To date, there is not a clear understanding of the pathogenesis of MA. MA is associated with, and probably caused by, friction and scratching with fingernails or implements such as towels or brushes. The amyloid deposits, which are keratinocyte derived, are restricted to the upper dermis in MA.
Lesions of MA are hyperpigmented, and this is the chief characteristic of the condition. Most often MA involves the upper back and arms. Sometimes there is a visible rippled pattern to the skin, similar to the "sea floor-like" rippling of lichen amyloidosis (LA), although it is generally far more subtle in appearance and may be absent in up to 50% of cases. Therefore, to a large extent, the distinction between MA and LA is simply a matter of the amount, appearance, and location of the deposition. In fact, up to 25% of patients with primary cutaneous amyloidosis have "biphasic amyloidosis," which has been described as coexisting lesions of MA and LA in the same individual.
MA may be asymptomatic, or the lesions may be pruritic. Asian patients are predisposed to MA, and the condition usually begins in adulthood. Like LA, MA may be associated with multiple endocrine neoplasia type 2A (MEN 2A).
E85.9 – Amyloidosis, unspecified
111032003 – Macular cutaneous amyloidosis
- Notalgia paresthetica (NP) – This condition is identified by a persistent itching sensation of the upper back, usually over one or both scapulae or in the interscapular area. Most cases of NP, even in entirely normal individuals, are thought to have a neurogenic origin. There are usually no primary skin lesions, but changes secondary to persistent rubbing and scratching may lead to hyperpigmentation, excoriation, lichenification, and MA. NP scratching leads to cutaneous deposition of keratin-derived amyloid in some chronic cases. Itch caused by NP may be the underlying trigger for the rubbing and scratching associated with MA.
- Post-inflammatory hyperpigmentation – Any preceding inflammatory condition may lead to pigment dropout from the dermoepidermal junction and hyperpigmented skin, but postinflammatory hyperpigmentation lacks amyloid accumulation in the papillary dermis.
- Atrophic lichen planus – In this condition, there is atrophy with skin wrinkling and also an interface process with pigmentary incontinence, but frank amyloid is not deposited in the papillary dermis.
- Drug-induced pigmentation – This is generally more extensive and symmetric than MA and is not associated with amyloid deposition.
- Erythema dyschromicum perstans – This is an interface process without amyloid deposition that is most common in persons of Latino ancestry; some believe it is simply a subtle or burned-out variant of lichen planus in Latino patients.
- Phototoxic contact dermatitis – This condition, caused by the interaction of phototoxic materials in plants, fruits, and flowers, assumes bizarre and sharply circumscribed patterns on the skin that suggest an exogenous source.
- Tinea versicolor – In darker skin types, tinea versicolor may produce hypopigmented but also hyperpigmented skin, but it does not result in the deposition of any amyloid, and the yeast and short hyphae of tinea versicolor are easily identified in a potassium hydroxide (KOH) preparation from the skin.