Sclera, Episclera, Limbus, Conjunctiva Flashcards
Which two structures make up the dense fibrous outer tunic of the eye?
Cornea and sclera
How is the cornea different from the sclera?
Interference theory - cornea needs to be clear to allow light to pass through
Cornea = multiple parallel layers (lamellae) of collagen fibres arranged in a regular lattice equidistantly apart from one another. This allows for transparency.
Sclera = interwoven collagen fibrils without this precise arrangement making it appear more white and dense.
Which embryological structure are the corneal epithelium, limbus and conjunctival epithelium derived from?
Surface ectoderm
Which embryological structure does the limbus stroma originate from?
Neural crest cells
What are the layers of the sclera?
Innermost layer = lamina fusca (elastic fibres) - separated from choroid by potential space (suprachoroidal space - involved in uveoscleral AH outflow), fine branching collagen fibres connect lamina fusca with choroid and long ciliary nerves/posterior arteries course within this space.
Middle layer = sclera proper (white fibrous layer, stroma, avascular)
Outer layer = loose connetive tissue (episclera) - external boundary of the sclera.
Outer layer = Episclera (thin collagenous and vascular layer)
Thickens anteriorally where it binds with Tenon’s capsule and the subconjunctival connective tissue towards the limbus.
Why does the sclera need to be rigid?
Provide resistance to IOP and maintain spherical shape of the globe along with the cornea.
What channels are found within the sclera?
Contains smal channels to accomodate the long and short ciliary nerves, long posterior ciliary arteries, vortex veins and anterior ciliary vessels.
While sclera generally a tough barrier it is through these channels that disease processes can enter/leave the globe.
What is the lamina cribrosa?
Sieve like opening of the sclera in the region of the exit of the optic nerve.
What is the intrascleral plexus?
Located in anterior portion of the scleral stroma
Interconnecting veins that receives aqueous humour from angular aqueous plexus.
Draining venous blood mixes with draining aqueous into the anterior ciliary vein or posterior vortex vein.
How is aqueous usually circulated in dogs/cats?
Majority = conventional route (97% in cats, 85% in dogs)
AH passes through pupil into anterior chamber there enters the corneoscleral trabecular meshwork in the iridocorneal drainage angle through to the venous angular aqueous plexus and ultimately systemic venous circulation through plexus of small veins in the sclera - scleral venous plexus.
What other alternate pathways are there of aqueous circulation?
Posteriorly into vitreous
Anteriorally within the iris stroma or across the cornea
Uveoscleral route (3% ouflow in cat, 15% in dogs) - unconventional route
Describe the uveoscleral route for AH flow.
Aqueous in the iris root passes through ciliary body and choroid via supraciliary and suprachoroidal spaces. From there passes through sclera and episcleral tissues into the orbit.
List the main differences between the sclera and cornea.
Opaque (due to arrangement of collagen)
Contains some blood vessels(whilst cornea avascular)
Rigid
Where is the sclera widest?
Widest near insertions of the EOM and at the intrascleral plexus
Where is the sclera thinnest/thickest?
Near equator and posterior to EOM insertions = thinnest
Limbus and lamina cribosa = thickest
What is the innervation to the sclera?
Ciliary nerves - arise from branching of the trigeminal (ophthalmic branch)
What is the blood supply to the sclera?
Branches of anterior ciliary arteries anterior to insertions of recti muscles form dense episcleral plexus - a rich blood supply existing beneath the conjunctiva
Posterior sclera - small branches from long and short ciliary arteries.
What tissues are neighbouring tissues to the episclera?
Cornea, conjunctiva and choroid - any inflammatory process within these tissues can affect and vice versa and inflammation within the episclera/sclera can affect those tissues.
Are the episcleral vessels located above or below the conjunctival vessels?
Episcleral vessels = below conjunctival
Important when differentiating hyperaemia.
Describe the conjunctiva.
Thin vascular mucous membrane lining the inner surface of the eyelids, inner and outer TEL and the anterior sclera.
Reflected at ventral and dorsal fornices - conjunctival sacs.
How can the conjunctiva be further divided into sections?
Palpebral conjunctiva = lines upper/lower eyelids
Bulbar conjunctiva = anterior aspect of globe and connects it close to the corneoscleral limbus (can be pigmented or non pigmented
Conjunctiva of leading edge of TEL tends to be pigmented.
Where is the conjunctiva most tightly adhered
Eyelid margins, leading edge of the third eyelid and at the limbus.
Otherwise elastic tissue to allow for ocular motility.
How is the conjunctiva prevented from dessication? What types of epithelium are there?
Tear film will coat the conjunctival epithelium - prevents dessication and provides nourishment.
Stratified squamous (non keratinised) &
stratified columnar
Which type of epithelium of the conjunctiva contain goblet cells - what do they produce?
Columnar epithelium
Goblet cells - produce mucin for trilaminar tear film
Abnormalities of conjunctiva may therefore lead to lack of mucin within tear film, decreased resistance to infectious agents and restriction of ocular motility.
Which species has the highest goblet cell density?
Cats - more numerous in palpebral zones and on anterior surface of TEL.
What is the blood supply to the conjunctiva?
Extensive network (vascular tissue)
Branches of dorsal and ventral palpebral and malar arteries
Terminal branches of the ciliary arteries
What is the innervation to the conjunctiva?
Branches of long ciliary, zygomaticotempora, intratrochlear and frontal nerves.
List the main functions of the conjunctiva.
Allow for ocular motility (mobile tissue generally)
Anatomical ocular barrier
Lining for the surrounding ocular structures - eyelids, TEL and anterior sclera.
Goblet cells - mucin of tear film (inner layer of tear film)
Immune defence - lymphoid tissue
What is the CALT?
Conjunctival associated lymphoid tissue in the stroma of conjunctiva (lamina propria)
Organised lymphoid tissue in the superficial stroma - diffuse layer + lymphoid follicles
Deeper - lymphatic vessels, nerves and blood vessels
Key role in protecting ocular surface by initiating and regulating immune responses.
Must also tolerate diverse conjunctival flora on ocular surface
Protects against infectious agents - defensive role of conjunctiva.
How does mucin produced by the goblet cells also contribute to immune defence of the eye?
Mucin - innermost layer of tear film
Coats ocular surface combining with the glycocalyx of the cornea
Traps and immobilises debris and microbes so they can be evacuated from the tear film.
Binds antimicrobial peptides and proteins - lysozyme and IgA.
What is tenon’s capsule?
Tenon’s capsule = fascial sheath
Thin membrane enveloping eye and separating it from the orbital fat
Tenon’s capsule blends with the episclera just behind corneoscleral junction and fuses with bulbar conjunctiva
Capsule fuses with and extends to the sheath of the optic nerve and the sclera around entrance of optic nerve.
Separates conjunctiva from episclera - should not include in conjunctival pedicle grafts as increases risk of graft contracture/failure.
Sub Tenons injections - technique for regionala anaesthesia - extraocular muscle akinesis and mydriasis
What is the limbus?
Transitional zone between the cornea and sclera
Usually clearly defined as narrow (1mm) pigmented region encircling the cornea periphery.
Junction of the conjunctival and scleral epithelia and corneal and scleral stroma.
Where conjunctiva, tenons capsule and episclera meet.
What type of cells are found at the limbus and why are they important?
Limbus = contains pluripotent limbal epithelium stem cells
Essential for the health and maintenance of corneal surface both under normal and wound healing conditions.
Acheive via amplification, proliferation and differentiation into corneal epithelium
Also act as a physiological barrier to the ingress of conjunctival cells across the cornea.
What can dysfunction/destruction to the limbal stem cells lead to?
Dysfunction (e.g in some cases of multiple ocular defects) or destruction (chemical, thermal, inflammatory)
= Can lead to conjunctivalisation of the cornea
May have issues with corneal wound healing/persistent corneal ulceration
Herpes-virus in cats = common cause of limbal stem cell damage.
How is the limbus associated with the drainage angle?
Outermost aspect of the drainage angle - termination of descemet’s membrane
Limbal border zone internally between corneal endothelium and anterior trabeculum also contains specialised cells that can be activated to migrate and repopulate the trabecular meshwork after trabecular injury.
What steps should be performed initially in the investigation of scleral, episcleral, limbal and conjunctival disease.
Hx - trauma, duration, change in appearance etc
Clinical exam - assess for trauma, neoplasia, signs of systemic disease
Routine ophthalmic examination
Remember only most anterior aspects of sclera/episclera may be examined and that these tissues lie beneath a translucent bulbar conjunctiva.
Compare conjunctival vessels to episcleral vessels
Position - conjunctival = superficial, episcleral = deep
Width - conjunctival = narrow/fine, episcleral = wider and larger
Colour - conjunctival = pink/red, episcleral = deep red
Pattern - conjunctival = form loops, more tortuous can be seen to cross limbus if extend onto adjacent cornea. Episcleral = enter sclera and disappear from view just before limbus - straight vessels
Movement = conjunctival = freely mobile , episcleral = fixed to globe
Individual vessels - conjunctival = not distinct, episcleral = distinct
Location = conjunctival = palpebral, bulbar, nictitans, episcleral = overlying sclera
Response to topical 0.25% phenylephrine = conjunctival = blanch rapidly, episcleral = blanch much more slowly
IOP = conjunctival alone (expected to be norma), episceral (glaucoma, uveitis or normal - episcleritis)
Pupil size = conjunctival - expected to be normal, episcleral (mydriasis = glaucoma, miotic = uveitis, normal = episcleritis)
Significance = conjunctival = surface disease or corneal disease, episcleral = deep intraocular disease (episcleritis, scleritis, uveitis, glaucoma, orbital disease)
When during fundic examination may you be able to visualise the underlying sclera (lamina fusca layer)?
Subalbinotic animals - lack of pigment in the retina and choroid - leads to transparency and visualisation of the white lamina fusca of the sclera with retinal and choroidal vessels superimposed on top.
Which conditions can we see an increased scleral show with?
Macropalpebral fissure (often brachys)
Exophthalmos e.g extraocular polymyositis
Proptosis
Hydrothalmos/buphthalmos due to chronic glaucoma
Strabismus
Microphthalmos/microcornea
Phthisis bulbi
How can the colour of the conjunctiva and sclera be altered in some disease states?
Conjunctivitis/episcleritis = red
Jaundice = yellow
Pigment accumulation (particularly melanocytic tumour or ocular melanosis = dark.
Anaemia = white
Thinning may also see homogenous dark - iris visible under lamina fusca
When may thickening/thinning of the conjunctiva/episclera occur?
Thickening - inflammatory disease and neoplasis
Thinning - glaucoma/staphyloma (stretching)
What do conjunctival follicles indicate and which age/species are the often common in?
Conjunctival follicles = chronic antigenic stimulation
Common in young dogs
When is subconjunctival haemorrhage most commonly seen?
Trauma
Other differentials = coagulopathy (e.g thrombocytopaenia) /vasculitis
What is chemosis?
Conjunctival oedema - sign rather than a condition
Caused by any stimulus that results in acute inflammation - trauma, toxin, acute allergic conjunctivitits, infectious agent
Can preclude eyelid closure and risk desiccation of the cornea.
What techniques are there for investigating conjunctival/episcleral disease?
Cytology
Microbial culture/sensitivity
Biopsy for histopathology (biopsy of the sclera or episclera can be performed as diagnostic procedure but may also be used to debulk/remove lesion)
(Needle biopsies of firm nodules less likely to yield diagnostic result unless lymphoma or infectious agent suspected).
How can conjunctival biopsy be performed?
Can be performed under local topical anaesthesia /sedated
Conjunctival sac cleaned with dilute 1:50 povidone iodine then topical anaesthetic (0.5% proxymetacaine) applied to area of anticipated biopsy site with cotton tipped applicator for 10 seconds - maximally effective after 1 minute and max effect 25 mins
Conjunctival biopsy - fine toothed forceps (Bishop Harman) used to raise affected conjunctiva then sectioned with tenotomy scissors.
How is episcleral/scleral biopsy performed?
Should be done under minimum of sedation + topical but GA generally advisable.
Prep with povidone iodine 1:50
Overlying conjunctiva resected from lesion
Sharp corneal scissors or No 64 Beaver blade used to remove representative sample
Sample placed in 10% formalin
Conjunctival defect should not need suturing unless >1cm squared
Gentle pressure applied to site and bleeding should stop within 2-3 minutes. If not adrenaline (ephinephrine can be applied to area at dilution of 1:10,000 to provide vasoconstriction.
Broad spectrum topical AB used for 3-5 days pending lab results.
What may cause an out of focus area on fundic examination?
Coloboma (congenital absence of normal ocular tissue) - sclera, retina, optic nerve
Scleral ectasia (thinning) - congenital, acquired with buphthalmos associated with glaucoma or due to medication (repeated triamcinolone injections)
When is ocular ultrasound useful in scleral disease?
If suspect scleral rupture - focal irregular contour with decreased echogenicity of the sclera in conjunction with intraocular haemorrhage is suggestive.
Most common site of rupture = posterior pole of sclera or adjacent to optic nerve head
What is the average thickness of the sclera?
Approx 0.7mm at limbus
0.24mm at equator
0.45 near optic nerve
What is a dermoid? Where is most common site in dogs?
Congenital choristoma (normal skin in an abnormal area) - usually cornea, conjunctiva or eyelids.
Full differentiated and non neoplastic but in inappropriate place.
Temporal limbus most common site extending onto cornea in dogs but can occur anywhere.
Young dogs- had since birth!
What clinical signs are associated with dermoids?
Ocular discharge (epiphora or mucopurulent discharge), conjunctival and corneal irritation, potentially corneal ulcerations
Which breeds are predisposed to dermoids?
French Bulldogs, Dachshund, Dalmatian, Shih Tzu, Dobermann, GSD, St Bernards
How would you treat a dermoid?
Needs to be surgically removed (usually once >6 months of age)
Lateral canthotomy - aid exposure of globe
Dermoids typically involve corneal stroma and superficial conjunctiva - superficial keratectomy/conjunctivectomy usually curative.
Generally does not require grafting after removal but may do in some instance and should be prepared for that possibility.
Surgical conjunctival defects <1cm heal by itself, larger defects - suture with 6/0 vicryl
Temporary tarsorrhaphy may be performed - >25% corneal surface removed often sensible
Curative surgery unless resection incomplete.
What is a staphyloma? Where do they occur most commonly?
Thinned bulging sclera with a lining of uveal tract - congenital
Occur most commonly in posterior pole and optic nerve usually also involved or at equator of globe.
May be a feature of Collie-Eye Anomaly or Ehlers-Danlos syndrome or part of multiple ocular abnormalities that occur with Merle Ocular Dysgenesis.
Can occur anteriorally and manifests as anteriorly protruding dark blue (due to uvea) mass with cystic appearance.
How can you distinguish a staphyloma from uveal melanoma?
Gentle palpation - softness of the mass
+/- ocular ultrasound
What treatments are there for staphylomas?
Posterior staphyloma - no treatment indicated
Anterior staphyloma (focal) - repaired with autografting procedure using scleral autograft or xenograft/other suitable tissue.
Specialist procedure
What is the prognosis for eyes with staphylomas?
Extensive staphyloma = usually blind
In these cases may consider enucleation, especially if the protrusion is preventing blinking.
What is a sclerocornea?
Congenital non progressive condition - corneoscleral limbus indistinct as there is an extension of the sclera into the peripheral cornea.
Thought to be due to disturbance of mesenchymal and surface ectodermal growth at the rim of optic cup.
May be seen with other congenital ocular defects and associated with Ehlers-Danlos syndrome.
What type of disorder is seen most commonly with the conjunctiva/episclera?
Inflammatory conditions = most common
Majority thought to be immune mediated!
Which breeds are most commonly affected by episcleritis?
American Cocker Spaniel, Shetland Sheepdog, Collies breeds
Lurchers
Airedales
English Springer
BUT any breed can be affected!
What are the 2 forms of canine episcleritis? What type of disease is it and why do we think this?
Diffuse and nodular
Primary inflammatory condition - immune mediated (based on response to immunosuppressives and histopathology - inflammatory reaction dominated by histiocytes, lymphocytes and/or plasma cells)
What types of infection/disease may episcleritis sometimes follow on from?
Ehrlichia, Ochocerca, Toxoplasma.
Occasionally related to ocular trauma or extension of opthalmitis.
How is diffuse episcleritis characterised?
Diffuse = generalised or regional episcleral vascular injection with thickening of surrounding episclera
Often accompanied by mild corneal oedema and peripheral neovascularisation adjacent to area.
