Lasers are powerful machines emitting monochromatic, monophasic, collimated light of high intensity. They are used both for diagnostic as well as therapeutic purposes. Recent studies have shown good effects of the pulsed dye laser in nail psoriasis. Onychomycoses are now treated with Nd:YAG lasers. The carbon dioxide laser is used to vapourise the lateral matrix horn in ingrown nails and can be successfully used to treat a variety of nail tumours.
Laser is an acronym for Light Amplification by Stimulated Emission of Radiation. The laser is a high-tech marvel based on Einstein’s theory of light having a certain mass, i.e. being photons. What makes it unique compared to other light sources is that its emitted light has three properties: it is monochromatic, monophasic, and collimated. Each of these characteristics potentiates the energy. Monochromatic light has only one colour, monophasic light oscillates in the same phase and amplitude, and collimated light runs absolutely parallel. This means that each single light wave’s amplitude is added to that of the other, making even relatively weak light extremely powerful.
There are many medical lasers spanning wavelengths from 10 200 nm in the infrared to 308 nm in the ultraviolet B range. Each laser wavelength is targeted to a specific absorption spectrum of a pigment or tissue component, which determines its specific indication. However, despite the same wavelength, there may be tremendous differences in indication that depend on whether the laser is continuous or pulsed, how the pulse is delivered, and the fluence. Furthermore, cooling devices and spot size have an important influence on non-specific heat damage and depth of penetration. The latter is also modified by reflection from skin surfaces, absorption by non-target pigments, scattering by the structures the light has to pass to reach the target, and the target’s size, which determines the thermal relaxation time. The most common targets for laser treatments are haemoglobin, melanin, water, and some other pigments that specifically absorb the laser light energy and transform it into heat. Non-specific laser effects are the result of perilesional damage. All these facts allow the laser to be used both for diagnostic purposes as well as for treatment. This principal is called selective photothermolysis 1.
Lasers in the diagnosis of nail conditions
Confocal reflectance laser scanning microscopy is a tool to non-invasively detect small objects in the skin and nail that not only absorb, but also reflect laser light. The laser is focused to a specific depth in the tissue and the reflected light is bidimensionally analysed, giving a picture of the absorbing structures in a certain plane of the tissue. Three-dimensional reconstruction of stacks of such examination planes permits even more information. This allows melanin, melanocytes, or fungi 2 to be analysed. In the proximal nail fold, it can be used as an ultrasensitive alternative to capillary microscopy 3. In research, it was used to accurately measure the thickness of the nail and to determine the integrity of single onychocytes 4. The major disadvantage of this technique is the low depth of penetration that ends at approximately 250 µm. In addition, it is an expensive tool, the images are difficult to interpret, and there is little experience with nails.
Laser Doppler perfusion imaging permits the user to exactly measure capillary blood flow, which is useful for the measurement of cold influence in scleroderma and vibration disease 5,6. Comparison of laser Doppler flowmetry with capillary microscopy demonstrates that the flow velocity is inversely related to the degree of capillary alterations 7.
Laser speckle imaging is capable of measuring tissue perfusion at high temporal and spatial resolution. It can be used to accurately quantitate microvascular reactivity following ischaemic and hyperthermic challenges 8.
Matrix-assisted laser desorption/ionisation time-of-flight mass spectrometry is a mass spectrometric peptide analysis technique. For the diagnosis of onychomycosis, it was characterised as a ‘precise, robust and fast tool in diagnostic investigation of nail disorders, which is superior to common standard methods’ 9. It even allowed dermatophyte species to be identified 10.
Laser-induced breakdown spectroscopy with discriminant function analysis (DFA) allowed users to investigate normal nails 11 and discriminate samples between the nails of healthy male subjects and opium addicts with the help of spectral line intensities of elements, including iron, carbon, titan, magnesium, silicium, aluminium, calcium, hydrogen, potassium, oxygen, and sodium12,13.
Laser nail ablation uses high-energy lasers to ablate portions of the nail plate surface, the fumes of which are analysed and allow multiple elements in the nail to be determined. This is achieved with the use of double-focusing sector field inductively coupled plasma mass spectrometry and studying intra-individual long- and short-term chronological variations, as well as depth distribution patterns14.