it drains to the lacrimal sac.
[*] The lacrimal canaliculi commence at the medial angle of the eye, where lacrimal fluid is drained from the lacrimal lake => lacrimal sac
[*] The fluid passes to the nasal cavity through the nasolacrimal duct that opens in the nasal cavity (into the inferior nasal meatus which is inferior to the inferior nasal concha) from which it passes into the nasopharynx and is swallowed.
[*] Tears production: parasympathetic fibres of facial nerve, CN VII
[*] Sensory supply via lacrimal branch of the ophthalmic division of CN V (also to eyelid and conjunctiva)
The eyeball has 3 layers. Describe the outer layer
[*] The outer protective layer, which comprises the sclera and the cornea, is fibrous and provides attachment for the extraocular muscles.
- The sclera forms the bulk of the fibrous layer eyeball. Its anterior part is visible through the conjunctiva as the “white of the eye” – white and relatively avascular.
- The cornea is the transparent part of the fibrous coat and is relatively avascular. It receives nourishment from lacrimal glands and vascular beds. It is sensitive – innervated by the ophthalmic division of the trigeminal nerve.
Describe the middle layer of the eyeball
[*] The middle coat comprises the choroid, ciliary body and iris. It has a rich network of blood vessels – vascular layer
- The choroid, which is a dark membrane between the sclera and the retina, forms the largest vascular layer of the eyeball and terminates anteriorly as the ciliary body. The choroid is the ‘red of eye’.
- The ciliary body (which is muscular as well as vascular) connects the choroid with the iris.
- Anterior muscle thickening
- Attaches and focuses the lens
- Ciliary process secretes aqueous humour into the anterior chamber
- The eyeball has 2 chambers. The anterior chamber is the space between the cornea and the iris. The posterior chamber is the space between the iris and the ciliary body and the lens.
- The ciliary body secretes the aqueous humour that fills the chambers of the eye.
- The iris, which lies on the anterior surface of the lens, is a thin contractile diaphragm with a central aperture (the pupil) for transmission of light; 2 muscles (sphincter and dilator papillae) control the size of the pupil.
The Parasympathetic Nervous System contracts the sphincter pupillae (causes circular muscles to contract)
The Sympathetic Nervous System contracts the Dilator Pupillae – stimulates the radial muscles to contract
- The lens, which is posterior to the iris, is a transparent biconvex structure enclosed in a capsule and is attached to the ciliary body by the suspensory ligaments.
- Contraction of the muscle fibres in the ciliary body changes the shape of the lens.
- The cavity behind the lens is filled with vitreous humour (a transparent jelly-like substance) that supports the lens and holds the retina in place.
Describe the inner layer of the eyeball
[*] The inner layer is the retina, consisting of optic and non-visual parts
- The optic retina, which receives the visual light (photo-sensitive), consists of a neural layer (light-receptive) and a pigmented layer.
- The non-visual part is anterior
- In the posterior part (called the fundus) of the eye is a circular depressed area, the optic disc where the optic nerve enters the eyeball. The fundus can be seen with a fundoscope/ophthalmoscope. The optic disc is known as the blind spot – convergence of sensory fibres
- Just lateral to the optic disc lies a small area (macula lutea) of the retina with photoreceptor cells specialised for acuity for vision.
- The fovea centralis, which is a depression in the centre of the macula, is the area of most acute vision.
- The retina is supplied by the central artery of the retina and drained by the corresponding vein.
What is meant by Myopia, Hyeropia and Presbyopia
The cornea is the primary refractive medium
- Myopia: short or near-sight. Image focussed in front of retina
- Hyeropia or hypermetropia: long or far-sight. Image focussed behind the retina
- Presbyopia: far sight due to age-related changes of lens
What produces aqueous humour? Where does it drain? What is its function?
- Produced by the ciliary body – specifically the ciliary processes of the ciliary body
- Drains into scleral venous sinus via trabecular network
- Responsible for intraocular pressure – glaucoma is caused by raised intraocular pressure (outflow of aqueous humour is blocked)
Describe the changes of the lens in near and far vision
Near Vision:
- Accommodation
- Parasympathetic activity in CN III => Sphincter like contraction of Ciliary muscle => lens more globe-like/fatter
Far Vision
- No parasympathetic activity to ciliary muscles => lens stretched => flatter
- Becomes thicker with age
Cataracts – clouding of the lens
What is Vitreous Humour? What are the 2 types of photoreceptors in the retina?
Transparent, jelly-like substance that occupies the cavity behinds the lens and supports the lens
Holds retina in place
NB: Retina – 2 types of photoreceptors
- Rods (more numerous, more sensitive): low light, black and white
- Cones: bright light, colour vision
What muscle is responsible for closing of the eyelid? What is Bell’s Palsy?
Closing of the eyelid involves the Orbiularis oculi which is innervated by the Facial Nerve. If damaged, it could lead to Bell’s Palsy (one-sided paralysis termed if there is no definitive cause found) => facial paralysis on affected side (muscles of facial expression weakened). You would get the loss of blink and corneal reflex, dry eyes (need eyedrops) and infection is likely
Describe the Recti muscles
[*] The recti muscles arise from a fibrous cuff, the common tendinous ring that surrounds the optic canal and attaches to the sclera on the anterior half of the eyeball.
- Superior rectus: look up
- Inferior rectus: look down
- Medial rectus: look medial (adduct pupil)
- Lateral rectus: look lateral (abduct pupil)
They are all innervated by the Oculomotor Nerve, apart from the Lateral Recutus which is innervated by the Abducent Nerve
Describe the Arterial Supply to the Orbit
The arteries supplying the orbit are mainly from the ophthalmic artery, which gives off the central artery to the retina. Short and long ciliary arteries supply external aspect of eye and some go to the back of eye, into the sinuses etc.
[*] Ophthalmic artery is branch of the internal carotid artery
[*] Central artery of the retina is an end artery – obstruction (e.g. by embolus) results in instant and total blindness.
[*] The other artery is the infraorbital artery, a branch of the maxillary artery which is a branch of the external carotid artery.
[*] Branches of the ophthalmic artery (supplies most of the orbit)
- Central retinal artery
- Lacrimal artery – lacrimal gland, eyelids, conjunctiva
- Posterior ciliary arteries – posterior external eye
- Muscular branches to extraocular muscles
- Other separate branches to ethmoidal and frontal sinuses, eyelids, forehead and scalp etc.
Describe the Venous Drainage of the Orbit. What is meant by Danger Triangle?
The veins of the orbit are tributaries of the ophthalmic veins that drain into the cavernous venous sinus lying within the cranial cavity.
[*] Superior ophthalmic vein – from inner angle to orbit to superior orbital fissure and drain into the cavernous sinus. (medial-superior direction)
[*] Inferior Ophthalmic vein drains from plexus on floor and medial wall to then drain either by superior orbital fissure or inferior orbital fissure. If it drains into the superior orbital fissure, it will drain into the cavernous sinus. (medial-inferior direction)
[*] Central vein of the retina drains into the cavernous sinus, either directly or via ophthalmic veins.
- Occlusion results in slow, painless loss of vision
- Infections may also spread by this route from the eye to the brain
DANGER TRIANGLE: communication between facial vein to cavernous sinus via ophthalmic veins
Cavernous sinus thrombosis, meningitis, brain abscess
What happens to the lens in old age?
During old age, the lens becomes harder and more flattened and these changes slowly reduce their focusing capacity (presbyopia). Some elderly people develop partial or complete opacity of the lens (cataracts).
[*] As people age, their lenses become harder and more flattened. These changes gradually reduce the focusing power of the lenses (a condition known as Presbyopia)
[*] Some people also experience a loss of transparency (cloudiness) of the lens leading to areas of opaqueness and decreased vision (Cataracts). Cataract extraction combined with an intra-ocular lens implant has become a common operation.
Describe examining the 6 Cardinal Positions of Gaze
Movements of the eyeball around the vertical, horiztonal and anteroposterior axes
[*] Adductors: MR, SR and IR
[*] Abductors: LR
[*] Elevators: IO, SR
[*] Depressors: SO, IR
[*] Medial Rotators (Intorsion): SR, SO
[*] Lateral Rotators (Extorsion): IO, IR
Describe normal ophthalmoscopy
The optic disc should be a nice rounded flattened structure with straight radiating artery branches. In a papillodema (optic disc swelling), the artery branches appear kinked and distorted and the optic disk may appear pointing forwards “bulging towards you”
If there is no red pupil reflex (reddish-orange reflection of light from the eye’s retina that is observed when using an ophthlamoscope), the person may have cataracts, retinboblastoma or retinal detachment
