1949

Meyer-Schwickerath’s sunlight photocoagulator

Gerhard Meyer-Schwickerath

Meyer-Schwickerath’s sunlight photocoagulator — the setup On the roof, a heliostat turns sunlight down through an aperture; in the room, a folding mirror, a Galilean telescope, a diaphragm and a shutter deliver the beam into the dilated eye of the patient. HOSPITAL ROOFTOP The mirror follows the sun; the optical axis does not move. clock drive TREATMENT ROOM headrest 1 2 3 4 5 6 7 8 9 THE SETUP, POINT BY POINT 1 Sun — a source of very high radiance, broad spectrum, from the ultraviolet to the infrared. 2 Heliostat — plane mirror on a clock drive: it holds the optical axis fixed despite the earth’s rotation. 3 Roof aperture — the beam travels down into the treatment room. 4 45° folding mirror — it brings the beam into the plane of the patient. 5 Galilean telescope — sets the angular magnification, and so the diameter of the retinal spot. 6 Diaphragm — defines the working beam. 7 Shutter — exposure time is the only dose variable that is really under control. 8 Viewing path — retinal whitening is watched throughout the exposure. 9 Dilated eye — the media transmit 400–900 nm; cornea and lens make the final focus. The whole apparatus does one thing: carry the sun’s image to the retinal plane. SCHEMATIC — NOT TO SCALE
The setup
Meyer-Schwickerath’s sunlight photocoagulator The apparatus on the clinic roof: the heliostat follows the sun, while the optical axis stays fixed.

An accidental retinal burn, seen after an eclipse, gave a thirty-year-old assistant the idea of surgery without an incision. Three years later, a mirror mounted on the roof of a Hamburg clinic was welding a retina with sunlight. The technique has disappeared completely; the indication it created has not moved.

A lesion read backwards

The eclipse of 9 July 1945 was partial over northern Germany. In the weeks that followed, the university clinic at Hamburg-Eppendorf saw patients who had watched it unprotected. Their maculae carried burns that were clean, round and sharply bounded. One such case was drawn in a thesis supervised under Oswald Marchesani, and it was that drawing which, in 1946, caught the attention of Gerhard Meyer-Schwickerath, then an assistant.

The reasoning turns on an inversion. A progressing detachment is known to halt at the edge of a scar; the surgery of the day produced that scar by transscleral diathermy, and therefore from outside the globe. If light could make the same scar through the pupil, the route became direct and the eye stayed closed. The story goes that he wrote the two words Licht and Koagulation on a scrap of paper in the middle of a sleepless night, for fear of losing them by morning.

An apparatus on the roof

The rig was built on the clinic roof. A heliostat — a flat mirror on an equatorial mount, driven by clockwork — compensated for the earth’s rotation and held the reflected beam on a fixed axis. The beam came down through an opening, a folding mirror brought it into the patient’s plane, and an optical bench shaped it. The last lens in the system was the patient’s own eye.

The detail that dates the period best is not an optical one. The surgeon worked to the calls of a colleague stationed on the roof, who shouted down the height of the sun and the movement of the clouds.

What the physics allowed — and forbade

No passive optic makes an image brighter than its source: radiance is conserved, at best. Sunlight photocoagulation therefore works not because the apparatus concentrates, but because the radiance of the sun already exceeds the retinal thermal threshold on its own. And there it stops. The only gain genuinely available is aperture — that is, the pupil: going from 3 to 8 mm multiplies the collecting area sevenfold, which is why maximal mydriasis was not negotiable.

A geometric floor comes with it. The solar disc subtends about 32 minutes of arc; its retinal image measures on the order of 160 µm. It can be enlarged, never reduced. And a telescope that multiplies the diameter by M divides the irradiance by M² — hence exposures of several seconds, immobility and anaesthesia included.

Then there is absorption. Light crosses the clear media and the neurosensory retina almost without loss; the melanin of the pigment epithelium and the choroid absorbs it and turns it into heat. Above roughly 60 °C proteins denature: whitening appears, the only dosimeter available in real time. Necrosis follows, then over one to two weeks a scar that welds retina to choroid. But a broad spectrum is absorbed everywhere, iris and lens included: no tissue selectivity is possible.

The path of the light — why sunlight was enough Conservation of radiance, a retinal image of the solar disc of about 160 micrometres, and heat deposited in the pigment epithelium above 60 degrees. A — CONCENTRATING IS NOT AMPLIFYING Source Optic Image L(image) ≤ L(source) No passive optic makes the image brighter than the source: radiance is conserved, at best. Sunlight photocoagulation works because the sun’s radiance already exceeds the retinal thermal threshold — but it stops right there. The only gain still available is the aperture: the pupil. × 7 in area 3 mm pupil 8 mm dilated pupil B — THE EYE IS THE LAST LENS 32′ ≈ 160 µm A geometric floor The solar disc subtends about 32 minutes of arc: its retinal image is on the order of 160 µm. It can be enlarged, never reduced. The Galilean telescope multiplies that diameter by M and divides irradiance by M²: hence exposures of several seconds, immobility included. Broad-spectrum light heats the iris and the lens too: no tissue selectivity at all. C — WHERE THE HEAT IS DEPOSITED VITREOUS NEUROSENSORY RETINA PIGMENT EPITHELIUM CHOROID SCLERA > 60 °C 1 The transparent media and the neurosensory retina let the light through, absorbing almost none of it. 2 Melanin in the pigment epithelium and the choroid absorbs it and turns it into heat. 3 Above roughly 60 °C the proteins denature: whitening appears, the only dosimeter available. 4 Necrosis follows, then within one to two weeks a pigmented, adherent scar. White light is absorbed everywhere — which is exactly what the laser will fix. SCHEMATIC — NOT TO SCALE
The path of the light

The procedure, 1949

The first coagulation took place on 22 August 1949, on an eye threatened with detachment. The results were reported that same year to the 55th meeting of the German Ophthalmological Society, in Heidelberg, under a title still admired for its plainness: “Coagulation of the retina with sunlight”.

The whole discipline lies in the placement: the burns go as a barrier around the tear, never on it. The hole is not treated, it is encircled — exactly as a laser retinopexy does today. The range of indications widened quickly: early detachments, traumatic macular holes, von Hippel angiomas, Coats disease, choroidal melanomas from 1952, then diabetic retinopathy.

Priority is not beyond dispute: in 2022 Ascaso and Grzybowski argued the case of the Spanish ophthalmologist José Morón, who they hold had run comparable trials in rabbit and human eyes before 1949. The claim has not shifted the consensus, but it deserves citing.

The sunlight photocoagulator in use — a barrier around a retinal tear The beam enters through the dilated pupil and focuses in the periphery; the burns are placed as a barrier around the tear, never on it. Conditions of the procedure. A — AIMING: THE BURNS GO AROUND THE TEAR DILATED PUPIL the only way in FUNDUS VIEW a barrier around the tear — never on it amber: fresh burns · blue: pigmented scars B — WHAT THE PROCEDURE DEMANDED Clear sky, high sun — the session depends on the weather and the hour. Anaesthesia and immobility — exposure is counted in seconds. Whitening — the only dose cue, judged by eye during the exposure. No selectivity — the iris and the lens heat up as well. It is not the light that treats: it is the scar the light leaves. A chorioretinal weld obtained without an incision — that is the whole break of 1949. SCHEMATIC — NOT TO SCALE
In use

Falling out of use

It was the weather that finished the heliostat. A carbon arc succeeded it as early as 1950: independent of the sky, but sooty, with short-lived electrodes, poorly reproducible burns, and a luminance too low for a small pupil. The break came from the collaboration with Hans Littmann at Carl Zeiss in Oberkochen: the xenon arc photocoagulator, presented in 1956-1957, became the first industrial instrument of its kind and spread worldwide. Its burns reached the order of 2 000 µm, and anaesthesia remained the rule.

The ruby laser brought the first coherent source into retinal practice in 1961-1963, and from 1968 the argon laser took hold: monochromatic, selectively absorbed by melanin and haemoglobin, spots of 50 to 500 µm, exposures of a few tens of milliseconds, delivery at the slit lamp under topical anaesthesia. Xenon fell out of use. The DRS and then ETDRS trials, between 1976 and 1985, made photocoagulation a quantified standard in diabetic retinopathy; the 2000s added pattern scanning and subthreshold micropulse; and from 2006 anti-VEGF agents took from it the rank of first-line treatment in macular oedema and exudative AMD.

From sunlight to laser — lineage 1945 to 2006 and a comparison of sources A timeline of retinal photocoagulation, from sunlight to the carbon arc, the xenon arc, the ruby laser and the argon laser, through to anti-VEGF. A table comparing spectrum, burn diameter, exposure and dose variables. 1945 Solar retinopathy seen after the eclipse: the macular burn, sharp and clearly delimited, suggests a therapeutic tool. 1947 First sunlight apparatus, Hamburg. A heliostat is added to keep the sun in the optical axis. 1949 First light coagulation on 22 August, on an eye facing detachment. The results are reported to the 55th meeting of the German Ophthalmological Society, Heidelberg. 1950–1956 Carbon arc: independent of the weather, but sooty, with short-lived electrodes and poorly reproducible burns. 1956 Xenon arc, with Littmann and Zeiss: the first industrial photocoagulator. Burns on the order of 2,000 µm, often under anaesthesia. 1961–1963 Ruby laser: the first coherent source in retinal practice. 1968 Argon laser: monochromatic, absorbed by melanin and haemoglobin, with spots of 50 to 500 µm. 1976–1985 DRS then ETDRS: photocoagulation becomes a quantified standard in diabetic retinopathy. 2000s Pattern scanning and subthreshold micropulse: exposures cut to 10–30 ms, with the dose under control. 2006 → Anti-VEGF: photocoagulation ceases to be first-line for macular oedema and exudative AMD. THREE SOURCES, ONE TISSUE EFFECT PARAMETER SUNLIGHT, 1949 XENON, 1956 LASER, TODAY Spectrum 300 – 2,500 nm broad, filtered 532 – 577 nm Burn diameter ≥ 160 µm, large in practice ≈ 2,000 µm 50 – 500 µm Exposure several seconds 0.2 – 1 s 10 – 200 ms Dose variables duration only duration, intensity power, duration, spot Availability clear sky, high sun permanent permanent The source changed three times. The principle — absorb, heat, scar — never did. Sources: Meyer-Schwickerath, Ber Dtsch Ophthalmol Ges 1949 · Kirchhof et al., Graefes Arch Clin Exp Ophthalmol 2020 · Ascaso & Grzybowski, Acta Ophthalmol 2022. LINEAGE
From sunlight to laser

What remains

The source changed three times in twenty years, the principle not once: absorb, heat, scar. What the laser corrected was not the idea but its coarseness — spectral selectivity, control of the spot, duration, reproducibility of the dose. The 1949 rig left no material descendants. It left something better: the demonstration that the retina could be operated on without opening the eye, and the barrier around a tear, which we still carry out in the same terms.

Techniques pass. The discipline that grounds the indication endures.

References

  1. Meyer-Schwickerath G. Koagulation der Netzhaut mit Sonnenlicht. Bericht über die 55. Zusammenkunft der Deutschen Ophthalmologischen Gesellschaft, Heidelberg 1949;55:256-259.
  2. Meyer-Schwickerath G. Lichtkoagulation. Eine Methode zur Behandlung und Verhütung der Netzhautablösung. Albrecht von Graefes Arch Ophthalmol. 1954;156(1):2-34.
  3. Meyer-Schwickerath G. Lichtkoagulation. Stuttgart: Enke; 1959.
  4. Meyer-Schwickerath G. The history of photocoagulation. Aust N Z J Ophthalmol. 1989;17(4):427-434.
  5. Littmann H. Der Zeiss-Lichtkoagulator nach Meyer-Schwickerath mit Xenonhochdrucklampe. Ber Dtsch Ophthalmol Ges. 1957;61:311-316.
  6. Zaret MM, Breinin GM, Schmidt H, et al. Ocular lesions produced by an optical maser (laser). Science. 1961;134(3489):1525-1526.
  7. Kirchhof B, Joussen A, Bornfeld N, Wessing A. Prof. Dr. med. Dr. h.c. mult. Gerd Meyer-Schwickerath, inventor of light coagulation, on his 100th birthday (July 10, 2020). Graefes Arch Clin Exp Ophthalmol. 2020;258(9):1837-1839.
  8. Ascaso FJ, Grzybowski A. José Morón was the first to introduce the retinal light photocoagulation. Acta Ophthalmol. 2022;100(2):234-236.

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