General Considerations

1. Keratinization disorder is a term used to describe the clinical signs associated with abnormalities in the process of epidermopoiesis, keratinization, apocrine or sebaceous gland function, intercellular lipid formation, cell cohesion, and cell desquamation.
2. Keratinization disorders are classified into primary and secondary causes. In primary causes, the excessive scaling is due to a direct defect in one or more steps in the formation of the stratum corneum. Known defects include mutations in genes which encode the structural proteins that form the corneocyte (e.g. transglutaminases), or enzymes involved in lipid formation or lipid transport. Secondary disorders are those where excessive scaling develops due to an underlying condition (e.g. allergic skin diseases, sarcoptic mange, hypothyroidism, epitheliotropic lymphoma, etc.). Most keratinization disorders arise from secondary causes.
3. Excessive scaling is the characteristic clinical sign of keratinization disorders.
4. Alterations in glandular secretions will result in either excessive sebum production (seborrhea oleosa) or deficient sebum secretion (seborrhea sicca).
5. Alterations in hair follicle function will result in follicular casts (excessive keratin produced in the follicle canal adheres to and surrounds the hair shafts) and comedones (black heads – the upper portion of the hair follicle (infundibulum) is dilated and filled with keratin).
6. Malassezia and/or bacterial overgrowth and bacterial infection are frequently observed in many keratinization disorders due to alterations in the surface microenvironment, the stratum corneum, and other components of the skin barrier.
7. The diagnostic challenge when managing a scaling disorder is to determine if clinical signs are secondary to an underlying dermatosis or are the result of a primary keratinization defect.
8. Therapeutic and prognostic factors for keratinization disorders are variable.
9. In general, if the keratinization disorder is secondary to an underlying disease and the underlying dermatosis is identified and treated successfully, the keratinization disorder has an excellent prognosis for complete cure and/or control.
10. If there is a primary keratinization defect, life-long therapy is usually required.

Classification

1. Keratinization disorders can be classified based on:
   1. Clinical signs.
   2. Etiology: primary and secondary causes.
2. Classification based on clinical signs:
   1. Seborrhea sicca:
      1. It is characterized by white and loose scales. These signs are typical of the “dry” form. The hair coat is typically dull and is not oily.
      2. Odor is usually not present.
   2. Seborrhea oleosa:
      1. It is characterized by greasy yellow/brown scales. This color results from sebum accumulation on the scale surface. The hair coat is oily. Scales adhere to hairs.
      2. “Rancid fat” odor may be present.
      3. Secondary Malassezia and bacterial overgrowth or infection are common.
      4. In some cases, you need to touch the animal to differentiate a seborrhea oleosa from a seborrhea sicca. If the seborrhea is oleosa, your fingers become coated with a thin layer of oil.

1. 3. Seborrheic dermatitis:
      1. It is the inflammatory form.
      2. Scaling (with or without greasiness) and clinical evidence of inflammation characterize this form.
      3. Multifocal hyperkeratotic plaques may be present.
      4. Investigate the presence of Malassezia overgrowth and bacterial overgrowth/infection because they are common problems in greasy skin.
   4. Ceruminous otitis externa:
      1. Excessive waxy debris in the ear canal.
      2. Odor and erythema are typically present.
      3. Secondary Malassezia and bacterial overgrowth in the external ear canals causing inflammation are common.
      4. The external ear canal is a modified extension of the skin; therefore, seborrheic otitis externa is common in seborrheic dermatitis especially the cases associated with atopic dermatitis.
   5. REMEMBER! Secondary Malassezia and bacterial overgrowth and bacterial infection are common.

3. Classification based on etiology:
   1. Secondary keratinization disorders:
      1. External and internal causes with effect on keratinization and/or sebum production will lead to the keratinization disorder.
      2. Inflammation associated with the primary disease and/or self-trauma result in the keratinization disorder. Therefore, scaling with or without excessive sebum secretion is observed in areas affected by the underlying disease.
      3. Secondary keratinization disorders may occur with most types of skin diseases including:
         1. Parasitic: cheyletiellosis, demodicosis, scabies.
         2. Allergic: canine atopic dermatitis, feline atopic skin syndrome, food allergy, fleabite allergy, contact allergy.
         3. Endocrine: hypothyroidism, hyperadrenocorticism, sex hormone dermatoses.
         4. Pyoderma: epidermal collarettes associated with superficial pyodermas.
         5. Dermatophytosis
         6. Neoplastic: epitheliotropic T-cell lymphoma

1. 2. Primary keratinization disorders:
      1. The diseases classified as primary keratinization defects are either hereditary, idiopathic, or nutritional in origin.
      2. Hereditary (confirmed or presumed) keratinization disorders:
         1. Ichthyosis.
         2. Familial nasal and footpad hyperkeratosis.
         3. Schnauzer comedo syndrome.
      3. Idiopathic keratinization disorders:
         1. Primary.
         2. Acne.
         3. Ear margin seborrhea.
         4. Nasodigital hyperkeratosis.
      4. Nutritional keratinization disorders:
         1. Zinc responsive dermatosis
         2. Fatty acid deficiency.