Multiple Minute Digitate Hyperkeratoses

Multiple minute digitate hyperkeratosis (MMDH) or spiny hyperkeratosis (SH) is a rare nonfollicular digitate keratosis affecting the trunk and limbs. ^{[1, 2]}  Although some confusion has existed in past terminology, more recent works have emphasized the separation of multiple minute digitate hyperkeratosis (spiny hyperkeratosis) from any association with porokeratosis. Keratosis pilaris ^[3]  also is often confused with multiple minute digitate hyperkeratosis (spiny hyperkeratosis) in clinical diagnosis. ^[4] Note the following important clarification of the most often interchanged terms in the literature:

• Spiny hyperkeratosis - Synonym for multiple minute digitate hyperkeratosis

• Spiny keratoderma - Different process that is included in the palmoplantar keratoderma disease group ^[5]

• Music box spines - Term applied to multiple conditions, including multiple minute digitate hyperkeratosis (spiny hyperkeratosis)

• Follicular spicules - Most often used to describe the myeloma-associated process; also used in keratosis pilaris and multiple minute digitate hyperkeratosis (spiny hyperkeratosis); multiple minute digitate hyperkeratosis (spiny hyperkeratosis) is not commonly follicular, as is described below; follicular spicules should not be confused with trichodysplasia, which can also be seen in myelomas and is described in more detail in Diagnostic Considerations

• Porokeratosis and orthokeratosis - Most classically associated with premalignant lesions and actinic processes; however, have also been used to describe multiple minute digitate hyperkeratosis (spiny hyperkeratosis)

A list of other similar-appearing entities that are easily confused can be found in Diagnostic Considerations.

The exact cause of multiple minute digitate hyperkeratosis (spiny hyperkeratosis) is not known, although cases have been associated with autosomal dominant genetic inheritance, postinflammatory changes, and sporadic occurrence.

As more reports of multiple minute digitate hyperkeratosis (spiny hyperkeratosis) are published, it will be confirmed whether the reported associations with systemic disease are real or whether they represent chance occurrences. It is also important to note that many of the above divisions may be associated with similar presenting clinical conditions and larger studies are required to validate such classifications before broad adoption of these concepts occurs. ^[6] So many of these proposed divisions/classifications are based on case reports, and the validity of use is still hard to determine.

No one cause of minute digitate hyperkeratosis (spiny hyperkeratosis) is known. The exact cause of multiple minute digitate hyperkeratosis (spiny hyperkeratosis) is not known, though cases have been associated with autosomal dominant genetic inheritance, postinflammatory changes, and sporadic occurrence.

Frequency

Multiple minute digitate hyperkeratosis (spiny hyperkeratosis) is a rare chronic disorder of keratinization. The incidence and prevalence have not yet been determined.

Race

No race predilection has been reported for minute digitate hyperkeratosis (spiny hyperkeratosis) in the evidence-based literature.

Sex

A slight male prevalence has been noted for minute digitate hyperkeratosis (spiny hyperkeratosis). ^[7]

Age

No age predilection has been reported for minute digitate hyperkeratosis (spiny hyperkeratosis) in the evidence-based literature. However, multiple minute digitate hyperkeratosis appears to be most reported in the second and third decades of life and in the fifth and sixth decades of life.

Classification

One group has proposed a classification based on certain characteristics of the multiple minute digitate hyperkeratosis presentation. ^{[8, 9]}

Familial form of multiple minute digitate hyperkeratosis

These are considered autosomal dominant in the majority of cases. The most common age at presentation is the second and third decade.

Postinflammatory/insult form of multiple minute digitate hyperkeratosis

This type is reported after various skin insults, including radiotherapy, sun damage, and use of systemic or topical drugs (eg, cyclosporin, simvastatin, etretinate). This category brings to light the question of trauma as a trigger or facilitator of multiple minute digitate hyperkeratosis, as is noted in other diseases of keratinization.

Sporadic multiple minute digitate hyperkeratosis

Patients with this form tend to present in the fifth and sixth decade. Sporadic multiple minute digitate hyperkeratosis may be associated with internal malignancy or other systemic disease. In particular, it is possible that the myeloma-related spicules are a variation of multiple minute digitate hyperkeratosis (spiny hyperkeratosis), although this differentiation is elaborated on in the sections below.

No reports in the evidence-based literature have examined the prognosis of minute digitate hyperkeratosis (spiny hyperkeratosis). However, the lesions themselves are believed to be benign in nature.

The larger unanswered question is the role of associated findings that may occur with minute digitate hyperkeratosis (spiny hyperkeratosis). Currently, multiple minute digitate hyperkeratosis appears to be found inconsistently with other disease processes. Any further associations would require a larger series of patients or pathogenic, molecular, or genetic studies to correlate the somewhat random findings noted to date. Anecdotal and clinical observations have suggested early-onset/congenital multiple minute digitate hyperkeratosis (spiny hyperkeratosis) differs from late-onset/acquired forms. Although not backed by a large body of evidence, the late-onset multiple minute digitate hyperkeratosis (spiny hyperkeratosis) is presumed to have an increased association with inflammatory or malignant disease in some cases. However, the evidence for these associations is anecdotal. ^{[7, 10]}