Transscleral LED illumination pen

Abstract

Existing light sources for intraocular illumination are often bulky and expensive and pose a risk for the patient, because light guides are inserted in the eye through incisions and if the tip of these light guides get too close to the retina, the retina can be damaged photochemically within minutes or even seconds. Therefore a new, safe and simple device for intraocular illumination is developed and evaluated for its thermal and photochemical risks to the patient. It consists of a white LED which is integrated into a pen like holder. This device is pressed against the sclera by the physician who seeks for illumination during surgery or for diagnostic purposes. The LED light is transmitted through the sclera without the need for an incision. Considering the relevant standards, the device poses no harm to the patient, and in tests with the authors’ own eyes a sufficient intraocular illumination is reached. The proposed device is quite simple but easy to handle and very gentle for the patient.

Introduction

For ophthalmologic fundus examination the physician requires adequate illumination. This could be achieved by different techniques. One of them is the application of an ophthalmoscope—invented by Helmholtz in 1851 [1]—shining light through the pupil and the transparent media of the eye. However there are situations, e.g. the investigation of patients with cataract, in which these kind of illumination causes distracting reflections or scattering. In vitreoretinal surgery, an ophthalmic field that includes the recognition and treatment of retina detachments, removal of the vitreous and epiretinal membrane peeling, this challenge is usually solved by light guides that are inserted in the vitreous cavity through small incisions while the patient is under anesthesia [2].

This illumination by optical fibers is linked with some risks besides infection like photochemical and thermal hazards [3, 4] and therefore a more gentle illumination technique was suggested that is based on the idea of transscleral illumination already mentioned by von Graefe in 1868 [5]. In its modern version a white LED is attached to an eye speculum and pressed against the patient´s eye at the pars plana [6]. The fundus illumination should be achieved transsclerally, which means by light passing through the sclera into the intraocular space. This was successfully tested on porcine cadaver eyes [6] but tests on human eyes haven´t been published so far.

This approach is feasible, because the sclera isn´t as opaque as it seems at first glance, however it exhibits a small but significant wavelength and pressure dependent transmission. This was first reported for a discrete number of wavelengths [7]. The values depicted in Fig. 1 are recalculated values of previous own measurements [6] with an increased actual accuracy and therefore an increased spectral range due to an improved spectrometer background correction. The pressures have not been determined exactly but the higher pressure was about 15 kPa or 110 mmHg and the lower pressure was far below this 15 kPa.

Fig. 1.

Human sclera transmission spectrum. Recalculated values of own earlier measurements [6]

In this paper a variation of this approach—a white LED integrated in a pen like holder (see Fig. 2)—is investigated. In comparison to a white LED integrated in an eye speculum [6] it would offer more flexibility for the physician and the chance to use it for diagnostic purposes on patients awake.

Fig. 2.

Scheme of transscleral illumination by LED Pen (modified according to [8])

Materials and methods

Prototype setup

For the first tests a white commercial 5 mm dome LED type NSDW510-GS-K1 of Nichia (Tokyo, Japan) was integrated in a prototype pen like holder of beige polylactid acid, produced by a 3D printer type Poetry 2 of IRAD3D (Borgomanero, Italy). A CAD drawing also showing an additional switch can be found in Fig. 3.

Fig. 3.

Transscleral illumination LED pen [9]. a Electric circuit, b CAD drawing with white LED on the left and on–off switch on the right hand side. The battery and the electric circuit are inside of the polylactid acid housing c Photograph of the LED illumination pen

The LED is powered by a 6 V button cell. An internal resistor limits the current to approximately 40 mA. The LED emission spectrum was measured by a calibrated combination of an Ulbricht sphere and a spectrometer. The customized Ulbricht sphere was delivered by Mountain Photonics (Landsberg am Lech, Germany) with an inner diameter of 100 mm and a Teflon coating. The employed UV–VIS spectrometer SensLine AvaSpec 2048 XL was produced by Avantes (Apeldoorn, Netherlands).

Hazard evaluation

Light poses a danger to the eye—especially the retina. The international standard DIN EN ISO 15007-2: 2014 “Ophthalmic instruments—Fundamental requirements and test methods—Part 2: Light hazard protection” gives guidance for judging the photochemical and thermal risks and establishes exposure limits to guarantee the patient´s safety [10].

This standard defines the “weighted retinal visible and infrared radiation thermal irradiance”E_VIR-R for judging the thermal burden of the retina as presented in Eq. (1) [10]:

$$E_{VIR-R}=\sum _{380}^{1400}{E}_{\mathit{\lambda }}·R(\mathit{\lambda })·\mathrm{\Delta }\mathit{\lambda }$$ 1

with the thermal hazard weighting function R(λ) shown in Fig. 5. E_VIR-R has to be smaller than 0.7 W/cm² [10].

Fig. 5.

Black line: calculated maximum intensity behind sclera; blue line: photochemical hazard weighting function; red line: thermal hazard weighting function [10]

The “weighted retinal irradiance” E_A-R is defined by Eq. (2):

$$E_{A-R}=\sum _{305}^{700}{E}_{\mathit{\lambda }}·A(\mathit{\lambda })·\mathrm{\Delta }\mathit{\lambda }$$ 2

with the photochemical hazard weighting function A(λ) presented in Fig. 5 leads to the “maximum exposure time” t_max given in Eq. (3) [10]:

$$t_{max}(E_{A-R})=\frac{10\frac{J}{{\text{cm}}^2}}{{\sum }_{305}^{700}{E}_{\mathit{\lambda }}·A(\mathit{\lambda })·\mathrm{\Delta }\mathit{\lambda }}$$ 3

Result and discussion

Thermal and photochemical hazard

With the Ulbricht sphere and the spectrometer the total LED emission was measured spectrally resolved. The intensity distribution can be found in Fig. 4. The calculated total optical output power was about 41 mW and the luminous flux 13.8 lm.

Fig. 4.

Measured total emission spectrum of the employed LED

With this data the weighted thermal and photochemical retinal irradiances E_VIR-R and E_A-R can be calculated with the assumption of a scleral transmission as presented in Fig. 1, which would lead to a reduced spectral intensity distribution as illustrated as black line in Fig. 5. Fortunately the photochemically dangerous violet and blue parts of the spectrum are attenuated the most.

For simplification the LED contact area is supposed to be equal to the LED housing cross section of about 0.2 cm² and all emitted light is assumed to be distributed homogenously over this area. This would lead to a weighted thermal retinal irradiance E_VIR-R of about 0.05 W/cm² which is more than one order of magnitude below the limit of 0.70 W/cm² given by the international standard [10].

For the weighted photochemical retinal irradiance E_A-R the result is 1 mW/cm². According to Eq. (3) this limits the maximum exposure time t_max to almost 3 h, which should be more than sufficient for almost all surgeries and diagnostic procedures. Otherwise the LED pen could be moved to another spot on the wall of the eye.

It should be mentioned that these values were calculated for the increased sclera transmission for high pressure in Fig. 1 which also leads to an increased hazard. In self-experiments with the LED illumination pen an intraocular pressure of 60 mmHg or 8 kPa was measured with a tonometer, when the LED pen was firmly pressed against the eyes of the authors. This value is below the assumed pressure in Fig. 1 and the same will be true for the sclera transmission resulting in an even lower photochemical and thermal risk.

Transscleral illumination

The application of the LED pen is demonstrated in Fig. 6 with a human eye of one of the authors. The LED is pressed against the eye on the author´s left hand side. The whole eye, or at least the parts that are not covered by skin or bone, seem to be glowing dark red. That is caused by LED light that has passed the sclera twice. The light coming out of the pupil is orange and much brighter.

Fig. 6.

Photograph of the eye of one of the authors illuminated transscleral by the LED pen pressed against the wall of the eye

The reason for the different colours of the original LED emission, light emitted through the pupil and light coming elsewhere out of the eye is partially influenced by the choroid´s transmission and reflection, but probably mainly determined by the sclera transmission properties presented in Fig. 1, because the sclera shows increased transmission for longer wavelengths and therefore favours the transmission of red light. In terms of colour temperatures the measured emission spectrum of the white LED (Fig. 4) leads a colour temperature of 4840 K that is red shifted to 3030 K behind the sclera (black line in Fig. 4). This red shift could be compensated by applying a colder white LED with increased blue emission as it is discussed in [11] but this results in an increased photochemical hazard and should therefore be avoided here.

Conclusion

The presented simple LED based device is able to illuminate the intraocular space homogenously and with sufficient brightness. The calculated intraocular luminous flux is about 3.5 Lumen which is below the values of a commercial Xenon system of 4 to 12 lm [12], but it is already in the same order of magnitude and the development has just started. The thermal hazard for the patient is almost negligible and even the photochemical hazard poses no large thread in usual surgeries or diagnostic procedures. This is a very significant improvement for the patient compared to other illumination techniques that often demand an incision and incorporate the risk of photochemical damage like the widespread rigid light guides [4].

Devices like this could be provide as sterile single use products for physicians in hospitals or doctor´s practices or for applications in the filed outside of medical facilities.

Conflict of interest

Frank Koch and Pia Klante declare to have no conflict of interests. Christian Lingenfelder, Philipp Koelbl and Martin Hessling have filed a German patent application in November 2014.

Informed consent

Informed consent has been obtained from all individuals included in this study.