ABNOM: What It Is and How It Differs from Melasma

What is ABNOM?

Look at both cheekbones in the mirror and you may see small gray-brown to blue-brown spots scattered symmetrically across them — this is usually where ABNOM (acquired bilateral nevus of Ota-like macules) is first noticed. Depending on the clinic or the paper it is also called Hori's nevus, and these are the same condition. Medically it is an acquired bilateral dermal melanocytosis: histologically, irregular spindle-shaped melanocytes are scattered through the papillary to mid dermis, with perivascular clusters and accompanying melanophages.

Onset is usually in adulthood — it appears in the twenties to forties and is more common in women. It is not a condition that fades on its own. Since the first report in 1984 it has been described widely in East Asian populations.

The largest clinical problem is that ABNOM is often mistaken for melasma. The assumption behind that mistake is simple: pigment on both cheekbones in an adult must be melasma. But whether the pigment sits in the epidermis or in the dermis changes both the depth a treatment has to reach and the energy it has to carry. That difference in depth is the axis this page turns on.

How is it treated?

Any approach to ABNOM starts from one requirement: conditions that can reach the dermis.

The low-energy epidermal toning used for melasma works by fragmenting melanosomes in the upper epidermis, and it does not reach the spindle-shaped melanocytes sitting in the dermis. Targeting pigment that deep requires energy capable of arriving at that depth.

But stronger energy raises the risk along with it. In a retrospective study of 110 patients treated with a 1064 nm Q-switched Nd:YAG laser for 2 to 9 sessions at 3 to 6 month intervals, post-inflammatory hyperpigmentation occurred in 11 of the 110 patients (10.0%). Among the 18 patients who also had melasma it occurred in 6 (33.3%), against 5 of the 92 patients without melasma (5.4%). These are small subgroups with few events, so the figures are better read as a direction than carried over as rates — and the direction they point is that when both conditions share one face, conditions meant for dermal pigment should not be applied as they stand.

Post-inflammatory hyperpigmentation (PIH) is the most common risk. The same retrospective analysis reported it as more likely with older age, with darker skin and darker lesion color, and with coexisting melasma. This pigmentation has been reported to fade over roughly 3 to 6 months, but during that interval the area passes through a phase of looking darker than it did before treatment.

Hypopigmentation, scarring, and telangiectasia. These are known risks of the strong lasers used for dermal pigment. The risk changes with higher energy and with the area being treated. A pale, color-stripped mark can remain at the treated site permanently or for a long period.

On a face that also has melasma, conditions meant for dermal pigment are not applied unchanged. When both conditions are present the order and the settings of treatment have to change, and even when they are addressed together the approach is staged. Applying strong conditions first, before the diagnosis is clear, can end up adding post-inflammatory hyperpigmentation and nothing else.

The largest risk is diagnostic error. If ABNOM is taken for melasma and only low-energy toning of the upper epidermis is repeated, time passes while the dermal pigment is never in the target; if strong conditions are used instead without regard to what is there, post-inflammatory hyperpigmentation increases. And if another condition — malignant melanoma among them — is taken for ABNOM and treatment is started first, the diagnosis is delayed. This is why accuracy of diagnosis accounts for so much of how treatment turns out.

Treatment is deferred while photosensitizing medication is being taken or while the skin is tanned. Photosensitizing drugs (doxycycline, isotretinoin and others), recent ultraviolet exposure, and sun tanning shortly before a session raise the risk of post-inflammatory hyperpigmentation, blistering, and delayed recovery. How long to wait varies between sources and devices, so this page does not put a number of months to it.

If there is an active skin infection (impetigo, herpes simplex and the like) or an active lesion of contact dermatitis in the treatment area, that condition is settled first. During pregnancy and breastfeeding, controlled data on the safety of strong lasers aimed at dermal pigment are lacking, and treatment is frequently postponed. Skin that has recently had a strong ablative laser or a dermal procedure is not treated again at the same intensity; before the epidermis and dermis have finished recovering, the risk of pigment change and scarring rises.

The Other Dermatology sets its conditions toward pigment deep in the dermis while aiming to reduce irritation to the epidermis. Even so, conditions that reach the dermis are not a method without effect on the surrounding tissue, so the decision is made alongside the reactions described above. The specific parameters, devices, and combinations depend on the extent of the lesion and the state of the skin, and are settled in an in-person consultation.

Depth treated — The pigment of ABNOM lies in spindle-shaped melanocytes from the papillary to the mid dermis, so laser conditions that reach the dermis are required. No standardized depth in micrometers has been reported in the literature, and the actual depth of delivery is set in consultation after looking at the depth and the location of the lesion.

Aftercare

After a treatment aimed at dermal pigment, avoiding ultraviolet exposure and irritation is the basis of recovery. A small dressing may be advised depending on the area treated, and sun protection during the recovery period is what largely decides both post-inflammatory hyperpigmentation and the result.

The details — when to resume washing, sun protection, and makeup — vary with the treatment conditions and the area, so follow the instructions given at the visit. As a general principle, harsh cleansing, excessive exfoliation, and rubbing are avoided, and a broad-spectrum sunscreen that blocks visible light as well as ultraviolet is advised. Conditions aimed at dermal pigment carry common reactions (redness, swelling, occasional bruising) along with the risk of post-inflammatory hyperpigmentation, and sun protection and gentle cleansing through the recovery period shape the outcome.

Other conditions that look similar

Melasma is what ABNOM is most often confused with. The two are usually written about together, but which one is the subject changes the role of the other: where melasma is the subject, ABNOM is the condition to be ruled out, while here, with ABNOM as the subject, melasma is both the look-alike and the coexisting condition that raises the risk of treatment. Either way the same axis — dermal pigment against epidermal pigment — is what decides the next step, and whichever condition is “the one being misdiagnosed” puts the other in the position of “the one causing the misdiagnosis.”

Other pigmentary conditions of the face — freckles (ephelides) appear from childhood as scattered dots on the cheeks and nose and fluctuate, fading in winter and darkening in summer, while lentigo (solar lentigo) appears from middle age onward as sharply demarcated brown macules on chronically photoaged skin. ABNOM, blue-brown to gray-blue and dotted in its distribution, has a different character from either.

Malignant melanoma and other conditions that darken also occur on the face. A lesion that has been changing recently, one with an irregular border, or one whose color runs in several directions within a single lesion may not be ABNOM, so confirming what it is comes before removing it. Treatment is not started before the diagnosis is settled. Looking at color, distribution, and time of onset together to determine what the lesion is comes first, and where necessary that is settled by examination. If another condition — melasma, flat warts, or malignant melanoma among them — is taken for ABNOM and treatment is begun first, the diagnosis is delayed.

Pigment change after treatment — a temporary darkening after treatment is reported to fade over 3 to 6 months. The area passes through a phase of looking darker than it did before treatment, so the point at which the effect of a session can be judged is explained at the visit.

Frequently asked questions

This page is general information about a treatment and is not a diagnosis or a recommendation. The right choice differs by skin condition, so treatment is decided after an in-person examination. Individual results vary.