Epidermal Differentiation Disorders (EDDs)
The new gene-based naming system for ichthyosis and related inherited skin conditions — what it means, how it works, and what changes for you.
What Are Epidermal Differentiation Disorders?
A simple way to understand EDDs: Think of your skin as a protective shield. In people with an epidermal differentiation disorder, the skin cells don't form or work the way they should — making the barrier weaker and leading to problems like scaling, thickening, redness, or blistering. EDD is the umbrella term that covers all of these conditions, grouped by which gene or biological pathway is affected.
"Epidermal Differentiation Disorders" (EDD) is the new umbrella classification covering ichthyoses, palmoplantar keratodermas (PPK), and related inherited skin conditions. It was formally published in 2025 in the British Journal of Dermatology by an international panel of experts and patient representatives.
This is a shift in how these conditions are categorised — moving away from what the skin looks like towards which gene or biological pathway is affected. Both the old and new names remain valid and both will appear in medical records during this transition period.
Why Change the Names?
Previous names were often based on appearance and led to some terms that are now considered offensive or inaccurate. For example, "ichthyosis" likens the skin to fish scales, "hystrix" compares it to a porcupine, and "harlequin" refers to a historic entertainer's outfit. Names based on appearance can also be scientifically inaccurate — limiting research and targeted treatment development.
Old approach: Appearance-based names
New approach: Gene-based names
How the New Naming System Works
Every new name has exactly two parts:
For example: FLG-nEDD (formerly Ichthyosis Vulgaris) · TGM1-nEDD (formerly Lamellar Ichthyosis) · SPINK5-sEDD (formerly Netherton Syndrome)
The Three EDD Subgroups
Non-syndromic EDD
Old name: Ichthyoses
Conditions that mainly affect the skin. The largest group — includes most ichthyosis types.
Examples: FLG-nEDD, TGM1-nEDD, KRT1-nEDD, ABCA12-nEDD
Palmoplantar EDD
Old name: Palmoplantar Keratodermas (PPK)
Conditions mainly affecting skin on the palms and soles.
Examples: FLG-pEDD, KRT1-pEDD, SLURP1-pEDD, KRT16-pEDD
Syndromic EDD
Old name: Syndromic Ichthyoses
Conditions involving the skin but also affecting other organs or systems.
Examples: SPINK5-sEDD, STS-sEDD
Old Names → New Names: Full Reference Table
Both names remain valid. During this transition period, you may see either term in your medical records or correspondence. This table shows how your existing diagnosis maps to the new name.
What If I Don't Know My Gene?
Genetic testing is not always available or conclusive — in approximately 10% of cases, the affected gene is not identified. This may be because the gene hasn't been discovered yet, or the variant is difficult to detect with current technology.
The term unspecified-nEDD (or unspecified-sEDD / unspecified-pEDD) can be used until genetic confirmation is possible. Dermatologists can make this clinical diagnosis based on your skin's appearance and whether other organs are affected — genetic information is not required to receive an EDD classification.
Under the EDD framework, knowing your gene mutation is central to your precise classification and determines which pathway is affected — and which emerging targeted treatments are most relevant to you. If you haven't had genetic testing, the new classification gives additional reason to request it from your dermatologist or clinical geneticist.
What's Covered Under the EDD Umbrella
Epidermal Differentiation Disorders (EDD)
All of the following share the same disease group under the new classification:
Key Biological Pathways Affected
The EDD framework groups conditions by which molecular pathway is disrupted. Understanding this is increasingly important for targeted therapy development:
Structural Protein Defects
Affects the process of normal skin cell maturation and cornification (formation of the skin's protective outer layer).
Epidermal Lipid Barrier Defects
Affects the lipid envelope surrounding skin cells. Most ARCI non-TGM1 subtypes fall here.
Lamellar Body Secretion Defects
Affects transport of lipids into the extracellular space. The most severe ichthyosis — Harlequin — is in this group.
Protease / Anti-Protease Imbalance
Affects the regulation of serine proteases in skin. Results in structural and immune abnormalities.
Nucleotide Excision Repair Defects
Affects repair of UV-damaged DNA. Results in photosensitivity, hair shaft abnormalities, and often neurological involvement.
Steroid Sulfatase Deficiency
Cholesterol sulfate accumulates in the stratum corneum, disrupting shedding.
What This Means for You
If you were diagnosed with Lamellar Ichthyosis, Netherton Syndrome, Ichthyosis Vulgaris, or any other specific type — that diagnosis remains completely valid. The EDD umbrella is a classification system, not a new condition. Patients who already have a diagnosis can continue to use their existing name alongside the new EDD name. For patients who receive a new diagnosis going forward, clinicians will use the new name from the outset.
The reclassification is academic and research-focused. Your emollients, medications, and management plan are unchanged. Treatment decisions continue to be based on your specific gene mutation and clinical presentation. Both names will appear in healthcare records during the transition period.
The broader EDD umbrella means that rare disease funding bodies and clinical trial eligibility criteria can encompass more patients. Trials targeting "EDD" may include PPK and ichthyosis patients together — this is essential for future targeted treatments and increases enrolment opportunities for everyone.
The Ichthyosis Support Group acknowledges that older condition names can be part of a person's identity or their role in the community. There is no pressure to abandon your existing diagnosis name. The expectation is a gradual transition period where both names are used together in clinical records and patient communications.
Timeline
2025
Appearance-Based Classification
Conditions named by clinical phenotype (how the skin looks). Multiple different names for the same underlying gene variant led to confusion — the same person might be told they have "lamellar ichthyosis", "ARCI", or just "ichthyosis" depending on who diagnosed them.
Four Landmark Papers Published — British Journal of Dermatology
An international panel of experts and patient representatives published four peer-reviewed papers formally proposing the EDD classification system, covering non-syndromic, palmoplantar, and syndromic subtypes. Both sets of names (old and new) are considered correct during the transitional period.
Gradual Adoption — Both Terms in Use
Clinicians, dermatologists, and patient groups are beginning to adopt the new terminology alongside existing names. Full integration into NHS rare disease pathways, NICE guidance, and clinical trial design is expected over the coming years. The new names are already being used in specialist centres and academic literature.
Academic References (Open Access)
All four papers on the new classification system are freely available. These are the authoritative sources for the EDD naming system:
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1
Classification proposal (overview paper)
Hernandez-Martin A, Paller AS, Sprecher E et al. A proposal for a new pathogenesis-guided classification for inherited epidermal differentiation disorders. Br J Dermatol 2025.
https://doi.org/10.1093/bjd/ljaf065 -
2
Non-syndromic EDDs (nEDD) — Ichthyoses
Akiyama M, Choate K, Hernandez-Martin A et al. Nonsyndromic epidermal differentiation disorders: a new classification toward pathogenesis-based therapy. Br J Dermatol 2025.
https://doi.org/10.1093/bjd/ljaf154 -
3
Syndromic EDDs (sEDD)
Paller AS, Teng J, Mazereeuw-Hautier J et al. Syndromic epidermal differentiation disorders: a new classification toward pathogenesis-based therapy. Br J Dermatol 2025.
https://doi.org/10.1093/bjd/ljaf123 -
4
Palmoplantar EDDs (pEDD) — Keratodermas
Sprecher E, Ishida-Yamamoto A, Schwartz J et al. Palmoplantar epidermal differentiation disorders: a new classification toward pathogenesis-based therapy. Br J Dermatol 2025.
https://doi.org/10.1093/bjd/ljaf054
The first draft of the patient-facing article was written by Mark Eisner with input from Edel A O'Toole, Neil Rajan, Mandy Aldwin-Easton, and the Reclassifying Epidermal Differentiation Disorders Initiative group. Source: Ichthyosis Support Group (ichthyosis.org.uk).