Lecture 11, Refractive Errors PDF

 The normal eye is like a camera with 3 focusing elements: ① Tear film ② Cornea ③ Crystalline lens + Retina acting as the receptive film.  Distant objects are focused by the eye as an inverted image on the retina which is re-inverted in the brain.  It is...

 The normal eye is like a camera with 3 focusing elements: ① Tear film ② Cornea ③ Crystalline lens + Retina acting as the receptive film.  Distant objects are focused by the eye as an inverted image on the retina which is re-inverted in the brain.  It is the normal optical condition of eye → incident parallel rays of light from infinity come to a focus on retina (fovea centralis) with accommodation at rest.  There is no error of refraction.  Emmetropic eye will have a clear image of a distant object without any internal adjustment of its optics.  The average power of a normal emmetropic eye is + 58 to + 60D & axial length (AL) is 24mm.  Each image passes through the cornea to the fovea in what is called the Optical axis.  Through the optical axis is the nodal point which is the optical center of refraction & it is just behind the lens.   State of refraction of in which parallel rays do not come to a focus on DEFINITION the retina (with accommodation completely relaxed) ① Myopia. ② Hypermetropia. ③ Astigmatism. ④ Anisometropia. ⑤ Aphakia. TYPES ① Manual retinoscopy:  can be performed by using a plain mirror and a source of light to retinoscope projects light directly into the eye → produce a red reflex (done after inducing cycloplegia by cyclopentolate in adults or atropine in children). DETERMINATION & ② Automated refractometers: QUANTIFICATION  For rapidly determining objective refraction. OF REFRACTIVE ERRORS (REFRACTION)  ① Axial ametropia:  Abnormal length of eyeball.  Too long → myopia.  Too short → hypermetropia. ② Curvature ametropia:  Abnormal curvature of the refracting surfaces of cornea or lens.  Too strong → myopia. ETIOLOGY  Too weak → hypermetropia. ③ Index ametropia:  Abnormal refractive index of cornea or nucleus of lens.  Too high → myopia.  Too low → hypermetropia. ④ Abnormal position of the lens  Like anterior dislocation of the lens  causing myopia.   Dioptric condition of eye in which with the accommodation at rest, DEFINITION incident parallel rays of light come to a focus anterior to the retina.  basically a disturbance of growth on which degenerative changes are superimposed. ① In axial myopia:  The part anterior to the equator is normal.  The increase in AL affects the posterior pole. CASE ② Index myopia:  Refractive index of the nucleus in nuclear cataract. ③ Curvature myopia:  Lens → lenticonus.  Cornea → keratoconus or corneal ectasia. HIGH OR SIMPLE CONGENITAL DEGENERATVE ONSET 14-16 years 5-10 years At birth Continues after TYPES PROGRESSION Till 20 years Stationary 20 DEGREE ↓6D ↑10D Around 10D DEGENERATIVE absent present absent CHANGES  Indistinct far vision.  Mid closure of eyelids to simulate a pinhole which increases the depth of focus.  Defective night vision (in progressive myopia).  Musca volitantes. SYMPTOMS   Apparent divergent squint may be present.  Fundus examination: ① OPTIC DISC  Temporal crescent: The retinal pigment epithelium fails to extend up to the temporal border of the disc. This leads to exposure of choroidal pigment.  Posterior staphyloma: The sclera may bulge out at the posterior pole due to thinning. ② MACULA  Chorioretinal degeneration is often present.  Foster Fuch’s spots: These are dark pigmented circular areas following old choroidal HGEs. ③ PERIPHERAL FUNDUS SIGNS  Tesselated (tigroid) fundus may be present.  Weiss ring may be seen due to posterior vitreous detachment. ① Vitreous degeneration (liquefaction), opacities and detachment are commonly seen. ② Tear (BREAK) and hemorrhages occur in the retina due to chorioretinal degeneration. ③ Retinal detachment (rhegmatogenous) due to retinal break through COMPLICATIONS which fluid seeps in, detaching the retina from its bed. ④ Complicated cataract (posterior cortical) is due to the disturbance to the nutrition of the lens. ⑤ High myopia is sometimes associated with chronic simple glaucoma. ⑥ Consecutive optic atrophy.   Spectacles  Prescribing suitable correcting spherical concave lenses.  Contact Lenses:  Minus (concave) lenses are obtained through entering the obtained refraction in a special conversion table.  Operative: ① Radial keratotomy:  Multiple peripheral cuts are made in the cornea in order to flatten the increased curvature of the cornea. MANAGEMENT ② Photorefractive keratectomy (PRK)  Excimer laser: It reshapes & flattens the central part of cornea. ③ LASIK (Laser-assisted in situ keratomileusis)  Correct myopia of –0.5D to –9.00D (according to recent guidelines) ④ Femtolaser(lasik & smile):  Allow surgeons to treat patients with thinner corneas and prescriptions higher than -10.0 D. ⑤ Phakic IOL or clear lens extraction or exchange:  For very high myopia.   Dioptric condition of the eye in which with the accommodation at rest the incident parallel rays of light come to a focus posterior to the light DEFINITION sensitive layer of the retina.  Most newborns are hypermetropic at birth (about +2D) & this hypermetropia vanishes with eye growth. ① Axial hypermetropia: Due to decrease of the axial length the eye. ② Index hypermetropia: Due to decrease of refractive index of the cornea or nucleus of the lens. ETIOLOGY ③ Curvature hypermetropia: Due to decreased curvature of the cornea or lens. ④ Abnormal position of the lens: e.g. Posterior dislocation of the lens. ⑤ Aphakia.  These are noticed specially in the evenings after close work. ① There is blurring of vision for near work. SYMPTOMS ② There may be frontal headache and eye strain (asthenopia). ③ Early presbyopia.  There is typical small eye as a whole. It is prone to cause primary closed angle glaucoma due to the shallow anterior chamber and narrow angle. SIGNS  Accommodative convergent squint may be present (esophoria or esotropia).  Glasses: Using convex spherical (plus) lenses. MANAGEMENT  Contact lenses: Less tolerated than in myopia.  Refractive surgery (LASIK): less satisfactory than in myopia.   The error of refraction in which, with accommodation completely relaxed, parallel rays come to many foci at different distances from the retina not to a fixed point.  Occurs when either the front surface of the eye (cornea) or the lens inside the eye has mismatched curves → the eye has different refractive powers in different meridians. DEFINITION  Corneal astigmatism:  The most common and is usually congenital.  It may be induced by surgical or traumatic scars. ETIOLOGY  It may also occur due to ectatic diseases of cornea as keratoconus.  Lenticular astigmatism:  as in subluxation of the crystalline lens or tilted IOL. ① FIRST CLASSIFICATION: a. With the rule: The vertical meridian → more curved. b. Against the rule: The horizontal meridian Early presbyopia → more curved. ② SECOND CLASSIFICATION: a. Simple:  One meridian is emmetropic and the other is ametropic, i.e. simple myopic or simple hypermetropic astigmatism. CLASSIFICATIONS b. Compound:  Both meridians are ametropic but of the same type, i.e. compound myopic or compound hypermetropic astigmatism. c. Mixed:  One meridian is myopic and the other is hypermetropic. The rule is the ↑ curvature of the vertical meridian → thought to be d.t pressure of the lids on the cornea.  ③ THIRD CLASSIFICATION: a. Regular. b. Irregular. ① Blurring of vision. SYMPTOMS ② Accommodative asthenopia. ① The patient reads some types on the visual acuity charts and cannot read other types on the same line. SIGNS ② Special tools as placido disc keratometer & corneal topography. ③ The letters in the book appear to be “running together”. CORRECTION OF ① Glasses using cylindrical lenses. REGULAR ② Soft toric contact lenses. ASTIGMATISM ③ Refractive surgery: LASIK is less satisfactory than in myopia. ① Rigid gas permeable (RGP) contact lens → These stable contact lenses CORRECTION OF essentially act as 2nd surface for eye, compensating for the irregularities IRREGULAR of the cornea and functioning as the eye's new reflective surface. ASTIGMATISM ② Keratoplasty is the end-stage treatment.   Difference of ˃ 1D in refraction between the two eyes.  Patients may have up to 3D of anisometropia before the condition DEFINITION becomes clinically significant due to headache, eye strain, double vision or photophobia. ① Dating since birth TYPES ② Acquired  in cases of aphakia or assymetrical keratoconus ① Asthenopia. SYMPTOMS ② Diplopia on wearing full power correcting glasses.. ① Glasses :  Can be used with under correcting the eye with a higher error at the expense of good vision to avoid anisekonia  Usually a maximum of 4d difference can be tolerated) CORRECTION ② Contact lenses:  Reduce the difference in retinal image size to about 6%. ③ Refractive surgery:  LASIK or intraocular lenses produce a difference of less than 1% (most convinient)   A group of symptoms noticed with fine visual tasks as close work, DEFINITION especially in artificial illumination conditions. ① Headache and eye ache. ② Burning sensation and frequent blinking. SYMPTOMS ③ Lacrimation. ④ Hyperemia of the conjunctiva and lid margin. ⑤ Recurrent styes, chalasia and blepharitis. A. Accommodative asthenopia: ① Hypermetropia. ② Astigmatism. ③ Presbyopia. CAUSES ④ Anisometropia. B. Muscular asthenopia:  Due to heterophoria (latent squint). C. Nervous asthenopia:  Due to vitamin deficiency and liver diseases.   Difficulty in near vision due to progressive weakness of accommodation DEFINITION with aging (after the age of 40).  Crystalline lens fibers  sclerosed with age. MECHANISM  Ciliary muscle  weaker with age.  Difficult near vision  Around the age of 40 in a previously emmetropic eye.  Earlier in hypermetropic patients. SYMPTOMS  Later in myopic patients.  Accommodative asthenopia.  Glasses:  A pair of glasses (Plus lenses added to the far correction).  Multifocal glasses. CORRECTION  Accommodative Intra-Ocular Lenses:  May be used during cataract extraction.  Multifocal Intra-Ocular Lenses:  May be used during cataract extraction.   Contact lens rest on the corneal surface→ changes the power of the PRINCIPLE anterior corneal surface. ① Hard lens (historical)  Consist of PMMA (Polymethyl methacrylate) a plastic, non-toxic material.  Advantage:  It is durable, firm and inert.  Disadvantage:  The corneal hypoxia leads to corneal oedema.  It may cause foreign body sensation. ② Soft lens  Consists of HEMA (hydroxyethyl methacrylate) or related polymer and is hydrophilic in nature. TYPES  Advantage:  It is comfortable and stable.  Disadvantage:  It is delicate and has a short lifespan. ③ Gas permeable lens (new hard lens)  Consists of mixture of hard and soft material, e.g. CAB (cellulose acetate butyrate), silicone, silicone with PMMA.  Advantage:  It causes minimum corneal hypoxia.  Disadvantage:  It tends to scratch and break. ① Refractive  In bilateral errors of refraction → patient not accepting glasses or glasses lenses are too thick or heavy in high errors.  Unilateral high errors (aphakia, myopia) → It prevents diplopia as INDICATIONS there is no retinal image magnification.  regular astigmatism → toric CL  Irregular astigmatism or Keratoconus → provides regular corneal surface and mechanical support.  ② Therapeutic  It has epithelial healing effect, e.g. as in corneal ulcers, filamentary keratitis.  It is used as a vehicle for drug delivery, e.g. soft hydrophilic lens.  It prevents symblepharon formation, e.g. as in chemical burn.  It encourages natural healing process, e.g. as in descemetocele and wound leaks ③ Occupational  In athletes—There is less chances of serious injury, better optics and wider field. ④ Cosmetic  Colored CL  Cosmetic CL for anirdia  Cosmetic CL for corneal opacity ① The size of the retinal image is near to normal. ② The field of vision is larger than spectacles as it moves with the eye and ADVANTAGES not restricted with a frame. ③ No spherical or chromatic aberrations. ④ Cosmetic. ① Special care is required for its cleanliness and storage. ② Some people do not tolerate them (foreign body sensation) and others develop allergy to contact lens solutions. DISADVANTAGES ③ Traumatic corneal abrasions may occur during manipulation. ④ Infection is always a risk with bad hygiene or lens contamination (acanthamoeba keratitis → catastrophic) ⑤ Economic burden in poor patients  Conjunctiva:  Allergic (giant papillary) or infective conjunctivitis may occur occasionally.  Cornea: COMPLICATIONS  Corneal epithelial edema → due to corneal hypoxia.  Vascularization → due to hypoxia or infection.  Ulcer → due to improper hygiene and infection by a high virulent organism.  DEFINITION  The correction of the refractive state of the eye by surgical interference.  When glasses and contact lenses cannot be used due to ① Optical reasons as in anisometropia with intolerance to contact lenses. ② Mechanical reasons as nose or orbit configuration are difficult to fit spectacles with intolerable contact lenses. INDICATIONS ③ Cosmetic reasons specially females not accepting there look with glasses. ④ Occupational reasons as people working in physically aggressive occupations, media and sport fields. ⑤ Contact lens intolerance (recurrent allergy).  Lenticular ① Phakic IOL implantation (AC or PC) ② Clear lens extraction +/- IOL implantion  Corneal ① Incisional (refractive keratotomy- astigmatic keratotomy) ② Corneal rings & inlays CLASSIFICATION ③ Conductive keratoplasty ④ Laser ablation:  flap surgery (LASIK – femtolasik)  surface ablation (PRK – PTK) ⑤ Femto-smile (small incision lenticule extraction)  Combined (bioptics)  Both corneal & lenticular surgeries are done to correct the refraction  A commonly performed surgical procedure in the 20th century.  This type of corneal surgery involves multiple linear incisions in the anterior cornea that cause central cornea flattening and thereby reducing its focusing power.  It is still an excellent procedure for low myopias, i.e. from –1.0 to –6.0 D in young adults.  RK is still a cheaper option for patients who have low to moderate myopia & cannot afford the more expensive laser and surgical correction procedures.  It decreases myopia by flattening the corneal curvature due to the PRINCIPLE peripheral scarring induced by the incisions.  The central optical zone measuring 3-4 mm in diameter is marked out & is spared.  With a specially calibrated diamond knife 4-8 radial incisions (depth METHOD varying according to the degree of myopia possibly reaching just above the Descemet’s membrane) in the area between the limbus and the pre-marked optical zone.  RK incisionsare now constructed by femto-laser (bladeless).  It is suitable for young adults with stable myopia of –1 to –6 D with INDICATION minimal astigmatic error.  The main advantage of RK is the spared 3mm optical center (unlike in laser refractive surgery where the central optical zone of 6 to 7 mm is ADVANTAGES the target with the possibility of PO-complications in that important area).  Cost factor → almost 1/4 of the cost of laser refractive surgery.  It is an extension of the principles of radial keratotomy.  The aim of astigmatic keratotomy is to flatten the more curved (steep) meridian by asymmetrical incisional surgery.  To achieve this various considerations are kept in mind such as the number and position of the transverse arcuate incisions done in this surgery which are designed according to a certain nomogram.  The main indication is in the management of post-keratoplasty induced astigmatism.  The results are less predictable (not so precise).  PRK is the treatment of choice for myopia of –1 to –10 D. It also treats HM up to +6D & astigmatism of 4D.  The central part of the cornea (optical zone) is reshaped by the laser after corneal epithelial debridement.  PRK uses the computer-controlled, high accuracy and precision excimer laser to sculpt the surface of the cornea, correcting myopia, hypermetropia and astigmatism.  As very thin layer of the cornea is removed, PRK does not weaken the cornea. ① The results are excellent with an accuracy of 95% in achieving a +/– 0.5D correction with nil to negligible corneal haze. ② Avoidance of suction cup use which pressurize the globe & can initiate ADVANTAGES retinal tears & vitreous detachment. ③ Avoidance of flap-related complications. ④ Only 80 microns of corneal tissue are sacrificed compared to about 120 microns for the lasik flap  ① Severe postoperative pain & inflamation due to bare corneal stroma which gradually recedes until full epithelization is achieved. ② There may be residual corneal haze affecting clear vision (specially with DISADVANTAGES high ablation values) but the use of mitomycin-c decreased post-PRK haze incidence & density. ③ Any corneal infection will be very destructive as the primary defensive mechanism of the cornea (epithelium) is removed.  LASIK is a modification of PRK. In this procedure a 100-130 micron hinged corneal flap is lifted from the central 8 to 9 mm of cornea with the help of a microkeratome.  This flap is folded to the side and the excimer laser (same for PRK) is then used to remove tissue from the exposed surface → correcting myopia, hyperopia and astigmatism.  The corneal flap is replaced back.  The maximum range of LASIK correction → -8 to +5D  Mechanical microkeratomes may not produce the intended actual flap thickness, with the average thickness being 120 microns. With the development of femtosecond lasers, flap creation is much more precise (femto-flap) allowing surgeons to use a flap thickness of between 100 - 120 microns. ① Patient has good vision at the end of the same day (rapid visual results). ADVANTAGES ② There is no pain, watering (RK) or corneal haze (PRK) as compared to RK and PRK respectively. ① Expensive procedure ② Requires greater surgical skill and precise microkeratomes. ③ Complications of LASIK are related to the corneal flap  Too thin a flap can cause flap wrinkling (striae) on repositioning.  Too thick a flap will leave very little of the corneal stromal tissue to DISADVANTAGES work on.  stromal flap interface complications (diffuse lamellar keratitis {sands of sahara} – Epithelial ingrowth)  during flap construction → free flap (cut of the hinge), incomplete flap, or flap buttonhole.  Intrastromal corneal ring segments (ICRSs) are placed in the mid-corneal periphery (leaving a central optical zone) at approximately two-thirds of stromal depth → reshape the anterior corneal surface.  It utilizes the theory demonstrated by Baraquer → when material is removed from the central area of the cornea or added to the periphery, a central flattening effect is achieved. In contrast, when material is added to the center or removed from the corneal periphery, the central surface curvature is steepened.  The corrective results are in direct proportion to the thickness of the implant and in inverse proportion to its diameter. The thicker and smaller the (device) → the higher the (refractive result).  ICRS correction of myopia has faded due to → limited range of correction, induced astigmatism, and slow visual recovery.  Nowadays, the use of ICRS has evolved into an important therapeutic intervention for corneal ectatic diseases such as keratoconus and post-laser surgery-induced ectasia.  CK is a noninvasive procedure using radio frequency without removal of any corneal tissue.  The probe is applied to the corneal periphery leading to heat production → peripheral shrinkage & central steepning according to the theory by Baraquer.  It treats patients with mild to moderate hyperopia & may be used in mono-vision treatment of presbyopia.  The results are usually regressive with time & re-treatment may be needed.  Induced astigmatism is the most troublesome complication & may require additional treatment.  Myopic & astigmatic patients are not candidates.  Lenticular targeted refractive procedure → for correcting myopia of –16 to –26 D.  In high myopia the image is refracted far in front of the macula usually due to abnormally tall axial length. PRINCIPLE  The lens is removed the same way as cataract extraction though it is clear to weaken the refractive power of the eye & render the image refracted far behind. It is now accepted that even a zero power posterior chamber IOL is better than no IOL at all. NOTES  It retards incidence of posterior capsular opacification (PCO).  It reinforces the posterior capsule to hold the vitreous phase thus minimizing incidence of retinal detachment (RD).  it has the same complications of cataract surgery as endophthalmitis & pseudophakic RD. DISADVANTAGES  post-operative refraction is not so precise leaving a diopter of over or under correction.  loss of natural accommodative power of the eye.  Phakic refers to the fact that the lens is implanted into the eye without removing the eye's natural lens.  During phakic lens implantation surgery, a small corneal incision is constructed through which the lens is implanted just in front of or just behind the iris.  >21 years old, have had stable refraction (change in vision 30 degrees, CANDIDATES  have acceptable central endothelial cell count as phakic IOL specially AC tends to decrease the count.  Anterior chamber depth (ACD) 3.00 mm or greater is required for implantation. ① Active anterior segment disease or recurrent or chronic uveitis ② Cataracts ③ Previous ocular surgery ④ Glaucoma or IOP >21mmhg CONTRAINDICATIONS ⑤ Preexisting macular pathology ⑥ Retinal disease ⑦ Systemic diseases classically associated with poor postoperative healing (DM, connective tissue disease, autoimmune disease). ⑧ Pregnancy. ① Anterior chamber angle-supported: (almost obsolete nowadays)  Complications: ① endothelial cell loss ② Pigment dispersion and lens deposits ③ chronic inflamation & anterior uveitis TYPES OF PHAKIC ④ pupil ovalization & irirs retraction IOL ⑤ IOL rotation ⑥ surgically induced astigmatsm ⑦ Glaucoma ⑧ Glare & haloes ② Anterior chamber iris-fixated (Iris claw):  Complications:  Same as that of angle supported except the IOL here is dislocated or decentered not rotated iris atrophy may occur on fixation sites. ③ Posterior chamber phakic IOLs:  Complications: ① Iatrogenic inverted implantation ② Endothelial cell loss ③ pigment dispersion ④ anterior uveitis ⑤ Glaucoma ⑥ Cataract development 1-3% at 5 years ⑦ Decentration  The most popular phakic implantable lens nowadays is the implatnable collamer lens (ICL)  The new versions had updates to overcome certain characteristic problems of the like:  Cataractogenesis → The ICL is vaulted anteriorly to avoid touch with the crystalline lens capsule.  Glaucoma → The ICL has multiple holes in the haptics & the optic to pass aqueous from the PC to the AC & prevent pupillary block glaucoma.   Femtosecond (FS) laser is an infrared laser (wavelength of 1053nm).  FS laser, like Nd: YAG laser works by → producing photodisruption of the optically transparent tissue such as the cornea. PRINCIPLES  Application of either FS laser or Nd: YAG laser → generation of a rapidly expanding cloud of free electrons and ionized molecules → acoustic shock wave generation → disruption of the treated tissue.  LASIK flap creation (femto-flap)  Astigmatic keratotomy (AK)  Small-incision lenticule extraction (SMILE)  Channel creation for implantation of intrastromal corneal ring segments (ICRS) CLINICAL  Intrastromal presbyopia correction (INTRACOR) APPLICATIONS  FS lasers are also being used in  laser-assisted anterior and posterior lamellar keratoplasty.  customized trephination in penetrating keratoplasty.  wound construction, capsulorrhexis, and nuclear fragmentation in cataract surgery.   FS laser offers several advantages over the conventional microkeratomes and is gaining popularity across the globe.  Reduced incidence of flap complications like buttonholes, free caps, incomplete cuts  Greater surgeon control over flap diameter and thickness, side cut angle, PRINCIPLE hinge position and length  Increased precision with improved flap safety and better thickness predictability  Capability of cutting thinner flaps (up to 90 microns) to accommodate thin corneas and high refractive errors  Opaque bubble layer (OBL): Gas bubbles routinely accumulate in the flap interface but occasionally they may dissect into the deep stromal bed or into the AC → interfere with the ability of the excimer laser eye tracker device for tracking and registration.  Rainbow glare due to micro-irregularities on the back surface of the FS laser LASIK flap.  Lamellar keratitis due to Photodisruption-induced microscopic tissue DISADVANTAGES injury and ocular surface inflammatory mediators.  Site → in the flap interface.  It differs from DLK → the location of stromal inflammation is in the flap anterior stroma and corticosteroid treatment seems to have little effect on outcomes.  Difficulty in lifting the flap if retreatment is required  Much more expensive  REFRACTIVE LENTICULE EXTRACTION (RELEX) SMALL-INCISION LENTICULE EXTRACTION (SMILE)  The procedure uses femto-laser to construct a corneal pocket with a PRINCIPLE stromal lenticule within it and a 2-4 mm small incision at the base of the pocket through which the lenticule is manually extracted.  No flap-related complications  Less postoperative dry eye ADVANTAGES  Less corneal denervation  Less biomechanical instability leading to corneal ectasia.  It is both a flapless & a bladeless procedure CORRECTION  It corrects myopia up to -10D & astigmatism up to -5D RANGE  Epithelial defect or even cap rupture due to vigorous manneuvers leading to damage of the epithelium or the cap above the corneal pocket (< 1%)  Suction loss  OBL COMPLICATIONS  Lenticule tearing during extraction  Foreign body inside the pocket  Diffuse lamellar keratitis  Ectasia