Melasma Pathophysiology

Melasma is best understood not as "extra pigment" but as a chronic, relapsing photo-hormonal disorder in which structurally normal-numbered but hyperfunctional melanocytes overproduce melanin against a multifactorial background of hormonal sensitivity, ultraviolet and visible-light exposure, a damaged dermo-epidermal junction, and an active vascular component. Each of these drivers is independently capable of reactivating the condition, which is precisely why melasma is so stubborn: clear the visible pigment and the underlying machinery is still there, waiting for a trigger. For the Fitzpatrick IV–VI patient who makes up the bulk of the Indian melasma caseload, the same melanocyte lability that drives the disease also makes aggressive treatment dangerous — a tension that defines the whole module.

Hyperfunctional melanocytes, not more of them

The central lesion in melasma is melanocyte hyperactivity. Melanocyte numbers are not dramatically increased; instead the existing melanocytes are larger, more dendritic, and upregulated — transferring more melanosomes to more keratinocytes. Melanin synthesis turns on tyrosinase, the rate-limiting, copper-dependent enzyme of melanogenesis, so the entire pathway is biased toward overproduction. This is why tyrosinase inhibition (kojic acid, arbutin, azelaic acid and similar agents) is mechanistically central to every melasma protocol: you are dialling down a hyperactive enzyme, not killing cells. It also explains the relapse pattern — the melanocytes are not destroyed by treatment, only quietened, and they resume overproduction once the brake (treatment or photoprotection) is released.

Where the pigment sits: epidermal, dermal, mixed

Melasma is traditionally classified by the depth of pigment, because depth predicts how treatable a lesion is:

The clinical consequence is honest expectation-setting. Epidermal pigment is what responds well to tyrosinase inhibitors, gentle resurfacing and turnover acceleration. Dermal pigment — melanin that has dropped below the basement membrane and been engulfed by dermal macrophages — is far slower to clear and sets a hard ceiling on results. In practice, most Fitzpatrick IV–VI melasma is mixed, so the realistic target is meaningful lightening and stabilisation, never guaranteed clearance.

The basement membrane: why pigment keeps falling into the dermis

A defining and under-appreciated feature of melasma is disruption of the dermo-epidermal junction (basement membrane). Chronic UV exposure and the melasma process itself degrade this barrier, creating gaps through which melanin and even whole melanocytes can drop into the dermis ("pigment incontinence"). This is a one-way problem: once pigment is in the dermis it is no longer accessible to epidermally-acting agents, and the leaky junction means even successfully treated epidermal pigment is prone to re-seeding the dermis under continued insult. The damaged basement membrane is a major reason melasma behaves as a chronic relapsing disease rather than a one-and-done pigment problem, and it is the strongest mechanistic argument against aggressive medium-depth peeling in these patients — injury that further inflames the junction can deepen, not lighten, the pigment.

The vascular component

Melasma is not purely melanocytic. A substantial proportion of lesions show an increased number and dilation of dermal blood vessels with raised vascular endothelial growth factor (VEGF), and melanocytes carry VEGF receptors — so the vasculature and the pigment are biologically coupled. Clinically this presents as melasma with an underlying erythematous or telangiectatic tinge, and it partly explains why some cases are refractory to pigment-only therapy and why heat and inflammation (including over-aggressive procedures) worsen them. Recognising a vascular-predominant case changes the plan: it argues for anti-inflammatory, anti-angiogenic adjuncts (azelaic acid, tranexamic acid, niacinamide) and against thermal or inflammatory procedures.

The reactivating triggers: UV, visible light, hormones, heat

The drivers above are the standing machinery; the triggers are what switch it on:

• Ultraviolet light directly stimulates melanogenesis and degrades the basement membrane — the single most important modifiable driver.
• Visible light (especially high-energy blue light) independently induces pigmentation in Fitzpatrick IV–VI skin, which is why UV-only sunscreens under-protect melasma patients and tinted (iron-oxide) photoprotection matters.
• Hormonal stimulation — pregnancy, combined oral contraceptives, hormonal therapy — upregulates melanogenesis, which is why melasma classically appears or worsens in these states.
• Heat and inflammation — including ill-advised aggressive procedures — independently flare melasma via the vascular and inflammatory pathways.

Because these triggers persist for life, melasma is managed, not cured. Every effective protocol therefore pairs pigment-directed treatment with relentless trigger control, the subject of the maintenance lesson.

Key takeaway

Melasma is chronic because it is multifactorial: hyperfunctional (not just more numerous) melanocytes overproduce melanin, that pigment distributes across epidermal and dermal compartments, a damaged basement membrane lets it fall into the slow-clearing dermis, a vascular component co-drives many cases, and UV, visible light, hormones and heat keep reactivating the whole system. No single agent addresses all of this — which is exactly why melasma demands multi-pathway protocols and lifelong trigger control rather than a one-time treatment.