Introduction

“A diagnosis of glaucoma requires not only spotting the condition’s characteristics and signs, but also determining the rate of structural and functional progression” (Robert Murphy, 2012).

Nowadays, it is estimated that structural changes in glaucoma most often precede functional changes. According to some opinions, this process could appear up to 5 years before functional damage. Thus, making feasible early structural damage could be key strategy for future successful glaucoma management. Imaging diagnostic methods that provide quantitative information regarding structural parameters in glaucoma (ONH topography, retinal nerve fiber layer thickness and macular thickness) had become una voidable and priceless tools in glaucoma diagnosing and progression assessment.

A number of glaucomatologists are reporting controversial results regarding diagnostic superiority and accuracy of different structural parameters, conducted with different imaging method (Optical Coherence Tomography, Confocal Scanning Laser Ophthalmoscopy or Scanning Laser Polarimetry).

Retinal ganglion cell (RGCs) loss has been clearly established as basic and crucial pathophysiological phenomenon in glaucoma development. Retinal nerve fiber layer (RNFL) and retinal ganglion cell together comprise about 40% of the retinal thickness, and about 50% of total RGCs are localized in the region of macula. In human and experimental primate models of glaucoma, RGCs loss was identified around the fovea at early disease stages. Loss of RGCs necessarily causes atrophy of the ganglion cell layer (GCL). Data emerging from some histological studies have reported that abnormalities in RGCs morphology and cell density precede clinically detectable VF loss. So, early detection of RGCs loss is considered essential for diagnosing glaucoma and monitoring glaucomatous progression. Retinal nerve fibers are the axons of the RGCs and emerge from the cells to form the optic nerve.

Therefore, circumpapillary retinal nerve fiber layer (cRNFL) thickness has been used to assess the extent of glaucomatous damage (1).

Number of total RGCs varies in range of 0.7-1.5 millions, in average about 1 million RGCs, with 50% localized within central 4-5 mm of the macular region and peak density of 15000 mm2. It makes only about 7.3% of total retinal area, the part which is not properly covered with visual field assessment tests. Macular ganglion cell loss is considered as early phenomenon in glaucoma. However, as glaucoma eventually involves the cell body of the RGCs, mea su rement of RGC thickness (if possible), rather than of the RNFL alone, would help estimate the glaucomatous damage accurately (1).

The main focus of this article is to emphasize the significance of macular thickness measurements in glaucoma. So, what are (if so) the advantages of macular thickness assessment over other structural parameters in glaucoma?

Macular thickness does not change over time in healthy subjects and in the presence of reduced macular thickness it is most probably glaucoma. Besides, evidence is stating that, the ganglion cell population in the macula, unlike the total number of ganglion cells in the entire retina, is relatively stable among normals. On the other hand, the optic nerve varies individually in terms of size, shape, sloping, etc. Furthermore, because the ganglion cells represent a huge portion of the thickness of the retina, it is estimated easier to detect changes in macular thickness, compared with changes in a thin layer, as it is RNFL. Those features make macula ideal parameter for comparison to a normative database (2).

The significance of macular thickness in glaucoma starts with the introduction of Retinal Thickness Analyzer (RTA) by Zimmer and co-workers (1998), suggesting that reduced macular thickness could be useful in glaucoma detection. The measurements were recorded on macular thickness map (3). The studies based on RTA and introduction of time-domain Optical Coherence Tomography (TD-OCT) had offered evidence promoting macular thickness as “surrogate” parameter in assessment of glaucoma (1).

Studies evaluating data from TD-OCT suggested better reproducibility of macular scans over RNFL scans, due to the fact that macular scans require internal fixation and are less dependent on the operator skills, which increases scan reproducibility. On the other hand, RNFL scans require correct positioning of the scan circle by the operator performing the method (4).

The role of Optical Coherence Tomography

The advent of posterior segment Optical CoherenceTomography had brought new prospective in numberof retinal diseases and optic nerve pathology. Opticalcoherence tomography (OCT) is a non-invasive, noncontactdiagnostic tool that can provide in vivo crosssectionalimages of the retina with high resolution, reproducibilityand repeatability, as well as low variability.The new imaging diagnostic method has proved itssuperiority in the diagnosis and assessment of glaucomaand its progression. OCT enables measurementsof structural parameters, identified through theoptic nerve head features (ONH), evaluation of circumpapillaryretinal nerve fiber layer thickness (RNFL)and assessment of central macular thickness andmacular ganglion cell complex (GCC).

In general, advantages of Optical Coherence Tomographyare due to the ability of providing quantitative information with high resolution of the scans. The obtainedmeasurements are objective, with high sensitivity,specificity and reproducibility.

The studies based on Time domain Optical CoherenceTomography (TD-OCT) are reporting controversial resultscomparing diagnostic accuracy of RNFL versusmacular thickness measurements (Medeiros, 2005;Wollstein,2005). Limitations of TD-OCT had been recognizedas the ability of full retinal thickness assessment,without software of ganglion cell separation,and also, insufficient depth resolution for accurateevaluation and ganglion cell segmentation. A numberof studies have demonstrated that automated macularGCC measurements obtained with SD-OCT havebetter diagnostic accuracy compared with macularthickness measurements provided by TD-OCT. However,the relationship between macular structuralchanges and topographic changes in the optic nervehead in glaucoma is still not completely clarified yet.5The upgrade of OCT technology and introduction ofSpectral domain (Fourier domain) Optical CoherenceTomography (SD-OCT) had enabled approach to moredetailed and sophisticated information regardingmacular thickness, compared with TD-OCT. Due to thehigh speed, large areas of the retina could be examinedwith great accuracy.

The most important and useful advantages of SD-OCTover TD-OCT were identified as:

• 2x better resolution depth (10 μm/5 μm)
• segmentation of ganglion cell layers
• improved scanning speed (50-70x)
• better reproducibility, sensitivity and specificity
• less artefacts due to eye movements
• software for evaluation of glaucoma progression (GPA- glaucoma progression analysis)

Those characteristics had made macular thicknessvaluable and reliable glaucoma marker (2).

Recent studies have found that glaucoma diagnosticaccuracy is improved if SD-OCT macular measurementsare focused on inner retinal layers. It remainsunclear whether the outer retinal layer (ORL), especiallythe photoreceptor layer, is involved in glaucoma.

Some electrophysiological reports are sug gesting thatthe outer retina is involved in glaucoma, and have appearedsome studies noticing the involvement of thefoveal photoreceptor layer in glaucoma (1).

It has been shown that circumpapillary retinal nervefiber layer (cpRNFL) measurements with OCT have goodability to distinguish glaucomatous from healthy eyes.Although cpRNFL thinning is a useful marker of glaucomatousdamage, there is growing evidence that measurementof the glaucomatous macula may also revealchanges in favour of diagnosing glaucoma. Measurementsof macular retinal ganglion cell-related structuremay therefore offer valuable adjunct or alternative toroutine circumpapillary measurements for glaucomadiagnosis. As the macula is generally liberated fromlarge vessels and has a readily identifiable center, assessmentof the macula may also overcome some limitationsof circumpapillary measurements, such as interferencefrom retinal and optic nerve head vasculature,parapapillary atrophy, and variable placement ofthe measurement circle around the disc (6).

Future improvement in OCT technology embracesability for evaluation of structures that have been inapproachableso far (lamina cribrosa and choroidalthickness measurements). The introduction of sweptsource (deep range OCT) obtains very high resolutionscans and choroidal volume measurements.Ultra-high resolution OCT (UHR-OCT), with an axial resolutionof approximately 3 μm or less, has the abilityto image retinal ultrastructure.

The repeatability and reproducibility of measurementsare very important features that provide reliability ofthe obtained data, which would be compared with the data form normative database. This informationenables the clinician to evaluate if observed changesare due to fluctuations in the methods or if they arereally valid changes in the structures. This is especiallytrue for the measurements of the intra-retinal layers,which are important morphometric parameters in thediagnosis of retinal and neurological diseases and themonitoring of the progression of these disorders (7).

The concordance between the patient’s measurementand normative database is another very importantand sensitive aspect of the interpretation of the results.

Also important to the issue of “normative” data is stratificationby age, ethnicity and even gender of the subjectswho make up the “normal,” since measurementsin healthy individuals may vary according to thesefactors. It is clear that large amounts of in vivo informationcan be acquired without invasive intervention,effectively allowing a “virtual biopsy” of the retina. Inpractice, clinicians and operators should stick to therule for quantitatively and qualitatively reviewing scansbefore comparing them to the macula or RNFL normativedatabases (8).

The macular Ganglion Cell Complex (GCC) significance

Glaucoma preferentially thins the ganglion cell complex(GCC) in macula, which includes the axons, cellbodies and dendrites of retinal ganglion cells. Humanand experimental studies have confirmed RGCs lossaround fovea occurring in early glaucoma. Moreover,the GCC has been identified as highly sensitive andspecific for diagnosing glaucoma and monitoring itsprogression (Rao et al, Ophthalmology 2010). Therefore,studies based on imaging methods had revealedthe possibility of using macular thickness as a markerfor glaucoma detection and progression assessment.Recent studies have found that glaucoma diagnosticaccuracy is improved if SD-OCT macular measurementsare focused on inner retinal layers (1).

The macular GCC thickness measurement incorporatesseveral retinal layers, including ganglion celllayer, inner plexiform layer and overlying retinal nervefiber layer. According to data emerging from somestudies, it is possible that finer segmentation of theganglion cell-containing retinal layers alone might facilitatebetter detection of glaucomatous damage,particularly as it is loss of retinal ganglion cells that isthe defining histological feature of glaucoma. Recentdevelopments in SD-OCT provide the ability for segmentationof the ganglion cell containing macularganglion cell inner plexiform layer (mGCIPL) (6).

There are few commercially available OCT technologiesthat have software algorithm capable of ganglioncell complex assessment. It is affordable with Optovue(RTVue), Carl Zeiss Meditec (Cirrus) and Heidelberg(Spectralis) OCT algorithm.

Several imaging technologies have included progressionanalysis software algorithms tending to assist theclinician in monitoring glaucoma progression. In orderfor progression analysis to be useful in clinical practice,three criteria must be met: the measurements must bereproducible and have minimal noise, follow-up imagesmust be accurately registered to each other, anda statistical test must distinguish between true biologicalchange and instrument measurement variability (9).

Relationship between diagnostic accuracy of different structural parameter measurements

It was mentioned previously that structural damage of the optic nerve head (ONH) precedes functional loss identified through visual field impairment.

Number of studies conducted with imaging devices,especially SD-OCT, that are assessing such relationshipare reporting good correlation between GCC thinningand visual field changes (VF) (5, 10, 11).

Recent studies have registered that eyes with SAP progressionhave significantly greater rates of macularthickness loss consistent with glaucomatous atrophy ofRGCs, compared with non-progressing eyes (12).

Combined Structure-Functional Indices (CSFI) are recentlyrecognized as more objective and relevant indicatorof glaucomatous damage assessment, comparedwith structural and functional parametersalone13 Combined structure and functional index describedby Medeiros and co-workers serves the purposeof merging the results of structural and functionaltests into a single index that could be used for diagnosing,staging and detection of progression in glaucoma.Limited agreement between structural andfunctional tests shows necessity of combined approachfor detecting and monitoring the disease.Also, it was found that macular thickness assessed onOCT is directly related to the best corrected visualacuity and contrast sensitivity, which is predictable toexpect (14).

But, speaking of diagnostic accuracy of differentstructural parameters, the key question is, could beone structural parameter (retinal nerve fiber layerthickness or macular thickness obtained through GCC)recognized as most relevant and superior, with best diagnosticaccuracy for early glaucoma detection?

And, the other issue, which one of the imaging methods(OCT, CSLO, SLP) has better diagnostic accuracyover the others? But, this is highlight for some other occasionas a challenging topic.

One of the most important features of SD-OCT is highreproducibility and low variability of the measurements.

Speaking of diagnostic accuracy of different parametersand measurements, the relevance of the performedstudies should be based on STARD Guideline -Standards for Reporting of Diagnostic Accuracy. It isinternationally accepted method of assessing a studyconducted on diagnostic equipment. The STARDGuideline contents 25 points that should be fulfilled forgood diagnostic accuracy of the study.

The majority of studies investigating correlation betweendifferent structural measurements are reportinggood inter-measurements agreement (5, 6, 7, 15–19).The reports are emphasizing that advanced imagingdevices with their sophisticated technology are capableof identifying glaucomatous damage at the earlystage. On the other hand, clinicians have always tohave in mind the objective limitations of imagingmethods that provide quantitative information withhigh resolution, reproducibility and repeatability. Asidefrom the real “revolution” that imaging methodsbrought to diagnostics in ophthalmology, those methodsshould not be interpreted without thorough clinicalexamination of the patients, complementary with thefunctional tests data.

Another challenging issue related to the diagnostic superiorityof imaging methods, is the assessment andmonitoring of glaucoma progression. High resolutionand low variability of measurements conducted withimaging devices are essential for detecting glaucomaprogression.

Some of the sophisticated and upgraded OCT technologieshad developed commercially available softwarealgorithm for detection and assessment of glaucomaprogression.

The Cirrus High definition OCT (HD-OCT), as well asRTVue 100 optical coherence tomography, have inbuiltganglion cell analysis algorithm (GCA), which allowssuccessful segmentation of inner macular layers,with reproducible measurements of the layers thickness.Studies performed with this algorithm are suggestingthat macular parameters, such as total macularthickness and ganglion cell analysis, obtained byOCT, could be found useful in detecting glaucomaprogression (1, 7, 9).

Finally, “an optimal method for detecting glaucomaprogression should not only give an indication ofwhether or not the eye is changing over time, but alsoshould estimate the rate of deterioration (20).

Comment and recommendation

Imaging technologies that evaluate the structure ofthe optic nerve head, peripapillary retinal nerve fiberlayer and macular thickness provide important anduseful quantitative information that could be adjunctand complement, but not substitute for thorough andcomprehensive clinical examination and functional investigationdata.

Advanced ocular imaging technologies are facilitatingobjective and reproducible quantification ofchange in glaucoma, but, at the same time, they putsubstantial challenge in front of the clinicians in orderto determine true structural change due to glaucoma.

But, on the other hand, clinicians should be fully awareof the objective limitations of imaging methods that insome case could even mislead inexperienced ophthalmologistand provoke false glaucoma diagnosis.

The advent and upgrading of Spectral domain OpticalCoherence Tomography technology has enabled advancedmacular protocols to play significant role inthe diagnosis and monitoring of glaucoma. OCT isuseful tool for detection of glaucomatous structuralprogression and quantification of the velocity of progressivemacular loss.

OCT measurements of macular structures such asmacular ganglion cell complex (GCC) has beenshown to be very useful adjunct for differentiatinghealthy and glaucomatous eyes. Additionally, thickness measurements of the intra-retinal layers havegood repeatability and reproducibility.

There are still controversial opinions regarding superiorityand overestimation of diagnostic accuracy of singleimaging device over the others. No relevant controlledclinical studies yet have clearly and undoubtedly reportedthat one single imaging device outperforms theothers in diagnostic ability and accuracy.

Nowadays, most imaging technologies have comparablediagnostic accuracies, but each of them has itsown objective limitations. Progression detection softwareis still relatively new, and long-term prospectivedata on progression analysis are limited. There is stillmissing definition of imaging obtained structuralchange as gold standard in glaucoma.

Also, the artefacts due to eye movements scan quality,ocular media transparency and some other pathologicconditions of the optic disc and macula couldsignificantly influence diagnostic accuracy. The normativedatabase in each imaging device is different;resulting in outcome as “outside of normal limits” messagethat could lead to false diagnosis.

Taking all these facts into account, one has to concludethat imaging methods are undoubtedly extremelyuseful in providing quantitative information regardingstructural damage as early phenomenon inglaucoma. But, for real, relevant assessment of eachglaucoma patients it is necessary to obtain and interpretcomprehensive data of thorough clinical examination,functional investigation and imaging devices.

The outcome should result in individually tailored therapeuticapproach for every glaucoma patient.

The revolution in diagnostic and investigative ophthalmology that was provoked with the invention of imaging diagnostic methods had brought to the light new evidence that have defined and resolved some dilemmas in a number of ocular diseases concerning retina and optic nerve. Nowadays it is accepted as scientific fact that imaging technologies (Optical Coherence Tomography, Confocal Scanning Laser Ophthalmoscopy, and Scanning Laser Polarimetry) are providing quantitative information confirming that structural damage in glaucoma most often precede functional changes, identified through the condition of visual field.

Assessment of the peripapillary retinal nerve fiber layer thickness (RNFL) and parameters of optic nerve head (ONH) conducted through the imaging methods has proved its importance in the recognition and detection of early glaucoma.

Different studies have previously reported controversial results comparing diagnostic accuracy between RNFL and macular thickness in glaucoma.

The invention of Spectral domain Optical Coherence Tomography (SD-OCT) and the ability of evaluation of macular thickness and macular ganglion cell complex (mGCC) has improved the potential of early glaucoma diagnosis and monitoring of glaucoma progression.

It is common knowledge that retinal ganglion cell loss (RGCs) is basic pathophysiological phenomenon in glaucoma. Retinal nerve fiber layer (RNFL) and retinal ganglion cells (RGCs) together comprise about 35-40% of the macular thickness.

Early detection and recognition of structural damage is considered to be crucial for diagnosis and management of glaucoma.

But, on the other hand, glaucomatologists should always take into account the objective limitations of imaging methods that should not be used as substitution for the clinical evaluation and assessment of visual field in patients with glaucoma.

Proper diagnosing of glaucoma should be made only with complementary and comprehensive evaluation and interpretation of all available methods, in order to make the right diagnosis and start immediate treatment if necessary.

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