The Nidek MP1 -

Occasionally a new instrument appears, usually designed and built overseas, which seems to await some niche market yet to be identified. There has been a stream of research papers relying on this instrument, however, and its potential in optometric practice is becoming clearer.

The MP1
The Nidek MP1 microperimeter is an innovative development in visual field measurement. The instrument is a hybrid between a fundus camera and a perimeter, capable of imaging the fundus while simultaneously measuring the visual field using either static or kinetic stimulus presentation. Stimuli are generated by means of an LCD display which is incorporated into the imaging system of the fundus camera. Landmark features on the fundus such as blood vessels are used to accurately track eye movements and precisely position light stimuli. The visual field results are superimposed over the fundus image, thus allowing for accurate mapping and monitoring of eye diseases ranging from optic nerve head disorders to maculopathy and more general diseases of the retina. The spatial resolution of visual field stimuli is much higher than can be obtained using conventional perimeters, making the instrument particularly useful for the investigation of maculopathy. The accurate and constant mapping of fixation enables the instrument to be used to train fixation for eccentric viewing in maculopathies which have damaged the foveal region.

Improvements
After the original launch, several improvements and configurations have been developed. The basic unit incorporates the following main features:

0.42 mega-pixel digital camera unit
Imaging and patient software
Microperimetry facility
Follow-up and low vision feedback training.

More advanced modifications included the facility for kinetic perimetry, peripapillary strategies and a differential grey-scale map. Kinetic perimetry allows a scotoma to be accurately related to an area of damage seen on the image, for example. Peri-papillary strategies allow the area immediately around the disc to be plotted such that individual sensitivities in this area may be measured. Any changes in the ability to detect stimuli repeatedly would be useful indications of glaucomatous progression. The grey-scale plot allows an easily interpreted graphic representation of the field that may be viewed in the same screen as a photographic image of the retina.
A later development still, the MP1 Pro, now allows the image to be displayed on a 14-inch screen and incorporates a higher resolution (1.4 mega pixel) camera.

Research Round-Up
One of the most useful applications of the MP1 so far identified has been its ability to monitor progress of patients subsequent to retinal surgery. Several such studies were reported on at the 2004 ARVO meeting. F Gora et al looked at 19 patients aged between 21 and 81 years of age who had had macular hole surgery. All had been assessed preoperatively for best corrected visual acuity, optical coherence tomography examination and MP1 microperimetry of the central 12 degree field. All patients then had a pars plana vitrectomy with inner limiting membrane peeling, injection of gas and a blood drop on the area of the macular hole (the concentration of thrombocytes thought to have therapeutic benefit). The same assessments were then carried out postoperatively at three months. The microperimetry assessment found a central scotoma reduction of 20 to 30 per cent among the subjects and the authors felt the MP1 an appropriate tool to use in post-macular surgery monitoring, 'especially due to the online eye tracking the exact scotoma size and the stability of the fixation can be determined precisely'. This study is an ongoing one.
Another ARVO-reported study looked at the MP1 in comparison with the scanning laser ophthalmoscope (SLO). The SLO had been developed years ago by Rodenstock and has proved a very effective instrument whenever there has been a need for accurate tracking and fixation.
In this study by Rohrschneider et al, 57 eyes were assessed with fundus controlled static perimetry using both the MP1 and the SLO. Comparable stimuli, a 4-2-1 strategy with Goldmann III stimuli and a red background illumination were used. The depth and size of the detected scotomas as well as the stability of the fixation were compared. They noted that the MP1 software made the test time longer for this instrument. There was a good concordance of the perimetric results, while the MP1 showed a larger field defect in 69 per cent of the cases, smaller in 10 per cent, and equal size in the remaining 21 per cent of eyes. They concluded that the MP1 is suitable for assessing retinal function along with the SLO and that the automated real-time alignment of the MP1 had advantages over the SLO where manual correction of eye movements was necessary. The MP1 also allowed a larger area of assessment.
A different use of the MP1 was reported in a study by Vingolo et al who used microperimetry in the assessment of postoperative refractive error in patients who had had foldable intraocular lenses inserted. A central 10-2 threshold strategy was used to evaluate single-point retinal sensitivity and a deviation of 2dB was taken as statistically significant.
They concluded that microperimetric data are more reliable than psychophysical data when assessing retinal function at a precise location. A further study of eyes with a variety of pathologies (such as diabetic retinopathy, macular degeneration, epiretinal membranes, macular holes and hypertensive retinopathy) by Lin et al found that microperimetry is a useful addition to visual acuity in monitoring visual impact and progression of disease.
In a report published in the middle of last year (Ophthalmic Surgery, Lasers and Imaging, (35) No 4, 2004), Shah and Chalam described the use of microperimetry in monitoring macular function in a case of idiopathic subretinal fibrosis. The condition is implicated in subsequent retinal detachment, macular degeneration, large naevi and melanoma. In this study, the condition was a rare one where acuity was barely affected. The microperimetric assessment was therefore useful in identifying, measuring and monitoring the extent and progress of the condition.
In another case study (Eye, April 23, 2004), the same authors looked at microperimetric monitoring of a case of sickle cell retinopathy where a vascular occlusion had resulted in severe vision loss. The tracking capability of the MP1 allowed accurate monitoring and measurement of the scotoma where other assessments would have been unreliable due to problems of fixation.

Optometric Applications beyond the Research Lab
The ability to use an accurate tracking system allows a stimulus threshold value to be accurately superimposed over a high-resolution image of the fundus. This would appear to be a useful facility for a number of areas of optometry:

Monitoring of a central scotoma
This could be of use in any vision therapy or rehabilitation programme where the position and nature of a central scotoma needs to be established in order to introduce a useful training model to encourage people to use eccentric viewing techniques and establish an preferred retinal location. Macular disease is significant and PCTs are moving towards an integrated approach in detecting and managing it. Anyone involved in a local scheme for AMD monitoring would find this instrument of use.

Monitoring maculopathy
Any inherited or drug/toxic induced maculopathy could be accurately monitored such that treatment might be accurately adjusted. This would be a useful facility for any optometrist allied to a general medical practice.

Glaucoma monitoring
Though relative scotoma depth assessment is limited, the ability to map scotomas and to look for sensitivity changes in the glaucoma-susceptible nerve fibre layers, which may then be related to the disc appearance, is useful and may have a role in glaucoma monitoring in future.

Monitoring field loss
The machine would prove effective in monitoring field loss and extent with progressive retinal diseases and detachment.

Accurate mapping
The instrument offers the ability to combine a static and kinetic plot, allowing for accurate mapping of, for example, the blind spot where an extension, perhaps in a swollen-looking disc, would be relevant.

AMD management
All clinics using the latest AMD treatments, such as PDT or retinal translocation, may find the MP1 excellent in monitoring vision change with the procedure.

High resolution camera
The MP1 offers a high resolution camera, so would allow for all diabetic and other lesion monitoring as well as its microperimetry use. It might be proposed as a '2 for 1' instrument in practices where a specialisation is not yet on the cards.

The author would like to thank Dr Robert Cubbidge for his help with this article