Cranial Nerves + The Eye PDF

The Eye The Bony Orbit Pyramidal structure bounded by: Roof: Frontal bone (with anterior cranial fossa above) Floor: Maxilla (with maxillary sinus below) Lateral: Zygomatic bone Medial: Maxilla, lacrimal + ethmoid bones The base of the pyramid is open It contains: Eyeball Eyelids...

The Eye The Bony Orbit Pyramidal structure bounded by: Roof: Frontal bone (with anterior cranial fossa above) Floor: Maxilla (with maxillary sinus below) Lateral: Zygomatic bone Medial: Maxilla, lacrimal + ethmoid bones The base of the pyramid is open It contains: Eyeball Eyelids Extra-ocular muscles Nerves (CN II-VI): Optic, Oculomotor, Trochlear, Trigeminal, Abducens Blood vessels: Ophthalmic artery and veins Orbital fat (cushions the eye and stabilises the extraocular muscles) Pathways into the Orbit There are three major openings in the orbit: 1. Optic canal ○ Optic nerve ○ Ophthalmic artery 2. Superior orbital fissure ○ Oculomotor nerve ○ Trochlear nerve ○ Superior ophthalmic vein ○ Lacrimal, frontal, nasociliary nerves (branches of V1) 3. Inferior orbital fissure ○ Zygomatic branch of maxillary ○ Inferior ophthalmic vein ○ Sympathetic nerves Fractures of the Bony Orbit Two major types: 1. Orbital Rim Fracture ○ Fracture of the bones forming the outer rim of the bony orbit ○ Occurs at the sutures joining the maxilla, zygomatic and frontal bones 2. “Blowout” Fracture ○ Partial herniation of the orbital contents through the walls of the bony orbit usually due to blunt force trauma to the eye ○ Most commonly occurs inferiorly into the maxillary sinus ○ Medial herniation into the ethmoid sinuses is also common Fractures may present with: Pain Exophthalmos from increased intraorbital pressure causing protrusion of the eye Involvement of surrounding structures e.g. haemorrhage into a neighbouring sinus Extraocular Eye Muscles There are 7 extraocular eye muscles: 4 Recti muscles originating from the common tendinous ring and attaching to the sclera ○ Medial rectus - adduction ○ Lateral rectus - abduction ○ Superior rectus - elevation (and adduction) ○ Inferior rectus - depression (and adduction) 2 Oblique muscles attaching to the posterior surface of the sclera ○ Inferior oblique - elevation and abduction ○ Superior oblique - depression and abduction Levator palpebrae superioris ○ Elevates the upper eyelid ○ Includes some sympathetic fibres - termed the superior tarsal muscle All are innervated by the oculomotor nerve (CN III) except: Superior oblique is innervated by trochlear (CN IV) Lateral rectus is innervated by abducens (CN VI) This can be remembered with LR6 and SO4 Eye Movements 1. Straightforward: ○ Abduction = Lateral rectus ○ Adduction = Medial rectus 2. Point of confusion #1: Recti vs. Oblique ○ From the abducted position, looking up and down uses the superior and inferior recti (even though these are involved in adduction) ○ From adducted position, looking up and down uses the inferior and superior obliques (even though these are involved in abduction) ○ This is because in the abducted position, the obliques are “slack” while the recti are “stretched” so elevation and depression will come from contraction of the stretched muscle 3. Point of confusion #2: The obliques do the opposite to their name ○ Inferior oblique elevates ○ Superior oblique depresses ○ Something about their angular approach 4. Rotation ○ Medial rotation = Superior oblique ○ Lateral rotation = Inferior oblique  Nerve Palsies of the Eye There are 3 cranial nerves involved in supplying the extraocular muscles. Remember LR6 - SO4 Abducens nerve (CN VI) palsy ○ Paralysis of lateral rectus ○ At rest the affected eye will turn inwards Trochlear nerve (CN IV) palsy ○ Paralysis of superior oblique ○ No obvious effect at rest ○ Butatient will have diplopia and head may tilt away from the site of the lesion Oculomotor nerve (CN III palsy) ○ Paralysis of everything except lateral rectus and superior oblique ○ At rest the affected eye will turn down and out Damage to the sympathetic trunk will cause Horner’s syndrome, a triad of: Partial ptosis (drooping of the upper eyelid) due to denervation of the superior tarsal muscle Miosis (pupil 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 This can be caused by a Pancoast tumour (apex of lung), aortic aneurysm or thyroid carcinoma The Eyeball 1. Fibrous layer ○ Sclera (white, covers most of the eye, attachment site for extraocular muscles) ○ Cornea (transparent, covers front of the eye, refracts light) ○ The sclera and cornea are continuous and provide shape and support to deeper structures 2. Vascular layer ○ Choroid (connective tissue and blood, nourishing the retina) ○ Ciliary body (controls the shape of the lens) Ciliary muscle - smooth muscle fibres Ciliary processes - attach the ciliary muscle to the lens of the eye ○ Iris (circular, with pupil in the centre) Central aperture (the pupil) is controlled by smooth muscle fibres, innervated by the ANS 3. Inner layer ○ Pigmented (outer) layer - single layer of cells attached to the choroid, absorbs light to prevent scattering, continuous all around the eye ○ Neural (inner) layer - photoreceptors, only located posteriorly and laterally (optic part of the retina) ○ Fovea centralis - particularly high concentration of photoreceptors, responsible for high acuity vision ○ Optic disc - where the optic nerve enters, no light detecting cells Other Structures in the Eyeball Anterior/Posterior Chamber ○ Between the cornea and lens, anterior and posterior chambers are separated by the iris ○ Both are filled with aqueous humor Aqueous humor ○ Clear fluid that nourishes and protects the eye ○ Constantly produced then drained via the trabecular meshwork at the base of the cornea Lens ○ Posterior to the pupil ○ Shape affects refraction of light and is altered by the ciliary body Vitreous humor ○ Similar to the aqueous humor but behind the lens Vasculature of the Eye Arterial Supply: Ophthalmic artery Branch of the internal carotid, arising immediately distal to the cavernous sinus Gives off many branches Most importantly, the central retinal artery which supplies the internal surface of the retina → occlusion = blindness Venous Drainage: Superior and Inferior ophthalmic veins Drain into the cavernous sinus The Eyelids Functions: Protection from excessive light or injury, distribution of tears of the eyeball for lubrication The upper and lower eyelids meet at the medial and lateral canthi and the opening between is the palpebral aperture The eyelid consists of 4 layers: Arterial supply: Skin and subcutaneous tissue Ophthalmic: Lacrimal, medial palpebral, supraorbital, Orbicularis oculi muscle dorsal nasal and supratrochlear branches Tarsal plates Facial: Angular branch Conjunctiva Superficial temporal: Transverse facial branch Venous drainage Medial: Medial palpebral vein → Angular and ophthalmic veins Lateral: Lateral palpebral vein → Superficial temporal vein Innervation Upper eyelid: Ophthalmic nerve (V1) Lower eyelid: Maxillary nerve (V2) Layers of the Eyelid The eyelid consists of 4 layers: Skin and subcutaneous tissue ○ No subcutaneous fat = easily show oedema/ blood ○ Ciliary glands of Moll = modified sweat glands ○ Glands of Zeis = sebaceous glands Orbicularis oculi muscle ○ Palpebral part → gentle closes eyelids ○ Lacrimal part → involved in drainage of tears ○ Orbital part → tightly closes eyelids ○ Innervation by the facial nerve Tarsal plates + Levator apparatus ○ Deep to palpebral region of orbicularis oculi ○ Dense connective tissue scaffolding ○ The levator palpebrae superioris and superior tarsal muscles attaches to the superior tarsal plate = Levator apparatus ○ Meibomian glands = sebaceous glands Conjunctiva ○ Palpebral conjunctiva (lining the eyelid) ○ Bulbar conjunctiva (reflected on the sclera of the eyeball) Lacrimal Gland Location: Located superolaterally in the orbit, within the lacrimal fossa of the frontal bone Structure: Compound tubuloacinar structure with lobules comprised of multiple acini. There are two main parts: Orbital - larger and sits on lateral margin of the LPS muscle Palpebral - smaller and located on the inner surface of the eyelid Function: Secretion of lacrimal fluid onto the surfaces of the conjunctiva and cornea of the eye: Lacrimal fluid functions to clean, nourish and lubricate the eyes Arterial Supply: Ophthalmic (V1) → Lacrimal artery Venous Drainage: Superior ophthalmic vein → Cavernous sinus Lymphatics: Superficial parotid lymph nodes → Deep cervical Innervation of the Lacrimal Gland Sensory - lacrimal nerve (→ ophthalmic → trigeminal) Parasympathetic: Stimulates secretion ○ Preganglionic:: Facial nerve → Greater petrosal nerve → Nerve of pterygoid canal → Pterygopalatine ganglion ○ Postganglionic: Pterygopalatine ganglion → Maxillary nerve → Zygomatic nerve Sympathetic: Inhibits secretion ○ Originate from superior cervical ganglion ○ Carried by the internal carotid plexus and deep petrosal nerve ○ Joins with parasympathetic fibres in the nerve of pterygoid canal and follows its route Lacrimal Apparatus The lacrimal fluid is secreted into excretory ducts, which empty into the superior conjunctival fornix 1. The fluid is then spread over the cornea by the process of blinking 2. After secretion, the lacrimal fluid accumulates in the lacrimal lake in the medial canthus of the eye 3. It then drains via the lacrimal puncta (the holes) and canaliculi (the tubes) into the lacrimal sac 4. The lacrimal sac refers to the dilated end of the nasolacrimal duct, located in a groove formed by the lacrimal bone and frontal process of the maxilla 5. The nasolacrimal duct then empties into the inferior meatus of the nasal cavity Pathology Glaucoma = obstruction of the drainage of the aqueous humour ○ Open angle = outflow of aqueous humor through the trabecular meshwork is reduced → gradual reduction of peripheral vision ○ Closed angle = iris forced against the trabecular meshwork preventing any drainage → rapidly leads to blindness Cataract = opacification of the lens in old age Papilloedema = swelling of the optic disc due raised intracranial pressure ○ High pressure in the cranium resists venous return from the eye ○ Fluid extravasates from blood vessels and collects in the retina ○ = Swollen optic disc ○ Causes: Intracranial mass lesions (e.g. tumour) Intracranial haemorrhage Meningitis Hydrocephalus Cataracts Tissues within the lens break down and clump together, clouding small areas within the lens Causes: Ageing and/or injury Past eye surgery Diabetes Other eye conditions Long term steroid use Early Late Signs Slight blurring of vision Blurred, hazy, foggy vision (like looking through a film or veil) Glare and sensitivity to bright light Aura/halo around lights especially at night Distortion of vision/Double vision Colours appear more faded or yellowish Correction Using stronger lighting Cataract surgery (to replace the lens with an artificial one) Corrective glasses Glasses usually still required (artificial lenses cannot focus) Diabetic Retinopathy Microvascular complication of diabetes resulting in bilateral retinal scarring Pre-proliferative stage: Mild or no symptoms Blood vessels in the retina start to leak fluid and blood into the eye = slight blurring of vision The retinal blood vessels may become distorted/blocked = loss of nutrient and oxygen delivery = ischaemia = cotton wool spots Proliferative stage New blood vessels grow within the retina but these are fragile and rupture easily → haemorrhage → scar tissue Vision changes will depend on where these haemorrhages occur ○ Peripheral: Scotomas (blurry patches) - Often compensated for by by additional head and eye movement ○ Macula: Partial or abrupt loss of acuity Early detection allows for the use of laser photocoagulation to stop growth of these new blood vessels Macular Degeneration Chronic, degenerative eye disease of the retina causing progressive loss of central vision (responsible for fine detail/acuity vision) with sparing of the peripheral vision Caused by progressive buildup of waste material (called “drusen”) under the retina. Risk Factors: Age Smoking Overweight Poor diet High cholesterol Diabetes 2 Main Types: Dry/Atrophic - most common, slow ○ Gradual atrophy of retinal cells = gradual loss of central vision Wet/Neovascular ○ Abnormal and fragile blood vessels grow under the macula and leak blood and fluid ○ Develops very quickly = rapid loss of vision  Glaucoma Progressive optic neuropathy, specifically damage to the retinal ganglion cells that carry information to the brain 2 Main types: Open-angle glaucoma - most common ○ Gradual clogging of the trabecular meshwork that drains the aqueous humour ○ = Raised intraocular pressure = Gradual loss of peripheral vision (scotomas) Closed-angle glaucoma ○ Peripheral part of the iris is forced against the trabecular meshwork preventing any drainage ○ = Rapid damage leading to blindness Other less important types: Normal tension glaucoma/Glaucoma without high eye pressure Secondary glaucoma ○ E.g. eye injuries, cataracts, diabetes, inflammation, steroid medications Congenital glaucoma - rare Near/Far Sightedness Nearsightedness (myopia) = Poor distance vision: Eye is too long Lens too rounded (Failure of ciliary muscles to relax) Cornea too curved Farsightedness (hyperopia) = Poor near vision Eye is too short Lens too flat (Failure of ciliary muscles to contract) Cornea too flat Normal Accommodation: Ciliary muscles contract → Suspensory ligaments relax → Lens becomes thicker and rounder → Good near vision Ciliary muscles relax → Suspensory ligaments tighten → Lens becomes flatter → Good distance vision