Use of fundus perimetry (microperimetry) to quantify macular sensitivity

Abstract

The advances in retinal imaging technologies have led to enormous innovation towards diagnostic in current ophthalmology, enabling the practitioner to detect early retinal changes and to document treatment effects. While, in the past, retinoscopy, visual acuity testing and perimetry played the major role in functional diagnostics, today, laser-based systems like laser scanning imaging systems especially for fluorescein-angiography, optical coherence tomography, electrodiagnostic systems and the analysis of retinal vessels may be used as well. However, the challenge to correlate subjective alterations or clinical changes with visual function, still remains. Micro- or fundus perimetry offers the option to test retinal sensitivity while directly observing the fundus.

In this paper, we review the literature encompassing the results of more than 25 years of fundus perimetry, i.e. perimetry under simultaneous visualization of the fundus. During this time, results on known diseases and reproducibility of the technique were published, but a lot of work was also performed on the combination of different examination methods, allowing a synopsis of long-term results and new approaches by combining different methods and improving each of them.

The first part of this review attends to improvements of the method. The second part addresses the clinical and diagnostic values. The final part is dedicated to diagnostic and long-term observation of fundus perimetric results beginning with common and rare diseases like age-related macular degeneration, macular holes and diabetic retinopathy, various types of macular dystrophies ending with challenges in conventional perimetry like glaucoma and malingering.

Due to the experience and progress in the field of fundus perimetry and retinal imaging, the method has long passed its role of observing and has all the potential for prediction, early detection and treatment-monitoring of macular diseases.

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.