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Eye
Oculomotor muscles, lacrimal apparatus, clinical
| Term | Definition |
|---|---|
| Features of CN III palsy | Mydriasis -> interrupted parasympathetics to sphincter papillae (dilation), down and out gaze -> intact SO (IV) and LR (VI), full ptosis -> interrupted LPS, compromised accomodation -> ciliary muscle paralysis |
| Features of Horner's syndrome | Miosis -> interrupted sympathetics to dilator papillae (constriction), partial ptosis -> compromised superior tarsal (Muller's) muscle but intact LPS, anhydriasis -> interrupted sympathetics to sweat glands |
| What is the orbital axis? | Angle of 23 degrees w/ visual axis (straight ahead) |
| What are the boundaries of the orbit? | Roof -> frontal bone, floor -> maxilla separating maxillary sinus, lateral -> zygomatic bone, medial -> ethmoid bone/lacrimal bone, anterior -> lacrimal bone/maxilla frontal process, posterior -> greater wing of sphenoid |
| Where does optic nerve enter? | Enters eyeball 3mm medial to posterior pole |
| What are the 3 layers of the eyeball? | Outer fibrous corneoscleral coat, vascular middle coat, inner neural layer |
| What is the corneoscleral coat? | Anterior collagen bundles w/ highly ordered // lamellae arrangement in transparent cornea and disordered arrangement in posterior sclera (opaque), contains the aqueuous humour |
| What does the vascular middle coat contain? | Vascular choroid -> blood vessels btwn retina/sclera, ciliary body, iris -> pupillary aperture controlled by dilator/sphincter papillae, ciliary muscles -> change lens shape during accommodation |
| What does the inner neural layer contain? | Retina -> retinal GCs continuous w/ optic nerve axons, macula lutea -> posterior pale yellow area w/ fovea for 90% optic nerve fibres, optic disc -> 15 degrees medial to macula (visual field blind spot - optic nerve exit) |
| Where does eye refractive power come from? | Cornea -> 2/3 refractive power, biconvex lens -> 1/3 eye refractive power |
| Extraocular muscles | Superior/inferior/lateral/medial rectus, superior/inferior oblique |
| Which muscles arise from the tendinous ring? | Recti muscles -> attach anterior to eyeball vertical equator |
| Superior rectus innervation/function | CN III superior division -> elevates/adducts/intorts eye |
| Inferior rectus innervation/function | CN III inferior division -> depresses/adducts eye |
| Medial rectus innervation/function | CN III inferior division -> adducts eyeball |
| Lateral rectus innervation/function | CN VI -> abducts eyeball |
| Superior oblique innervation/function | CN IV -> depresses/abducts eyeball, narrow tendon superomedial to optic foramen -> through cartilagenous pulley (trochlea) -> eyeball |
| Oblique muscle insertions? | Attach posterior to eyeball vertical equator |
| Inferior oblique innervation/function | CN III inferior division -> elevates/abducts eyeball -> medial to infraorbital margin -> eyeball |
| LPS innervation and location | CN III superior division -> lesser wing of sphenoid at orbit apex -> upper lid skin crease aponeurosis |
| Muller's muscle | Superior tarsal muscle -> assists LPS in eyelid elevation -> sympathetic innervation -> underside of LPS -> upper edge of tarsal plate of eyelid |
| Superior tarsal muscle | Muller's muscle -> assists LPS in eyelid elevation -> sympathetic innervation -> underside of LPS -> upper edge of tarsal plate of eyelid |
| Courses of cervical ganglions | Sympathetic fibres -> superior -> ICA/ECA, middle -> superior thyroid artery, inferior -> vertebral artery |
| Eye blood supply | Ophthalmic artery (1st major ICA branch) -> enters orbit (gives off retina/extraocular muscle branches) -> ascends on lateral surface of CN I -> gives off lacrimal artery -> gives off recurrent meningeal branch (anastamoses w/ middle meningeal artery) |
| Eye blood drainage | Superior/inferior ophthalmic vein -> leave orbit via SOF -> drain into cavernous sinuses |
| What is the lacrimal apparatus function? | Tear production/drainage -> corneal nourishment from tear film, medium for O2 diffusion, wash out foreign material, antibacterial enzymes |
| Eyelid layers | Skin, loose connective tissue, orbicularis oculi fibres, tarsal plates of dense connective tissue, Meibomian glands, conjunctiva |
| What are Meibomian glands? | Modified sebaceous glands producing oily secretion for waterproof lid margins to reduce corneal tear evaporation -> open onto skin posterior to eyelashes |
| What is the conjunctiva? | Delicate mucus membrane lining anterior sclera into superior/inferior fornices -> reflected onto inner eyelid surface, thick/highly vascular over lids, thinner over sclera -> microvascular haemodynamics affected by diabetic retinopathy |
| What is conjunctivitis? | Inflammation of conjunctiva due to virus/bacteria/allergies -> symptoms = red eye, swelling, watering, reactive pupils, normal visual acuity -> highly infectious -> resolve w/out treatment |
| What are the medial/lateral canthi? | Angles btwn upper/lower lids |
| What is the palpebral fissure? | Distance btwn medial/lateral canthi |
| What is the lacrimal gland location/function? | Superolateral orbit in lacrimal fossa -> 8-12 ducts open at lateral superior conjunctival fornix -> tear production |
| What are the fornices? | Superior/inferior spaces btwn eyelids and eyeball |
| What is the lacrimal gland innervation? | Parasympathetic supply -> superior salivatory nucleus -> CN VII greater petrosal nervus intermedius branch -> pterygopalatine ganglion -> zygomaticotemporal branch of lacrimal branch of Vb -> lacrimal gland |
| What is the function of orbicularis oculi muscle? | Contraction -> blinking -> squeeze tear production medially towards lacrimal puncta |
| What is the tear drainage pathway? | Lacrimal puncta on medial eyelid -> canaliculi -> lacrimal sac (medial orbit) -> nasolacrimal duct -> anteroinferior nasal meatus |
| What are the causes of bitemporal hemianopia? | Optic chiasm lesions -> pituitary adenomas, meningiomas (diaphragma sellae), anterior communicating artery aneurysm (superior) -> tunnel vision (lose bilateral temporal fields) |
| What is the investigation of bitemporal hemianopia? | Verify endocrine gland performance (blood test), contrast-enhanced brain MRI |
| What is the treatment for pituitary tumours? | Transphenoidal surgical access from nose into sphenoid sinus cavity and through thin plate of bone into pituitary fossa -> no brain effects/visible scar but may damage pituitary (xcs ADH secretion -> thirst) |
| What are the causes of CN III palsy? | Lesions/tumours (gliatomous cerebri), autoimmune disorders (myasthenia gravis), cavernous sinus thrombosis, posterior communicating artery aneurysm (basal cistern) |
| What might be the cause of restriction of upward gaze? | Orbital floor fracture from head trauma -> entrapment of inferior rectus into maxillary sinus -> teardrop sign |
| What are the symptoms of orbital floor fracture? | Diplopia when looking up, enophthalmos (sunken eyball), cheek paresthesia (CN Vb infraorbital nerve through infraorbital fissure in orbital floor) |
| What is the investigation/management for orbital floor fracture? | CT scan -> axial/coronal/Water's view, full neurological examination, surgical intervention after swelling abates -> prevent diplopia/enophthalmos |
| What are the causes of CN VI palsy? | Increased ICP (emerges from brain bottom - 1st nerve compressed against ridge of petrous temporal bone), intercavernous ICA aneurysm (CN VI against clivus), lesions compressing brainstem downwards |
| What are the symptoms of CN VI palsy? | Diplopia, turning head in affected eye direction (compromised LR) |
| What are the investigations for CN VI palsy? | H tracking of eyes -> check other extraocular muscles, complete neurological examination, MRI |
Created by:
vykleung
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