Use of fundus perimetry (microperimetry) to quantify macular sensitivity
Introduction
Perimetry, the quantification of the visual field, represents an important diagnostic examination in ophthalmology.
Macular diseases as well as pathologies of the optic nerve typically result in the deterioration of visual function. Macular function is not fully characterized by central visual acuity; major visual tasks such as reading rely on the central visual field as well. Testing visual acuity alone disregards paracentral or central scotomata, which strongly affect the patient's self-assessment of visual function.
Already in 1856 the German ophthalmologist Albrecht von Graefe mentioned that central visual acuity is only a single aspect of visual function but additional knowledge of the visual field is of the same importance (Graefe, 1856). In the meantime, a number of different methods for visual field measurement have been developed. Perimetry is very important especially in glaucoma management, in neuroophthalmological disorders and in diseases leading to peripheral visual field defects. Conventional methods of visual field testing such as kinetic perimetry with the Goldmann perimeter and static computerized perimetry serve well for these tasks. Standardized automated full threshold static cupola perimetry has been established into clinical practice during the past decades and it represents the gold standard in visual field testing today (Anderson, 1987). However, for precise evaluation of macular disorders, conventional perimetry appears often insufficient, because the accuracy of the conventional visual field is based on the assumption that fixation during the examination time is stable and located central at the fovea.
With the desire of an exact correlation between retinal pathology and functional alteration, various instruments have been invented in order to perform perimetry under simultaneous fundus control (Enoch, 1978, Kani and Ogita, 1978). The major problem of very high light levels necessary for retinal illumination in fundus observation could be overcome with the use of infrared light sources. With the invention of the scanning laser ophthalmoscope (SLO), it was possible to simultaneously visualize the fundus and perform fundus-correlated functional tests (Timberlake et al., 1982, Webb and Hughes, 1981). As a result, kinetic and static perimetry techniques have been implemented into clinical use as fundus perimetry or microperimetry, permitting simultaneous fundus observation and compensation of eye movements during the examination. By achieving precise examination conditions of patients with small retinal or choroidal lesions as well as with poor fixation, an exact correlation between retinal pathologies and functional defects was rendered possible.
Following the initial presentation of fundus perimetry with a scanning laser ophthalmoscope by Timberlake and coworkers more than 20 years ago, the value of this technique and other fundus-related function tests in the diagnosis and follow-up of patients with macular diseases has been investigated by a large number of researchers (Acosta et al., 1991, Ergun et al., 2003, Fletcher et al., 1994, Guez et al., 1998, Ishiko et al., 1998, Jarc-Vidmar et al., 2006, Midena et al., 2004, Orzalesi et al., 1998, Rohrschneider et al., 2001, Rohrschneider et al., 2005, Sunness et al., 1995a, Varano and Scassa, 1998, Vujosevic et al., 2006, Wolf et al., 1994).
Although the new perimetric procedure has often been referred to as “microperimetry”, neither the stimulus size nor the test grid qualify for this term. Like in conventional perimetry stimulus size varies from Goldmann size I to V, i.e. 6.5′ to 103′, and visual field examination is possible up to 15° or 20° from centre with the SLO and MP 1, respectively. Rather, than microperimetry fundus-correlated perimetry or even fundus perimetry may be the more appropriate term to entitle perimetry with simultaneous visualization of the fundus.
In the wake of expanding surgical options and an increasing number of therapeutic approaches for macular diseases, accurate functional testing of the macular region becomes more and more important. With the convenience of precise documentation of the actual test location on the retina and a real-time compensation for eye movements, fundus perimetry is the only reliable method of visual field testing in patients with instable or eccentric fixation caused by macular pathologies.
Unfortunately, the scanning laser ophthalmoscope (Rodenstock Instruments, Ottobrunn, Germany) is no longer available on the market. Even maintenance and repair of the existing instruments becomes more and more challenging. About 5 years ago, another comparable instrument has been developed: the Micro Perimeter 1 (MP 1), designed and manufactured by Nidek Instruments Inc., Padova, Italy. Just recently, another scanning laser ophthalmoscope has been combined with a microperimetric technique, the spectral OCT/SLO system built by Ophthalmic technologies Inc., Canada. To our knowledge, no publications with this new instruments exist, till now.
The SLO as well as the MP 1, are capable of quantifying macular sensitivity with direct visualization of the fundus under real-time conditions.
This review addresses instruments, perimetric techniques, comparison between different methods and clinical application of fundus perimetry.
Section snippets
Scanning laser ophthalmoscope
The principle of the Scanning laser ophthalmoscope (SLO 101, Rodenstock, Ottobrunn, Germany) has been described in detail earlier (Webb and Hughes, 1981). The SLO 101 projects a Helium–Neon laser beam (632.8 nm) and an infrared diode laser (780 nm) simultaneously onto the fundus through a slightly confocal aperture with a field size of 33 by 21°. The optics consists of a Maxwellian view, which enables high quality retinal images even in eyes with circumscribed opaque optic media. The amount of
Static threshold fundus perimetry
As in conventional perimetry, static fundus perimetry is the major technique used during fundus-controlled function testing. Several groups have developed automated landmark-driven techniques for accurate static threshold perimetry thus improving the original software of the Rodenstock SLO (Rohrschneider et al., 1995d, Sunness et al., 1995c, Toonen et al., 1996).
With the Heidelberg University perimetry software for the SLO, any stimulus can be projected exactly onto a predefined fundus position
SLO perimetry versus MP 1 perimetry
Fundus-controlled perimetry has been established as an important technique to evaluate visual function in macular diseases over the last years. Unfortunately, using the SLO with different set-ups of the instrument itself, and different software options ranging from the original Rodenstock software to advanced solutions like those from Sunness and coworkers or our own software led to results which were not easily comparable between different instruments and investigators (Rohrschneider et al.,
Clinical application
Advances in medical and surgical options of retinal diseases have changed treatment modalities and therefore improved the prognosis of macular diseases. Therefore exact evaluation and documentation of macular function has become more and more essential. Scientific evaluation is based upon two columns: morphologic and functional parameters. Documentation of morphological alteration of the posterior pole has been rapidly changed by new techniques as laser scanning tomography, optical coherence
Future directions
With the implementation of numerous high-definition diagnostic systems into clinical routine an enormous advance has been implemented towards the understanding and treatment of retinal and neuroretinal diseases. Especially optical coherence tomography (OCT) and fluorescein-angiography assisted by laser scanning devices have opened the doors to fundamentally different treatment procedures (Hogg and Chakravarthy, 2006). Recent developments such as the retinal vessel analyzer or a 360° fundus
Acknowledgement
Supported in part by Deutsche Forschungsgemeinschaft DFG Ro 973/11-1 and Ro 973/11-2.
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2021, Progress in Retinal and Eye ResearchCitation Excerpt :Future directions). Whilst FCP (or fundus, fundus-driven perimetry) is commonly referred to as “microperimetry”, it must be noted that the stimulus sizes typically used are identical to stimuli applied in standard automated perimetry (Rohrschneider et al., 2008). Here, we use the term fundus-controlled perimetry (“FCP”) as originally suggested (Kani and Ogita, 1979).