By Walid Haddad,MD
The tear film is ~ 7 Um thick,7Ul in volume
The tear film
It is made of 3 layers, from anterior to posterior :
1.The lipid layer ; secreted by the the ~ 30 Meibomian glands in the inner
leaflet of the eyelid. This layer prevents evaporation of the tear film and
lubricates the eyelid.
2.The aqueous layer ; constitutes 90% of the thickness of the tear film and
provides oxygen and nutrition for the cornea. This layer also has antibacterial
properties and washes away debris. It is secreted by 2 kinds of lacrimal glands:
-The main lacrimal gland in the anterolateral portion of the roof of the
This gland is responsible mainly for reflex tearing,but was found recently to
contribute to basal tear secretion also.
-The acccessory lacrimal glands of Krause and Wolfring located mainly in the
superior fornix.They are responsible for basal tearing .
3.The mucus layer ; secreted by goblet cells distributed throughout the
palpebral conjunctiva.This hydrated glycoprotein layer makes the corneal
hydrophyllic and thus wettable and decreases surface tension of the tear
A deficiency in "2" results in a +ve Schirmer test. (Ex: Sjogrenís
A deficiency in "3" results in a +ve Break up time test(Ex:
The cornea :
-transparent anterior 1/6th of the globe
-vertical diameter ~ 11mm, horizontal ~ 11.7mm
-central thickness 0.5mm, 1mm peripherally
-axial refractive power of 43 diopters
-avascular, oxygen supply mainly from the tear film,metabolic requirements from
the aqueous humor.
-composed of 5 distinct histologic layers:
-Non keratinized stratified squamous type of epithelium
-thickness of ~60Um.
-itself is 4 layers thick:
a. superficial layer of flattened cells rich in 64D keratin (not found in
b. intermediate layer of polyhedral cells called wing cells.
c. basal germinal layer(one cell thick) .Here the columnar cells regenerate
by mitosis,move superficially,become flatter,more keratinized and then are
shed after seven days.This process also happens in wound healing.
d. basement membrane of the germinal layer ; PAS +ve, type IV
collagen,firmly attached to bowmanís layer.(A disturbance at this level
results in recurrent corneal erosion syndrome)
2. Bowmanís layer :
-acellular anterior condensation of the corneal stroma.
(Does not regenerate after injury).
-or substantia propria, 90% of thickness of cornea
-mostly type I collagen (55%) and type VI (35%)
-composed of ~ 300 lamella of collagen fibrils of uniform diameter and
regular spacing. Fibers in any one lamellae are parallel but perpendicular to
fibers in adjacent lamellae.
- ground substance is made up of proteoglycans (keratan and chondroitin
4. Descemetís membrane :
-PAS +ve glassy basement membrane of the
-Does regenerate after injury
-Forms at the periphery the line of Schwalbe, referred to clinically when
assessing the the anterior chamber by gonioscopy.
-a single layer of flattened cells facing the anterior chamber
-highly active in maintaining corneal transparency by regulating the water
of the corneal stroma and ion transport via Na-K pumps.
-a minimum of 700 cells/mm2 is required for endothelial integrity function
- Physical or metabolic damage to the endothelium leads to corneal
swelling,overhydration and opacification. For example,after cataract
surgery,the corneal stroma is overhydrated and fluid may collect in small
cysts thus the naming pseudophakic bullous keratopathy.In many
instances,physycians use hypertonic saline (3-5 % NaCl) for deturgescence of
- Specular microscopy is a modality used to measure endothelial cell
density,size and morphology. In corneal transplantation, an eligible corneal
graft is one that has a minimum of ~1500 cells/mm2.
- Keratometry mesures the average corneal curvature of the central 3mm along
the vertical and horizontal meridians.Clinically used to calculate the power
of the IOL(intraocular lens) needed to replace the cataractous one being
- Keratoscopy provides a color-coded topographical map of the whole cornea.
Useful in the management of keratoconus,irregular and severe astigmatismÖ
5. Pachymetry provides measurement of the central corneal
thickness,useful in the diagnosis and monitoring of several
The aqueous humor
-volume ~ 0.6ml, turnover every 100minutes.
-main metabolic supply for the cornea and lens.
-secreted actively by the non- pigmented innermost layer of the ciliary
processes of the ciliary body.
-osmolarity ;same as plasma
-glucose ; 80% of plasma glucose
-proteins ; 1% of plasma ,mostly albumins
-lactate,ascorbate and chloride ;higher concentration than in plasma.
-The alb/glob ratio in aqueous is usually higher than in plasma because the
heavy globulins are excluded; in Retinoblastoma,the globulin content of aqueous
increases so the ratio A/G decreases,as opposed to all non neoplastic and/or
-The aqueous is thoroughly inspected on slit lamp examination for gaging the
activity of ocular diseases such as uveitis; abundance of cells(mostly
lymphocytes) and the presence of significant glare in the anterior
chamber(reflecting protein exudation) indicates high disease activity thereafter
affecting management and prognosis.
-a transitional zone between the cornea and the sclera
-the corneal lamellae become less regular,more oblique ,more hydrated and thus
much more opaque.
-abrupt transition occurs from avascular cornea to vascular limbus
-this region contains the outflow apparatus of the aqueous humor (the trabecular
meshwork and the canal of schlemn ) .
-The limbus can be involved in many ocular diseases,ex:
In vernal catarrh(allergic conjunctivitis), whitish elevations containing
eosinophils are seen at the limbus and called Trantas dots.
In late stage trachoma (caused by chlamydia trachomatis),a unique feature is
the presence of limbal follicles which on resolution leave characteristic
yellowish brown depressions
called Herbertís pits.
-thin translucent mucous membrane that starts at the limbus and covers the
sclera (bulbar conjunctiva) and the inner surface of the eyelid (palpebral
-statified columnar epithelium 3-7 layers thick,never keratinized normally.
-contains goblet cells that secrete the mucin layer of the tear film
-contains the accessory lacrimal glands of Krause and Wolfring(basal
-In viral conjunctivitis , numerous lymphoid follicles are observed in the
-In vernal keratoconjunctivits,giant papillae having a cobblestone appearance
are usually seen in the upper eyelid.
-Soft contact lens wearers may also develop GPC (giant papillary
-Excessive exposure to outdoor conditions especially UV radiation along with
heat,dust and wind, may lead to degenerative conjunctival changes, two of which
1.pingueculum: a raised yellowish patch of elastotic degeneration that abutts
but does not
encroach upon the cornea,usually nasally.
2.pterygium: a raised triangular area of bulbar conjunctiva which actively
invades the cornea,usually from the nasal side to produce visual symptoms .It
may be surgically removed if it is reaching the central pupillary area affecting
vision, if it is causing astigmatism,recurrent inflammation,affecting
motility,or even for mere cosmetic reasons.
-White of the eye, posterior five sixths of the globe.
-Irregular size and arrangement of collagen fibrils.
-Thickness is 1 mm posteriorly near the optic nerve and 0.3 anteriorly
where the EOM attach.
-The posterior scleral foramen is a canal which transmits the optic nerve,the
central retinal artery and vein,and the sympathetic plexus to eye.This canal is
2 -3 mm in diameter and is bridged by a sieve-like structure called the lamina
-The sclera can be subdivided into 3 layers:
1.episclera: external layer,loose connective tissue adjacent to the
periorbital fat,well vascularized
2. sclera proper:also called tenonís capsule. It is the dense investing
fascia of the eye composed of dense type I and III collagen. Avascular.
3.lamina fusca: the inner aspect of the sclera,located adjacent to the
choroid and contains thinner collagen fibers and pigment cells.
Episcleritis is a benign self limiting inflammation common in young
May be bilateral ,diffuse or nodular.
Symptoms:redness and mild discomfort. Unlike scleritis, photophobia and
severe pain are unusual in episcleritis.
Usually blanching of the red eye occurs with phenylephrine (unlike scleritis)
Treatment with steroids or NSAIDS.
Scleritis: more serious and destructive than episcleritis
Severe pain and photophobia and even visual disturbances are common.
Deep red coloration with bluish hue,no blanching with phenylephrine.
Usually associated with systemic conditions(inflammatory, autoimmune ,C.T
Treatment: systemic and topical NSAIDS and steroids .
-a contractile diaphgram that controls the degree of retinal illumination.
-has a central aperature ,the pupil,located slightly nasally.
-consists of the following layers from anterior to posterior:
1.stroma : a thin avascular layer with fibroblasts and melanocytes.Heavily
pigmented in persons with brown eyes,less pigmented in green and hazel
irises,and least in blue.
Posteriorly,the stroma contains the sphincter pupillae
2.Pigment epithelium: consists of 2 layers of cells:
-anterior layer which intermingles with the dilator pupillae
-posterior layer which is continuous with the pigment epithelium of the
ciliary body and RPE of the retina, all having same embryologic origin.
-the color of the eye depends on the degree of
pigmentation in the anterior stroma
Abnormal hyperpigmentation may result in iris hyperchromia,nevi and
-albinos have deficiency in the enzyme tyrosinase essential for melanin
production,so they have deficient iris pigmentation demonstrable by its
retroillumination through the pupil.The choroid and retina are also
hypopigmented in albinos.
-Rubeosis irides is a clinical sign referred to in advanced diabetes;
starting at the pupillary boder, neovessels proliferate and migrate
centrifugally .Occasionally,the vessels reach the angle and obstruct the
meshwork thereby causing neovascular or red glaucoma.
-In uveitis (inflammation of the middle coat of the eye),the iris may show
pathognomonic granulomatous nodules at the pupillary border(Koeppeís) and in
-in 95% 0f patients with neurofibromatosis,small bilateral melanocytic
hamartomas are found on the surface of the iris,called Lisch nodules.
-transparent biconvex structure held in position by the ciliary zonules (of
-very rich in proteins (35% w/w),mostly beta-crystallins, the insoluble
portion of which increases with age contributing to cataract formation.
-metabolism is mainly anaerobic, glucose and nutrients from aqueous humor
-composed of 6 layers from anterior to posterior:
1.anterior capsule: basement membrane of the anterior epithelium(thickest BM
2.anterior epithelium:single row of cuboidal cells responsible for metabolism
of the whole lens
3. anterior cortex
Cataract is the most common pathologic change in the lens,can affect any and
all its layers and can have different etiologies.
- Patients with posterior subcapsular cataract are particularly troubled by
sunlight and headlights of incoming cars,and might have their near vision
diminished. Common causes of PSC are aging,trauma or steroid intake.
-Nuclear cataract(age-related,senile) causes myopic shift by increasing the refractive index of
the lens,so elderly patients who had been wearing presbyopic lenses may once again be able to read without
-Anterior subcapsular catacract is often associated with atopy.
-PAM or potential acuity meter is a device used to assess the maximum
potential vision that could be achieved if media opacities,like cataracts, are
removed. The device shines a bright and focused beam of light that can make its
way through opacities, to the macula, thus unmasking any potential visual reserve.
The ciliary body
-part of the uvea = choroid+ciliary body +iris
-extends for 6mm from the end of the retina (ora serrata) till the scleral
-its epithelial portion ( adjacent to vitreous ) consists of a posterior
portion (pars plana) and an anterior portion (pars plicata).The latter has
60-70 folds called the ciliary processes which secrete the aqueous humor into the posterior
-The pars plana is the area through which surgical instruments are usually
-its uveal portion have the ciliary muscle which has 3 parts,all under
Contraction of the radial and circular muscles relaxe the zonules allowing
for increased curvature of the lens ,thereby increasing its refractive power in
the process of accomodation (near vision)
Contraction of the longitudinal muscle stretches mainly the choroid but also
applies forces on the scleral spur thereby opening the canal of schlemn and
facilitating the aqueous drainage.Pilocarpine is a cholinergic drug clinically
used to decrease intraocular pressure in glaucoma patients, by the above
- hold the lens in position immediately posterior to the iris.
-extend from the basal lamina of the NPE of the ciliary process and insert into
the capsule at the equator of the lens.
-composed of pure fibrillin.
-referred to as tertiary vitreous.
-dark brown vascular sheet,0.25mm thick, lying between the sclera and the retina.
-outer vascular bed have large vessels (layer of Haller)
- inner bed consists of an extensive network of fenestrated vessels, the
choriocapillaris which is the major blood supply to the outer layers of the
retina and to the whole macula .(The inner layers, up to the middle of the EPL are
supplied by the central retinal artery.
- Being rich in melanocytes,the choroid may be the site of benign choroidal
nevi and maligant choroidal melanoma which is the most common intraocular
tumor in adults.
-a relatively thin (1-4Um) and refractile connective tissue membrane lying
between the choriocapillaris of the choroid and the RPE of the retina.
-also called lamina vitrea.
- composed of 5 different layers by electron microscopy:
1 .(innermost) :basement membrane of the RPE.
2. a layer of collagen fibers.
3. a layer of elastic fibers.
4. another layer of collagen fibers
5. (outermost): basement membrane of the endothelial cells of choriocapillaris.
-In AMD (age related macular degeneration),which is the most common cause of
blindness above 65 years of age, there are yellow depositions in Bruchís
membrane called drusens,usually at the macula.
-Most internal layer of the eye, facing the vitreous.
-ends at the ora serrata anteriorly.
-consists of 2 basic layers
1.neural retina: inner layer, itself has 9 layers including the photoreceptor
2.RPE(retinal pigment epithelium):outer layer that rests on Bruchís
-The neural retina and RPE arise from distinct embryologic layers and the
potential space in between is the site of retinal detachment. RD is a clinical
emergency because the blood supply of the outer retina and the macula is from
the choroid ,which in RD ,is detached and remote.
-RPE cells provide metabolic and functional support to the
photoreceptors.They are versatile cells that can transform into more primitive
mesodermal cells,fibroblasts,myoepithelial and angioblastic cells and can
contribute to pathologic processes like CNVM ( choroidal neovascular membrane)of
myopes and AMD, and PVR (proliferative vitreoretinopathy) of RD.
-Photoreceptors are the outermost layer of the sensory retina, andare divided
1.cones : responsible for color vision and daytime high discrimination
vision, They are highly concentrated at the fovea.
2. rods : responsible for night vision (black and white) cruder perception and
-Retinoschisis is fluid accumulation within the neurosensory retina to
form intraretinal cysts.It may progress to retinal tears and retinal detachment.
-Retinal dialysis is caused by dynamic vitreoretinal traction usually at
the inferotemporal ora serrata (or superonasal in case of trauma).It is similar to a
flap tear exept that the vitreous gel is attached to the posterior rather than
the anterior margin of the tear.
-largest chamber of the eye (4.5ml).
-transparent gel composed of random network of thin collagen fibers in a
highly dilute solution of salts, proteins and hyaliuronic acid (99%
-The vitreous adheres firmly to the margin of the optic disc and to the
peripheral retina at the ora serrata and the pars plana(vitreous base)
-With age, the gel component of the vitreous degenerates and liquifies (syncherisis senilis), collagen fibers tend to aggregate and fluid may
detach the gel from the retina, starting a PVD (posterior vitreous detachment).The
patientís symptoms include photopsia or flashes of light ( implying traction on the
retina) and floaters frequently described as black flies .PVD has a benign
natural course usually, but may progress and cause retinal tears and detachment
in approximately 10% of patients.
-Sometimes the epipapillary glial tissue can become avulsed from the disc margin
during PVD,and the patientís floateris described as a cobweb or a
spider, moving with the eye. In such cases ,ophthalmoscopy reveals a small ring of
tissue swirming above the optic nerve head, called Weiss ring.
The optic nerve
-formed by the axons of the 1.2 million ganglion cells while exiting the eye.
-contains within its fibers the central retinal artery and the central
retinal vein which
emerge from a central depression called the physiologic cup
-can be divided into 4 portions:
1.intraocular : itself is divided into
-optic nerve head = optic papilla=optic disc with diameter of 1.5mm
-prelaminar portion, anterior to the lamina cribrosa
-laminar, within the lamina cribrosa
-post laminar,posterior to the lamina cribrosa
2. intraorbital portion extending from the globe until the apex of the orbit
3.intracanalicular portion within the optic canal.
4.intracranial portion that merges into the optic chiasm and
then optic tract.
The optic disc is a major ophthalmoscopic landmark of the ocular fundus.
Special attention is given to the color, the margins, the cup to disc ratio
and the neuroretinal rim.
A healthy ONH (optic nerve head) is a yellowish pink one,with distinct well
defined margins,nonelevated, with a cup to disc ratio of less than 0.3.
-ONH has blurred margins in papilledema, which is disc edema secondary to an
elevated intracranial pressure,as in space occupying tumors, meningitis...
The mechanism is the following : the central retinal vein pierces the dural sheathing
of the optic nerve ~ 12mm behind the globe and courses in the subarachnoid space
before heading to the globe.Here ,the increased ICP exerts external compressive
pressure on the vein.Elevated venous pressure is in turn transmitted backward to the optic disc to
cause swelling ,elevation,and ,in severe cases, hemorrhages and cotton wool
spots ( infarcts in the nerve fiber layer )).
The nasal margin is the first to blur in papilledema, the last being the
The latest stage of papilledema is atrophic, where the disc is white , no
hemorrhages or infarcts seen.This is called secondary optic atrophy (atrophy
-In glaucoma, there is increased cup to disc ratio.C/D which is usually <
0.3, can go up to 0.7, 0.8, or even to subtotal or total cupping.
-In diabetes, when the retinopathy advances to its proliferative stage,
numerous fragile neovessels can be seen over the optic disc. Such a subtle
sign can warrant intervention like PRP(panretinal photocoagulation),by which a
laser beam is used to "ablate" the ischemic retina secreting the
angiogenic factors.Usually PRP is done peripherally in hope to decrease
neovascularisation in the posterior pole including the ONH and
-Hyperopia is associated with a small ONH ,myopia with a large ONH.
-The blood supply of the optic nerve is derived from several sources:
1. The surface of the optic disc recieves capillaries from
the central retinal artery.
2. The prelaminar and laminar portions receive capillaries
from the ~12 short posterior ciliary arteries that penetrate thesclera and make the anastomotic
circle of Haller-Zinn. Occlusion of these vessels and infarction of the nerve
causes AION (anterior ischemic optic neuropathy).The patient presents with sudden
and painless loss of vision,mostly an
altitudinal defect affecting the inferior visual