Ординатура / Офтальмология / Учебные материалы / Color Atlas of Ophthalmology The Quick-Reference Manual for Diagnosis and Treatment
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17 Ophthalmic Instruments and Diagnostic Tests 491
To detect abn orm al ret in al correspon den ce object ive angle (OA) an d th e su bject ive angle w hich gives th e angle of an om aly (AOA).
(ARC) w ith th e syn optop h ore, th e (SA) of squ in t are first determ in ed,
AOA = OA – SA
In th e case of n orm al ret in al correspon den ce (NRC), th e SA is equ al to th e OA, an d th e AOA w ill be zero. In un h arm on iou s ARC, th e su bject ive angle w ill be less th an th e object ive angle, an d th e differen ce bet w een th e t w o sh ou ld be at least 5 degrees or m ore. In th e case of h arm on ious ARC, th e su bject ive angle w ill be zero. Th us th e angle of an om aly in a h arm on iou s ARC w ill be equ al to th e object ive angle.
Fusion: For test ing fu sion , t w o slides are used: on e is th at of a rabbit w ith out a tail an d th e oth er is th at of a rabbit w ith ou t ears. Th e t w o slides are kept in each
arm of th e syn optoph ore an d th e arm s are fixed at th e angle of squ in t . If th e pat ien t sees both th e ears an d th e tail, th en fu sion is presen t . If th e pat ien t sees eith er th e tail or th e ears, fusion is absen t .
Stereopsis: Stereop sis can be tested w ith slides con tain ing parat roopers w ith a plan e in th e backgrou n d . Th e pat ien t sh ou ld be able to tell w h eth er th e para- t roopers are in fron t of th e plan e or n ot , w h ich in dicates good stereopsis.
After im ages: After im ages can also be don e.
Visual-Field Testing
Th e visu al field is th e por t ion of sp ace th at is visible to th e fixat ion eye. Visual-field exam in at ion is th e exam in at ion of th e fun ct ion of th e visu al system in th e field an d n ot on ly th e determ in at ion of th e lim its of th e field . Th e differen ce th resh old is th e sm allest m easu rable differen ce in lum in an ce bet w een a st im ulu s an d th e backgroun d (Fig. 17.3).
Automated Static Perimetry
Th e differen t test s in autoperim et r y are as follow s:
Suprathreshold test: Th is test is u sed as a screen ing device for severe or m oderate defect s.
Threshold related st rategy: Th e act ual th resh old is determ in ed at a sm all n u m - ber of p oin t s an d th ey are u sed to ext rapolate th e h ill of vision .
Threshold test ing: Th resh old test ing is th e stan dard of care for glau com a m an - agem en t . Many p oin t s are tested an d th ere are differen t st rategies used to accurately defin e th e visu al field . Th ese tests can be long an d th e pat ien t s can becom e fat igu ed .
SITA: Becau se fu ll th resh old can be t im e con su m ing, sh or ter algorith m s h ave been developed . Th e Sw edish In teract ive Th resh olding Algorith m (SITA) uses m ath em at ical m odeling an d presen t un derstan ding of th e visu al field to in - crease accu racy w h ile speeding up th e test .
Short-w avelength autom ated perim etry (SW AP): Th is select ively tests th e sh or t- w avelength path w ay by presen t ing a blue t arget on a yellow backgroun d . SWAP m ay be able to sh ow defect s 1 to 3 years before st an dard tech n iques.
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Fig. 17.3 Humphrey autoperimetric 30–2 test.
Th e various field defects seen in glaucom a are gen eralized depression , baring of th e blin d spot , isolated p aracen t ral scotom a, Seidel scotom a, Bjerru m scotom a or arcuate scotom a, dou ble arcuate scotom a, Ron n e n asal step (w h ich respects th e h orizon tal m idlin e), tem poral w edge defect , p eriph eral breakth rough , alt it udin al defect , cen t ral an d tem poral islan ds, an d split fixat ion .
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Automated Corneal Topography
Im aging tech n iqu es of th e corn ea are developing rapidly, m ain ly because of con t in - uou s advan ces in refract ive an d cat aract su rger y. It is crucial to u n derst an d th e sig- n ifican ce of n ew im aging tech n iques an d th e relevan t prin ciples of corn eal opt ics. Th e discu ssion of th e m ost com m on clin ical m eth od of Placido-based corn eal topography em ph asizes im por tan t con cept s of it s clin ical in terpret at ion (Fig. 17.4).
A
B
Fig . 17.4 (A) Astigmatic corneal topography. (B) Orbscan topography of an eye with keratoconus.
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Optical Properties of the Cornea
Several con cepts are u sed to ch aracterize opt ical proper t ies of th e corn ea, such as cur vat ure, sh ape, local su rface, p ow er, expressed as refract ion in diopters, th ick- n ess, an d th ree-dim en sion al st ru ct u re. Th e keratom et ric value is a con cept in h erited from keratom et r y an d is calculated sim ply from radii of cu r vat u re as follow s:
K = refract ive in dex of 337.5/radiu s of cur vat u re
Th e in tact cen t ral corn eal th ickn ess of ~560 µm is con sidered en ough to en su re long-term m ech an ical stabilit y of th e corn ea. Th e periph eral th ickn ess (~600 µm ) is cer t ain ly clin ically im por tan t in som e refract ive procedures su ch as in t racorn eal rings, ast igm at ic keratotom y, an d cat aract surger y. W ith th e advan ces in corn eal im aging an d w idesp read refract ive su rgeries, corn eal beh avior w ill likely be bet ter un derstood . Corn eal topography in st ru m en ts u sed in clin ical pract ice m ost often are based on Placido reflect ive im age an alysis. Th is m eth od of im aging of th e an - terior corn eal surface uses th e an alysis of reflected im ages of m u lt iple con cen t ric rings projected on th e corn ea.
Interpretation of Topographic Maps
Ever y m ap h as a color scale th at assign s par t icu lar color to a cer tain keratom et ric diopt ric range. Never base an in terpret at ion on color alon e. Th e value in kerato- m et ric diopter is crucial in th e clin ical in terpret at ion of th e m ap an d h as to be looked at w ith th e in terpret at ion of ever y m ap . Absolute m aps h ave a preset color scale w ith th e sam e diopt ric step s an d diopt ric m in im um an d m axim u m assign ed to th e sam e colors for par t icu lar in st ru m en t .
Norm alized m aps h ave differen t color scales assign ed to each m ap based on in - st ru m en t soft w are th at iden t ifies th e act u al m in im al an d m axim al keratom et ric diopt ric valu e of a par t icu lar corn ea. Th e diopt ric range assign ed to each color is gen erally sm aller com pared w ith th e absolu te m ap, an d, con sequ en tly, m aps sh ow m ore det ailed descript ion of th e su rface. Th e disadvan tage is th at th e colors of t w o differen t m aps can n ot be com pared directly an d h ave to be in terpreted based on th e keratom et ric valu es of th eir differen t color scales.
Specular Microscopy
Th is is an im port an t par t of th e preoperat ive evaluat ion , especially if corn eal guttata or oth er sign s of a low en doth elial cell coun t are fou n d . It is clin ically an d m edical-legally im por t an t to docu m en t a low en doth elial cell coun t before LASIK, rath er th an w orr y later w h eth er LASIK caused it . It m u st be don e before any exam in at ion th at m igh t rough en or dr y th e corn eal su rface. Alth ough en doth elial cell ch anges follow ing lam ellar surger y (in cluding LASIK) h ave n ot been reported clin ically, th ey h ave been seen experim en t ally w h en excim er ablat ion of greater th an 90% depth w as ach ieved . Th is depth is of cou rse en t irely con t rain dicated in pat ien t s, bu t t w o cases of corn eal perforat ion du ring LASIK h ave recen tly been reported, an d on e w ou ld assu m e th at oth er cases w ith deep ablat ion m ust also exist (Fig. 17.5).
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Fig . 17.5 A normal specular microscopy.
Corneal Pachymetry
St u dy of th e cen t ral corn ea w ith ult rasoun d pachym et r y is fu n dam en tal. Th e surface of th e u lt rason ic pachym eter p robe is w iped w ith alcoh ol an d does n ot n eed to be sterilized . Th is in st ru m en t is used to take a ver y carefu l an d accurate reading of th e th ickn ess of th e cen t ral corn ea. Th e pachym eter probe is placed on th e cen ter of th e corn ea, perpen dicu larly, to determ in e corn eal th ickn ess. Th e pachym eter h as a con sole displaying th e corn eal th ickn ess reading (Fig. 17.6).
Clinical Importance
Ideally, w e m ust preser ve a m in im um of 250 µm in th e posterior st rom al bed . Oth er invest igators con sider th at preser vat ion of a m in im u m of 50%of th e preop - erat ive cen t ral pachym et r y is essen t ial. Th is is referred to as th e Barraquer law of corneal thickness. Th is is par t icu larly im por t an t in th e t reat m en t of h igh refract ive errors w ith th e excim er laser an d in “en h an cem en t” procedures. Un less sufficien t corn eal st rom a rem ain s, as determ in ed by Barraqu er’s basic prin ciple, th ere is an in creased risk of developing corn eal ect asia. It is gen erally est im ated th at 10 µm of ablat ion corrects on e diopter of m yopia.
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Fig . 17.6 Illustration of a corneal pachymetry in a patient undergoing LASIK. P, ultrasound pachymeter; A, flap depth in case of LASIK; B, amount of strom a ablated for correction of refractive error; C, resid-
ual bed thickness. (From Boyd, BF. Pre - operative evaluation and considerations. In: Atlas of Refractive Surgery. Highlights of Ophthalm ology, 2000, 45. Courtesy Jaypee Highlights Medical Publishers Inc., Panama.)
A-Scan Ultrasonography
Ocu lar biom et r y m u st be perform ed prior to cat aract su rger y. Biom et r y is th e discip lin e in ch arge of th e physical param eters of th e eyeball an d in clu des t w o fu n - dam en tal explorat ion s, keratom et r y an d th e a xial length m easurem en t of th e eye. Usu ally, in clin ical pract ice, th e term biom et ry refers to th e lat ter an d con siders keratom et r y to be a separate procedure.
Th ere is n o qu est ion th at w h en w ell selected an d properly don e th e m odern m eth ods of part ial coh eren ce in terferom et r y (opt ical coh eren ce biom et r y), th e pen t acam (Ocu lu s, In c., Lyn nw ood, WA) an d th e advan ce con t act an d im m ersion ult rason ography afford u s th e best w ay of ach ieving th e desired postoperat ive refract ion . Determ in at ion of in t raocular len s pow er th rough m ean ingful keratom etric readings an d a xial length m easu rem en ts h as becom e th e stan dard of care. Th is is a ch allenging tech n iqu e to obt ain good visual resu lts an d pat ien t sat isfact ion (Fig. 17.7).
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Fig . 17.7 Determination of intraocular lens power in patients with normal axial length (normal eyes)—mechanism of how ultrasonography measures distances and determines axial length. The use of ultrasonography to calculate the intraocular lens power takes into account the variants that may occur in the axial diameter of the eye and the curvature of the cornea. The ultrasound probe (P) has a piezoelectric crystal that electrically emits and receives high-frequency sound waves. The sound waves travel through the eye until they are reflected back by any structure that stands perpendicularly in their way (represented by arrows). These arrows show how the sound waves travel through the ocular globe and return to contact the probe tip. Knowing the speed of the sound waves and based on the time it takes for the sound waves to travel back to the probe (arrows), the distance can be calculated. The speed of the ultrasound waves (arrows) is higher through a dense lens (C) than through a clear one. Soft-tipped transductors (P) are recomm ended to avoid errors when touching the corneal surface (S). The ultrasonography equipment computer can automatically multiply the time by the velocit y of sound to obtain the axial length. Calculations of intraocular lens power are based on program s such as SRK-II, SRK-T, Holladay, or Binkhorst, am ong others, installed in the com -
puter. (From Boyd, BF. IOL power calculation in normal cases. In: The art and science of cataract surgery. Highlights of Ophthalmology, 2001, 41. Courtesy Jaypee Highlights Medical Publishers Inc., Panam a.)
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Fig . 17.8 Mode B-scan showing retinochoroidal colobom a.
B-Scan Ultrasound
Lin ear A scan s are sum m ated in th e B-scan . Th is gives a t w o-dim en sion al crosssect ion al im age of th e eye an d orbit . It is of great valu e in evaluat ing th e posterior segm en t in eyes w ith opaque m edia (Fig. 17.8).
Potential Acuity Meter Testing
Th e Poten t ial Acu it y Meter (PAM; Marco Oph th alm ic In c., Jacksonville, FL) is an in st r um en t th at at tach es to a slit lam p . It ser ves as a vir t ual pin h ole by project ing a regu lar Sn ellen visual acuit y ch ar t th rough a ver y t iny aerial p in h ole aper t u re ~0.1 m m in diam eter. Th e ligh t carr ying th e im age of th e visu al acu it y ch art n arrow s to a fin e 0.1-m m beam an d is directed th rough clearer areas in cataracts (or corn eal disease), allow ing th e pat ien t to read th e visual acu it y ch ar t as if th e cat aract or corn eal disease w ere n ot th ere. Th e PAM is taken from it s st an d an d placed directly on to th e slit lam p in th e sam e m an n er as th e detach able t ype of Goldm an n ton om - eter. Th e exam in at ion takes from 2 to 5 m in utes per eye, depen ding on th e den sit y of th e cat aract (Fig. 17.9).
As poin ted ou t by Guyton , for th e PAM to w ork adequ ately, th ere m ust be som e sm all h ole in th e cat aract for th e ligh t beam to pass th rough . You m ay fin d su ch a h ole even in cat aract s th at h ave m edia clou ding of up to 20/200 an d bet ter. W h en you fin d it , th en you can avoid th e ligh t scat tering produ ced by th e op acit ies. It is th is ligh t scat tering th at w ash es ou t th e ret in al im age an d decreases vision beh in d cataracts. By project ing th e im age of th e visual acuit y ch art th rough on e t iny area, w e avoid th e scat tering effect , an d th e pat ien t can see th e ch art .
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Fluorescein Angiography
Flu orescein angiography can be a ver y u sefu l procedure for assessing ret in al disease by delin eat ing areas of involvem en t , gu iding t reat m en t , an d form u lat ing a progn osis for ch anges in th e pat ien t’s vision .
To in terpret fluorescein angiograph ic im ages, kn ow ledge of ret in al/ch oroidal an atom y an d circulat ion is essen t ial. Arterial an d ven ou s circu lat ion differen ces, as w ell as th e ret in a’s barriers again st th e passage of sodium fluorescein (NaFl) dye, in cluding th e ret in al pigm en t epith eliu m (RPE) (ou ter blood –ret in al barrier) an d th e ret in al vascu lar en doth elium (in n er blood–ret in al barrier), m ust be un - derstood . Kn ow ledge of fu n dus path ophysiology an d an atom y h as been greatly en h an ced by research using flu orescein angiograp hy (Fig. 17.11).
For purposes of angiogram in terpretat ion , th e sen sor y ret in a can be divided in to vascu lar an d avascu lar por t ion s. Th e vascu lar port ion is com posed of th e in tern al lim it ing m em bran e (ILM), n er ve fiber layer (NFL), ganglion cell layer (GC), in n er plexiform layer (IPL), an d in n er n uclear layer (INL). Th ese por t ion s of th e ret in a receive direct m etabolic support from ret in al blood vessels.
A
B
Fig. 17.11 (A) Fundus photo of neovascularization.
(B) Fluorescein angiography of neovascularization.