GENERAL DERMATITIS PATTERNS

 

Pattern analysis

Pattern analysis is the foundation of Dermatopathology of inflammatory skin disease. Pattern analysis is based on the pathogenesis of the inflammatory disease; it is directed by the target of the inflammatory reaction. It allows the histological section to speak to you. Yes, you can be one with the tissue and the dermatopathology force can be with you.

Pattern analysis is used in parallel with the histological correlate to the clinical categories of lesions - pruritic, scaling, crusting, ulceration, alopecic and nodular skin diseases.

The inflammatory (Dermatitis) patterns were designed by Drs Danny Scott and Julie Yager. The patterns are evolving with greater knowledge, but the logic remains.

I gave an educational session to dermatology residents on this topic and the text is here.

I divide them into 8 inflammatory patterns. In reality I use 7 patterns and list numbers 7 and 8 as subpatterns of Vesiculopustular Dermatitis. I also dropped the atrophic pattern from the inflammatory patterns and list it in the noninflammatory patterns.

Practical approach to dermatopathology of inflammatory skin disease.

I list the patterns in the order I look for them in a histological section. While not as intuitive as the top down approach (epidermis, dermis, subcutaneous tissue approach), this system means you will not miss critical lesions. This is the order in which I look at skin histologically:

  1. Blood vessels of the subcutis and deep dermis.
  2. Panniculus
  3. Follicles including follicular cycles
  4. Sebaceous glands
  5. Dermal perivascular lesions and nodules of inflammation
  6. Epidermis
  7. Epidermal surface

Patterns of Dermatitis

  1. Vasculitis
  2. Panniculitis
  3. Nodular dermatitis
  4. Folliculitis, Furunculosis, Sebaceous adenitis
  5. Perivascular Dermatitis
  6. Cytotoxic Interface Dermatitis
  7. Intraepidermal Vesiculopustular Dermatitis
  8. Subepidermal Vesiculopustular Dermatitis

You will notice that I took some liberties with the order. This is very different to the order that most people place them into. Many people start with the generic Perivascular Dermatitis and then list patterns from the skin surface down to the subcutis. Most Dermatopathogists look deep and scan the skin section from the bottom to the top. This is good form - so this is how I listed them.

Correlation with clinical categories of skin lesions

There is a wide overlap between the clinical appearance of skin disease and the underlying target of the inflammation. Here I list the clinical signs and lesions and indicate the pathogenesis.

Pruritic skin disease

Dermatitis pattern: Perivascular dermatitis

The pathogenesis of pruritus involves a typical nervous system arc (like a reflex arc) with

  1. Sensor/detector system
      1. Intraepidermal nerve fibres (IENF) are C fibre nerves, unmyelinated nerves that are within the epidermis.
      2. Dermal sensory nerves

    2. These are activated

    1. directly (Scratch itch cycle, dry skin - xerosis)
    2. by chemical mediators of inflammation - histamine, serotonin either from mast cells or insect venum
    3. by keratinocyte products - cytokines
    4. by lymphocytes (as in epitheliotrophic lymphoma)
  2. Sensory / Afferent nerve

    3. phantom limb, traumatic neuroma, nerve sheath tumors can all active the sensory nerves

  3. Control center ( ganglia and central nervous system)

    4. Mutilation syndromes that result from central nervous system disease like arnold chiari syndrome and syringomyelia of Cavalier King Charles spaniels, psychogenic alopecia, pain induced alopecia

  4. Motor / efferent nerve
  5. Muscle of skeletal and digestive systems.

In determining the cause of pruritis, look at the detector, sensory nerve, and control center.

Scaling skin disease

Scale is accumulation of loose fragments of keratin (corneocytes) from diseases that cause hyperkeratosis and parakeratosis. Noninflammatory (keratin disorders) and inflammatory (epidermal hyperplasia) skin diseases cause scaling. Many practitioners incorrectly use scaling and crusting as descriptors.

Dermatitis pattern: Perivascular dermatitis

Noninflammatory pattern: keratisation disorder

Crusting skin disease

 

A crust is dried inflammatory exudate on the surface of the skin. Dark crusts are hemorrhagic, yellow green crusts

Dermatitis pattern: Intraepidermal vesiculopustular dermatitis

Nodular skin disease

Inflammatory nodules are in the subcutis, dermis or both.

Inflammatory nodules are either infectious or noninfectious (immune mediated). The cell type usually reflects the cause.

 

Dermatitis pattern: Panniculitis

Dermatitis pattern: Nodular to diffuse dermatitis

Dermatitis pattern: Folliculitis, furunculosis, sebaceous adenitis

Ulcerative and depigmenting skin disease

Ulceration reflects either

  1. reduced production of keratinocytes as occurs in ischemic skin disease or
  2. death of keratinocytes by vasculitis or epidermally targeted diseases

Dermatitis pattern: Epidermal cytotoxic dermatitis

Dermatitis pattern: Vasculitis

Dermatitis pattern: Subepidermal vesiculopustular dermatitis

Noninflammatory pattern: physicochemical/necrotis disorders

Noninflammatory pattern: Pigment disorders

Alopecic skin disease

Alopecic disease results from a failure to produce hair. This is either from a lack of follicle cycle (acquired follicular disease), or abnormal hair production (congenital/genetic). These are mostly discussed in the noninflammatory disease section

Dermatitis pattern: folliculitis, furunculosis sebaceous adenitis

Noninflammatory pattern: Hair follicule disease