"Gross Anatomy of Orbit and Eyeball" PDF

GROSS ANATOMY OF THE ORBIT, EYEBALL AND LACRIMAL GLAND BY DR PAUL A. ODEY The orbit The orbits are bilateral bony cavities in the facial skeleton that resembles hollow quadrangular pyramids with their bases directed anterolaterally and their apices, posteromedially. BOUNDEDE...

GROSS ANATOMY OF THE ORBIT, EYEBALL AND LACRIMAL GLAND BY DR PAUL A. ODEY The orbit The orbits are bilateral bony cavities in the facial skeleton that resembles hollow quadrangular pyramids with their bases directed anterolaterally and their apices, posteromedially. BOUNDEDED: Superiorly by anterior cranial fossa Inferiorly by maxillary sinus Medially by nasal cavity and ethmoidal air cells Laterally by medial cranial fossa and temporal fossa, Diagram showing bones of the orbit DIMENSIONS Volume: 30cm3 Rim: Horizontally – 4cm vertically – 3.5cm Intra orbital width: 2.5cm Extra orbital width: 10cm Depth: medially - 4.2 cm Laterally – 5.0cm Ratio of vol. of orbit: vol. of globe – 4.5:1  The Bony Orbit vSeven bones makes up the bony orbit: Frontal bone Zygomatic bone Maxillary bone Ethmoid bone Sphenoid bone Lacrimal bone Palatine bone  WALLS OF THE ORBIT The bony orbit has four walls: 1. The roof 2. The floor 3. Lateral wall 4. Madial wall  The Roof (superior wall) q - formed by: Orbital plate of frontal bone Lesser wing of sphenoid bone q It is triangular in shape. q It underlies the frontal sinus which separates the orbit from the anterior cranial fossa. q Has a landmark; supraorbital notch which transmits the supraorbital nerve and blood vessels.  The floor (inferior wall) q It is the shortest orbital wall. qTriangular in shape. qIt is formed by: qthe maxilla medially, qpalatine posteriorly and qzygomatic bone laterally. qThe maxilla separates the orbit from the underlying maxillary sinus. qThe floor has 3 landmarks; infraorbital groove, infraorbital canal, and infraorbital foramen. qIt transmits infraorbital nerves and blood vessels. Medial wall qformed antero-posteriolly by Frontal process of maxilla Lacrimal bone Orbital plate of ethmoid bone and Body of sphenoid bone. The ethmoid bone separates the orbit from the ethmoid sinus. Lateral wall It is the thickest and strongest orbital wall. It is formed by: The zygomatic bone Greater wing of the sphenoid. DIAGRAM SHOWING THE WALLS OF THE ORBIT qApex – located at the opening to the optic canal, the optic foramen. Average length is 6- 11mm. It connects the orbit to the middle cranial fossa and transmits the optic nerve and ophthalmic artery. qBase – also known as the orbital rim, opens out into the face and is bounded by the eyelids  Contents of the orbit Contains; the eyeballs, and accessory visual structures which includes: The eyelids which bound the orbits anteriorly, controlling exposure of the anterior eyeball. extraocular muscles, which positions the eyeballs and raises the superior eyelids. Nerves, and vessels in transit to the eyeballs and muscles. Orbital fascia surrounding the eyeballs and muscles. Mucous membrane (conjunctiva) lining the eyelids and anterior aspect of the eye b a l l s a n d m o st o f t h e l a c r i m a l apparatus, which lubricates it. Other spaces free of theses structures is filled with orbital fat; thus forming a matrix in which the structures of the orbit are embedded. APPLIED ANATOMY OF THE ORBIT Orbital rim fracture – This is a fracture of the bones forming the outer rim of the bony orbit. It usually occurs at the sutures joining the three bones of the orbital rim – the maxilla, zygomatic and frontal. ‘Blowout’ fracture – This refers to partial herniation of the orbital contents through one of its walls. This usually occurs via blunt force trauma to the eye. The medial and inferior walls are the weakest, with the contents herniating into the ethmoid and maxillary sinuses respectively. Any fracture of the orbit will result in intraorbital pressure, raising the pressure in the orbit, causing exophthalmos (protrusion of the eye). There may also be involvement of surrounding structures, – e.g haemorrhage into one of the neighboring sinuses.  Orbitral Tumors: due to the closeness of the optic nerve to the sphenpoidal and posterior ethmoidal sinuses, a malignant tumor in these sinuses may erode the thin bony walls of the orbit and compress the optic nerve and orbital contents. This also produces exophthalmos. THE EYEBALL It is a bilateral and spherical organ measuring approximately 25mm in diameter that lies in the bony orbit. It contains the optical apparatus of the visual system. It can be divided into three parts; the fibrous, vascular and inner layers.  FIBROUS LAYER It is the outermost (coat) layer of the eyeball. Consists of the sclera and cornea which are continuous with each other. The main functions of these structures is to provide shape and resistance to support the deeper structures of the eye. The sclera: It is the toughest opaque part of the fibrous layer. Comprises approximately 85% of the fibrous layer It provides attachment to the extraocular and intraocular muscles responsible for movement of the eye. It is visible as the white part of the eye. It is relatively avascular.  The cornea: It is transparent and positioned centrally at the front of the eye. Has a greater convexity than the sclera, hence it appears to protrude from the eyeball when viewed laterally. Refraction of light occurs at the cornea. Receives nourishment from capillary beds around its periphery and fluids from its external and internal surfaces (lacrimal fluid and aqueous humor respectively) as it is completely avascular. The corneal limbus is an angle formed by the intersecting curvatures of the cornea and sclera at the corneoscleral junction. The corneoscleral junction includes numerous capillary loops involved in nourishing the avascular cornea.  VASCULAR LAYER It is also called the uvea or uveal tract. It lies underneath the fibrous layer. It consists of the choroid, ciliary body and iris. Øchoroid – a dark reddish brown layer of connective tissue and blood vessels (choriocapillaris) between the sclera and retina that forms the largest part of the vascular layer. It lines the sclera and provides oxygen and nourishment to the outer layers of the retina. ØCiliary body – comprised of two parts – the ciliary muscle and ciliary processes. The ciliary muscle consists of a collection of smooth muscles fibers. These are attached to the lens of the eye by the ciliary processes. The ciliary body controls the shape of the lens, and contributes to the formation of aqueous humor ØIris – circular structure, with an aperture in the center (the pupil). The diameter of the pupil is altered by smooth muscle fibers within the iris, which are innervated by the autonomic nervous system. It is situated between the lens and the cornea.  THE INNER LAYER The inner layer of the eye is formed by the retina; its light detecting component. The retina grossly consists of two functional parts, the optic part and non-visual retina. The optic part is sensible to visual light rays and is composed of two layers: Ø Pigmented (outer) layer – formed by a single layer of cells. It is attached to the choroid and s u p p o r t s t h e c h o ro i d i n a b s o r b i n g l i g h t (preventing scattering of light within the eyeball). It continues around the whole inner surface of the eye. Ø Neural (inner) layer – is light receptive. It consists of photoreceptors, the light detecting cells of the retina. It is located posterior-laterally in the eye. The non-visual retina is an anterior continuation of the pigmented layer and a layer of supporting cells. It extends over the ciliary body and the posterior surface of the iris to the pupillary margin.  The optic part of the retina can be viewed during ophthalmoscopy. The center of the retina is marked by an area known as the macula. It is yellowish in colour, and highly pigmented. The macula contains a depression called the fovea centralis, which has a high concentration of light detecting cells. It is the area responsible for high acuity vision. The area that the optic nerve enters the retina is known as the optic disc – it contains no light detecting cells. STRUCTURE OF THE EYEBALL THE VITEROUS BODY: The vitreous body is a transparent gel which fills the posterior segment of the eyeball (the area posterior to the lens). It is marked by a narrow canal called the hyaloid canal which runs from the optic disc to the lens. This is a fetal remnant. The vitreous body has three main functions: Contributes to the magnifying power of the eye Supports the lens Holds the layers of the retina in place  LENS The lens of the eye is located anteriorly, between the vitreous humor and the pupil. The shape of the lens is altered by the ciliary body, altering its refractive power. In old age, the lens can become opaque – a condition known as a cataract. Anterior and Posterior Chambers There are two fluid filled areas in the eye – known as the anterior and posterior chambers. The anterior chamber is located between the cornea and the iris, and the posterior chamber between the iris and ciliary processes. The chambers are filled with aqueous humor – a clear plasma-like fluid that nourishes and protects the eye. The aqueous humor is produced constantly, and drains via the trabecular meshwork, an area of tissue at the base of the cornea, near the anterior chamber. If the drainage of aqueous humor is obstructed, a condition known as glaucoma can result. Vasculature The eyeball receives arterial blood primarily via the ophthalmic artery. This is a branch of the internal carotid artery, arising immediately distal to the cavernous sinus. The ophthalmic artery gives rise to many branches, which supply different components of the eye. The central artery of the retina is the most important branch – supplying the internal surface of the retina. Occlusion of this artery will quickly result in blindness. Venous drainage of the eyeball is carried out by the superior and inferior ophthalmic veins. These drain into the cavernous sinus, a dural venous sinus in close proximity to the eye.  Glaucoma refers to a group of eye diseases that result in damage to the optic nerve. There are two main clinical classifications of glaucoma: Open angle – where the outflow of aqueous humor through the trabecular meshwork is reduced. It causes a gradual reduction of the peripheral vision, until the end stages of the disease. Closed angle – where the iris is forced against the trabecular meshwork, preventing any drainage of aqueous humor. It is an ophthalmic emergency, which can rapidly lead to blindness.  Papilloedema: refers to swelling of the optic disc that occurs secondary to raised intracranial pressure. The optic disc is the area of the retina where the optic nerve enters and can be visualised using an ophthalmoscope. Common causes include: Intracerebral mass lesions Cerebral haemorrhage Meningitis Hydrocephalus In papilloedema, the high pressure within the cranium resists venous return f ro m t h e eye. T h i s ca u s e s f l u i d to extravasate from blood vessels and collect in the retina, producing a swollen optic disc. EXTRAOCULAR MUSCLES OF THE ORBIT The extraocular muscles are located within the orbit, but are extrinsic and separate from the eyeball itself. They act to control the movements of the eyeball and the superior eyelid. There are seven extraocular muscles – the levator palpebrae superioris, superior rectus, inferior rectus, medial rectus, lateral rectus, inferior oblique and superior oblique. Functionally, they can be divided into two groups: v Movement of the eyeball – Recti and oblique muscles. vMovement of the eyelids – Levator palpebrae superioris. MOVEMENT OF THE EYEBALLS There are six muscles involved in the control of the eyeball itself. They can be divided into two groups; the four recti muscles, and the two oblique muscles. RECTI MUSCLES Recti muscles; superior rectus, inferior rectus, medial rectus and lateral rectus. These muscles characteristically originate from the common tendinous ring. This is a ring of fibrous tissue, which surrounds the optic canal and part of the superior orbital fissure at the apex of the orbit. From their origin, the muscles pass anteriorly to attach to the sclera of the eyeball.  Superior Rectus Attachments: Originates from the superior part of the common tendinous ring, and attaches to the superior and anterior aspect of the sclera, posterior to the corneo-scleral junction. Actions: Main action is elevation. Also contributes to adduction and medial rotation of the eyeball. Innervation: Oculomotor nerve (CN III). Inferior Rectus Attachments: Originates from the inferior part of the common tendinous ring, and attaches to the inferior and anterior aspect of the sclera. Actions: Main movement is depression. Also contributes to adduction and lateral rotation of the eyeball. Innervation: Oculomotor nerve(CN II).  Medial Rectus Attachments: Originates from the medial part of the common tendinous ring, and attaches to the antero- medial aspect of the sclera. Actions: Adducts the eyeball. Innervation: Oculomotor nerve (CN III). Lateral Rectus Attachments: Originates from the lateral part of the common tendinous ring, and attaches to the anterolateral aspect of the sclera. Actions: Abducts the eyeball. Innervation: Abducens nerve (CN VI).  OBLIQUE MUSCLES There are two oblique muscles – the superior and inferior obliques. They attach to the posterior surface of the sclera. Superior Oblique Attachments: Originates from the body of the sphenoid bone. Its tendon passes through a trochlea changes its direction, and inserts to the sclera of the eye, posterior to the superior rectus muscles. Actions: Depresses, abducts and medially rotates the eyeball. Innervation: Trochlear nerve (CN IV). Inferior Oblique Attachments: Originates from the anterior aspect of the orbital floor. inserts to the sclera of the eye, posterior to the lateral rectus Actions: Elevates, abducts and laterally rotates the eyeball. Innervation: Oculomotor nerve (CN III).  MOVEMENT OF EYELIDS LEVATOR PALPEBRAE SUPERIORIS The levator palpebrae superioris (LPS) is the only muscle involved in raising the superior eyelid. It is opposed most times by gravity and is an antagonist of the superior half of the orbicularis oculi, the sphincter of the palpebral fissure. A small portion of this muscle contains a collection of smooth muscle fibres known as the superior tarsal muscle that produces additional widening of the palpebral fissure, especially during sympathetic responseAttachments: Originates from the lesser wing of the sphenoid bone, immediately above the optic foramen. It attaches to the superior tarsal plate of the upper eyelid (a thick plate of connective tissue). Actions: Elevates the upper eyelid. Innervation: The levator palpebrae superioris is innervated by the oculomotor nerve (CN III). The superior tarsal muscle (located within the LPS) is innervated by the sympathetic nervous system.  APPLIED ANATOMY Horner’s Syndrome: Horner’s syndrome refers to a triad of symptoms produced by damage to the sympathetic trunk in the neck: Partial ptosis (drooping of the upper eyelid) – Due to denervation of the superior tarsal muscle. Miosis (pupillary constriction) – Due to denervation of the dilator pupillae muscle. Anhidrosis (absence of sweating) on the ipsilateral side of the face – Due to denervation of the sweat glands. Horner’s syndrome can represent serious pathology, such as a tumour of the apex of the lung (Pancoast tumour), aortic aneurysm or thryoid carcinoma. Cranial nerve palsies: The extraocular muscles are innervated by three cranial nerves. Damage to one of the cranial nerves will cause paralysis of its respective muscles. This will alter the resting gaze of the affected eye. Thus, a lesion of each cranial nerve has its own characteristic appearance: Oculomotor nerve (CN III) – A lesion of the oculomotor nerve affects most of the extraocular muscles. The affected eye is displaced laterally by the lateral rectus and inferiorly by the superior oblique. The eye adopts a position known as ‘down and out’. Trochlear nerve (CN IV) – A lesion of CN IV will paralyse the superior oblique muscle. There is no obvious affect of the resting orientation of the eyeball. However, the patient will complain of diplopia (double vision), and may develop a head tilt away from the site of the lesion. Abducens nerve (CN VI) – A lesion of CN VI will paralyse the lateral rectus muscle. The affected eye will adducted by the resting tone of the medial rectus. THE LACRIMAL APPARATUS The tears system is what is known as the lacrimal apparatus. It is a group of glands, ducts and sacs responsible for production and drainage of lacrimal fluid (tears) from the orbit. It is a complex system that provides lubrication and protection to the eye After secretion, lacrimal fluid circulates across the eye, and accumulates in the lacrimal lake – located in the medial canthus of the eye. From here, it drains into the lacrimal sac via a series of canals. The lacrimal sac is the dilated end of the nasolacrimal duct, and is located in a groove formed by the lacrimal bone and frontal process of the maxilla. Lacrimal fluid drains down the nasolacrimal duct and empties into the inferior meatus of the nasal cavity.  LACRIMAL GLAND T h e l a c r i m a l g l a n d i s a compound tubuloacinar gland, comprised of lobules formed by multiple acini. The acini contain serous cells and produce a watery serous secretion (lacrimal fluid). Lacrimal fluid acts to the clean, nourish and lubricate the eyes. It forms tears when produced in excess. Production of the lacrimal fluid is stimulated by the parasympathetic impulses from CN VII. The lacrimal fluid produced by the gland is secreted into 8-12 excretory ducts, which empty into the superior conjunctival fornix. The fluid is then ‘spread’ over the cornea by the process of blinking.  ANATOMICAL STRUCTURE AND POSITION OF LACRIMAL GLAND STRUCTURE The lacrimal gland is approximately 2cm long. It can be divided into two main parts: Orbital (superior) part – larger and sits on the lateral margin of the levator palpabrae superioris muscle. Palprebral (inferior) part – smaller and is located along the inner surface of the eyelid. LOCATION Its anatomical relations include: Superior The lacrimal gland is located anteriorly in the superolateral aspect of the orbit, w i t h i n t h e l a c r i m a l fo s s a – a depression in the orbital plate of the frontal bone. – zygomatic process of frontal bone Anterior – orbital septum Posterior – orbital fat I nfe ro l ate ra l – l ate ra l re c t u s muscle  VASCULATURE The main arterial supply to the lacrimal gland is from the lacrimal artery, which is derived from the ophthalmic artery, a branch of the internal carotid. Venous drainage is via the superior ophthalmic vein, and ultimately empties into the cavernous sinus. Lymphatic drainage is to the superficial parotid lymph nodes. They empty into the superior deep cervical nodes.  INNERVATION The sensory innervation to the lacrimal gland is via the lacrimal nerve. This is a branch of the ophthalmic nerve (in turn derived from the trigeminal nerve). The lacrimal gland also receives autonomic nerve fibres: Parasympathetic: Preganglionic fibres are carried in the greater petrosal nerve (branch of the facial nerve) and then the nerve of pterygoid canal, before synapsing at the pterygopalatine ganglion. Postganglionic fibres travel with the maxillary nerve, and finally the zygomatic nerve. Stimulates fluid secretion from the lacrimal gland Sympathetic Fibres originate from the superior cervical ganglion, and are carried by the internal carotid plexus and deep petrosal nerve. They join with the parasympathetic fibres in the nerve of pterygoid canal, and follow the same route to supply the gland. Inhibits fluid secretion from the lacrimal gland APPLIED ANATOMY DACRYOADENITIS Dacryoadenitis refers to inflammation of the lacrimal glands. It can present acutely or chronically: Acute dacryoadenitis – typically due to a viral and bacterial infection such as mumps, Epstein-Barr virus, staphylococcus and gonococcus. Chronic dacryoadenitis – usually due to a non-infectious inflammatory condition such as sarcoidosis or thyroid eye disease associated with Grave’s disease. Clinical features include swelling, pain and excess tear production. The swelling can lead to visual impairment, secondary to pressure on the eye. If the cause is viral, simple rest and warm compresses can be helpful. For other causes, treating the underlying cause is necessary and effective. NERVE SUPPLY OF ORBIT Optic nerve (CN II):The large optic nerve conveys only sensory nerves that transmits impulses generated by optical stimuli. Though it is cranial nerve by convention, it develops as paired anterior extensions of the forebrain and are actually central nerous system fiber tracts formed of second-order neurons. Other nerves that supplies the orbit are those that enter through the superior orbital fissure and supply the ocular muscles. They include; Oculomotor nerve (CN III) Trochlear nerve (CN IV) Abducent nerve (CN VI) NERVE SUPPLY TO THE ORBIT  VASCULATURE OF THE ORBIT Arterial supply: Blood supply is mainly from the ophthalmic artery, a branch of the internal carotid artery. The infra-orbital artery, a branch of the external carotid artery also contributes to supply structures related to the orbital floor. Venous drainage: Venous drainage of the orbit is via the superior and inferior ophthalmic veins that passes through the superior orbital fissure and enters the caverbous sinus.  QUIZZ 1. LIST THE BONES THAT CONSTITUTES THE ORBIT A 25 years old lady was involved in a fight with her counterpart following an argument about a man. She was hit severely with a metal object around her orbit leaving her with a severe orbital injury and associated exophthalmos. Brain CT revealed some contents of the eyeball in the ethmoid and maxillary sinus. what is likely the problem? Give a brief Anatomical description.  1. DESCRIBE THE LOCATION AND POSITION OF THE LACRIMAL GLAND 2. A 40 year old man presented with severe pain, swelling and excessive tearing in both eyes. No known organic cause was noticed but it was accessed to be of viral origin. What is the likely cause? Give the anatomical overview of this pathology.