While the cause of MA is incompletely understood, it is associated with, and probably caused by, friction and scratching with fingernails or implements such as towels or brushes.
Clinically, MA manifests as hyperpigmentation. The most common locations are the upper back and extensor upper extremities. A rippled pattern is sometimes seen. MA and lichen amyloidosis (LA) are believed to exist on a spectrum and are mainly differentiated by the nature of the primary lesion (macules and patches in MA and thin plaques in LA) and histopathologic findings. Some patients also display features of both MA and LA, which is termed "biphasic amyloidosis."
MA is generally a pruritic condition, but it may sometimes be asymptomatic. MA has been described in the setting of connective tissue diseases, primary biliary cholangitis, and multiple endocrine neoplasia type 2a.
E85.9 – Amyloidosis, unspecified
111032003 – Macular cutaneous amyloidosis
Differential Diagnosis & Pitfalls
- 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 otherwise healthy 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.
- Postinflammatory 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 (LP) – 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 people of Latino ancestry; some believe it is simply a subtle or burned-out variant of LP 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 phototypes, 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.