Xanthelasma

Cutaneous conditions

There are many conditions of or affecting the human integumentary system—the organ system that covers the entire surface of the body and is composed of skin, hair, nails, and related muscle and glands. The major function of this system is as a barrier against the external environment. The skin weighs an average of four kilograms, covers an area of two square meters, and is made of three distinct layers: the epidermis, dermis, and subcutaneous tissue. There are two main types of human skin: glabrous skin, the non-hairy skin on the palms and soles (also referred to as the "palmoplantar" surfaces), and hair-bearing skin. Within the latter type, there are hairs in structures called pilosebaceous units, each with hair follicle, sebaceous gland, and associated arrector pili muscle. In the embryo, the epidermis, hair, and glands form from the ectoderm, which is chemically influenced by the underlying mesoderm that forms the dermis and subcutaneous tissues.

The epidermis is the most superficial layer of skin, a squamous epithelium with several strata: the stratum corneum, stratum lucidum, stratum granulosum, stratum spinosum, and stratum basale. Nourishment is provided to these layers via diffusion from the dermis, since the epidermis is without direct blood supply. The epidermis contains four cell types: keratinocytes, melanocytes, Langerhans cells, and the Merkel cell. Of these, keratinocytes are the major component, constituting roughly 95 percent of the epidermis. This stratified squamous epithelium is maintained by cell division within the stratum basale, in which differentiating cells slowly displace outwards through the stratum spinosum to the stratum corneum, where cells are continually shed from the surface. In normal skin, the rate of production equals the rate of loss; it takes about two weeks for a cell to migrate from the basal cell layer to the top of the granular cell layer, and an additional two weeks to cross the stratum corneum.

The dermis is the layer of skin between the epidermis and subcutaneous tissue, and comprises two sections, the papillary and reticular dermis. The superficial papillary dermis interdigitates with the overlying rete ridges of the epidermis, between which the two layers interact through the basement membrane zone. Structural components of the dermis are collagen, elastic fibers, and extrafibrillar matrix (previously called ground substance). Within these components are the pilosebaceous units, arrector pili muscles, and the eccrine and apocrine glands. The dermis contains two vascular networks that run parallel to the skin surface—one superficial and one deep plexus—which are connected by vertical communicating vessels. The function of blood vessels within the dermis is fourfold: to supply nutrition, to regulate temperature, to modulate inflammation, and to participate in wound healing.

The subcutaneous tissue is a layer of fat between the dermis and underlying fascia. This tissue may be further divided into two components, the actual fatty layer, or panniculus adiposus, and a deeper vestigial layer of muscle, the panniculus carnosus. The main cellular component of this tissue is the adipocyte, or fat cell. The structure of this tissue is composed of septal (i.e. linear strands) and lobular compartments, which differ in microscopic appearance. Functionally, the subcutaneous fat insulates the body, absorbs trauma, and serves as a reserve energy source.

Conditions of the human integumentary system constitute a broad spectrum of diseases, also known as dermatoses, as well as several nonpathologic states (like, in certain circumstances, melanonychia and racquet nails). While only a small number of skin diseases account for most visits to the physician, thousands of skin conditions have been described. Classification of these conditions often presents many nosological challenges, since underlying etiologies and pathogenetics are often not known. Therefore, most current textbooks present a classification based on location (for example, conditions of the mucous membrane), morphology (chronic blistering conditions), etiology (skin conditions resulting from physical factors), and so on. Clinically, the diagnosis of any particular skin condition is made by gathering pertinent information regarding the presenting skin lesion(s), including the location (such as arms, head, legs), symptoms (pruritus, pain), duration (acute or chronic), arrangement (solitary, generalized, annular, linear), morphology (macules, papules, vesicles), and color (red, blue, brown, black, white, yellow).

Resulting from errors in metabolism

Skin conditions resulting from errors in metabolism are caused by enzymatic defects that lead to an accumulation or deficiency of various cellular components, including, but not limited to, amino acids, carbohydrates, and lipids.

• Acute intermittent porphyria
• Adrenoleukodystrophy (Schilder's disease)
• Alkaptonuria
• Aminolevulinic acid dehydratase deficiency porphyria (Doss porphyria, Plumboporphyria)
• Angiokeratoma corporis diffusum (Anderson–Fabry disease, Fabry disease)
• B-mannosidase deficiency
• Carotenosis
• Cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy syndrome (CADASIL syndrome)
• Cerebrotendinous xanthomatosis
• Citrullinemia
• Congenital erythropoietic porphyria (Gunther's disease)
• Diabetic bulla (Bullosis diabeticorum, Bullous eruption of diabetes mellitus)
• Diabetic cheiroarthropathy
• Diabetic dermopathy (Shin spots)
• Dystrophic calcinosis cutis
• Eruptive xanthoma
• Erythropoietic protoporphyria
• Familial alpha-lipoprotein deficiency (Tangier disease)
• Familial apoprotein CII deficiency
• Familial combined hyperlipidemia (Multiple-type hyperlipoproteinemia)
• Familial defective apolipoprotein B-100
• Familial dysbetalipoproteinemia (Broad beta disease, Remnant removal disease)
• Familial hypertriglyceridemia
• Farber disease (Fibrocytic dysmucopolysaccharidosis, Lipogranulomatosis)
• Fucosidosis
• Gaucher's disease
• Gout (Podagra, Urate crystal arthropathy, Urate deposition disease)
• Hartnup disease (Pellagra-like dermatosis)
• Hepatoerythropoietic porphyria
• Hereditary coproporphyria
• Heredofamilial amyloidosis
• Hunter syndrome
• Hurler syndrome (Gargoylism, Mucopolysaccharidosis type I)
• Hyaluronidase deficiency (Mucopolysaccharidosis type IX)
• Iatrogenic calcinosis cutis
• Idiopathic scrotal calcinosis (Idiopathic calcified nodules of the scrotum)
• Lafora's disease
• Lesch–Nyhan syndrome (Juvenile gout)
• Lichen amyloidosis
• Limited joint mobility
• Lipoid proteinosis (Hyalinosis cutis et mucosae, Urbach–Wiethe disease)
• Lipoprotein lipase deficiency (Chylomicronemia)
• Macular amyloidosis
• Medication-induced hyperlipoproteinemia
• Metastatic calcinosis cutis
• Morquio's disease (Mucopolysaccharidosis type IV)
• Necrobiosis lipoidica (Necrobiosis lipoidica diabeticorum)
• Niemann–Pick disease
• Nodular amyloidosis
• Nodular xanthoma
• Normolipoproteinemic xanthomatosis
• Obstructive liver disease (Xanthomatous biliary cirrhosis)
• Ochronosis
• Osteoma cutis
• Palmar xanthoma
• Phenylketonuria
• Phytosterolemia (Sitosterolemia)
• Porphyria cutanea tarda
• Primary cutaneous amyloidosis
• Primary systemic amyloidosis
• Prolidase deficiency
• Pseudoporphyria (Pseudoporphyria cutanea tarda)
• Sanfilippo syndrome
• Scheie syndrome
• Secondary cutaneous amyloidosis
• Secondary systemic amyloidosis
• Sialidosis
• Sly syndrome (Mucopolysaccharidosis type VII)
• Subepidermal calcified nodule (Solitary congenital nodular calcification, Winer's nodular calcinosis)
• Transient erythroporphyria of infancy (Purpuric phototherapy-induced eruption)
• Traumatic calcinosis cutis
• Tuberoeruptive xanthoma (Tuberous xanthoma)
• Tumoral calcinosis
• Variegate porphyria (Mixed hepatic porphyria, Mixed porphyria, South African genetic porphyria, South African porphyria)
• Verruciform xanthoma
• Waxy skin
• Wilson's disease (Hepatolenticular degeneration)
• Xanthelasma palpebrarum (Xanthelasma)
• Xanthoma diabeticorum
• Xanthoma planum (Plane xanthoma)
• Xanthoma striatum palmare
• Xanthoma tendinosum (Tendinous xanthoma)
• Xanthoma tuberosum