Nevus

A nevus is a pigmented skin lesion that appears as a brown or black spot. This abnormal proliferation of cells is usually benign and does not cause complications.

Depending on when they appear, nevi are classified into two main groups:

• Congenital nevus: these are present at birth or appear before the first year of life. They are classified according to their size:
• Small nevi: they are estimated to measure less than 1.5 cm in adulthood.
• Medium-sized nevi: they will measure between 1.5 and 20 cm.
• Giant nevi: they are estimated to exceed 20 cm during adulthood. They are the rarest, but their progression should be monitored because they are more likely to become malignant.
• Acquired nevus: these appear from the age of two and continue to proliferate until approximately the age of forty.

The prognosis of a nevus is generally good, as the lesions are usually benign. However, in some cases, the cells that form it may be cancerous (malignant nevus) and lead to melanoma, which is the most aggressive type of skin cancer.

Symptoms

The main symptom of nevi is the appearance of dark spots, with a higher incidence in areas most exposed to the sun. Depending on the layer of the skin in which they are located, they may be of the following types:

• Junctional nevus: these appear at the dermoepidermal junction, the area where the epidermis (the outer layer of the skin) and the dermis (the inner layer) meet. They are common in childhood and adolescence. They are homogeneous, flat, brown in colour and usually do not exceed 6 mm in size.
• Compound nevus: these are the natural evolution of the former, which is why they are more common in adulthood. Over time, the cells move into the superficial layers of the dermis and become slightly raised. They may show variations in colour, although they always remain within brown tones.
• Intradermal nevus: the lesion moves completely into the dermis, so it loses pigment and becomes pink or similar in colour to the skin. It takes on a dome-shaped appearance, commonly known as warts, and hair may appear on the surface.

Occasionally, they may occur in other areas of the body:

• Nevus in the eye: in most cases, ocular nevus is congenital and appears before the age of twenty. Although they are usually benign, periodic follow-up is recommended to avoid complications. They usually form in the following areas:
  • Conjunctiva: conjunctival nevus is the most common and appears in the transparent tissue that covers the eye. It is yellow or light brown in colour.
  • Iris: coffee-coloured spots in the coloured area of the eye.
  • Under the retina: choroidal nevus is only detected during an eye examination. It is a grey, yellow or brown lesion. When it develops an orange tone or becomes protruding, there is a risk of transformation into melanoma.
  • Nevus on the scalp: sebaceous nevus of Jadassohn has the highest prevalence. It is a congenital formation present at birth that may go unnoticed until adolescence, when hormonal changes promote its growth. It presents as orange-coloured alopecia measuring between 1 and 10 cm in diameter and hyperplasia of the sebaceous glands.

Taking histological characteristics into account, the following types exist:

• Melanocytic nevus: this is the most common type, usually known as a mole. It occurs when a group of melanocytes, the cells responsible for producing melanin, cluster in a localized area of the skin instead of spreading evenly.
• Atypical or dysplastic nevus: this is a mole with an unusual appearance, generally brown, slightly raised and with irregular borders. Although they are benign, they have an increased risk of developing into melanoma. The most notable types are:
• Familial dysplastic nevus: it has a hereditary component.
• Sporadic dysplastic nevus: these appear in patients with no family history.
• Spitz nevus: this is the abnormal proliferation of epithelioid cells (similar to the epithelial cells that form the outer layer of the skin) or spindle cells (cells shaped like a grain of rice, that is, longer than they are wide. The most common in the skin are fibroblasts, which are responsible for producing collagen). It usually appears in childhood and has characteristics similar to melanoma, but is benign in nature: dome-shaped, pink or brown in colour and rapid-growing. They are more common on the face and neck.
• Reed nevus: this is a variant of the previous type. It is a dark brown or black mole, less than 1 cm in size, with rapid growth. It commonly appears on the legs of women and children.
• Sutton nevus: also called halo nevus, it is surrounded by an area of depigmented skin resulting from an immune system reaction. Its evolution follows these steps:
• The halo appears around a pre-existing mole.
• The nevus loses pigment and gradually decreases in size.
• The mole disappears, leaving only a light-coloured macule.
• The skin regains its normal pigmentation.
• Cherry nevus or cherry angioma: also known as ruby nevus, it is the most common cutaneous vascular tumour. It forms due to dilation of the superficial blood vessels. It is a sign of ageing that appears as red freckles.
• Nevus spilus or speckled lentiginous nevus: this appears in the first years of life. Its main characteristic is that it forms a cluster of brown macules that overlap one another.
• Nevus of Ota: this is a large bluish patch that usually appears on the cheeks, forehead or around the eyes.

Causes

In most cases, nevi form because melanocytes cluster irregularly, meaning that some areas of the skin or eye contain more than the normal amount and show hyperpigmentation. The exact reasons why this occurs are still unknown.

In some types of nevus, the causes are clearer:

• Sutton nevus: the immune system mistakenly attacks melanocytes.
• Cherry nevus: with ageing, blood vessels lose elasticity and dilate.
• Nevus on the scalp: during embryonic development, a mutation occurs in the HRAS, gene, which is responsible for producing the protein that controls cell proliferation.

Risk factors

The risk of a nevus forming increases in the following cases:

• Genetic predisposition.
• Hormonal changes, such as puberty or pregnancy.
• Exposure to the sun and ultraviolet (UV) radiation.
• Fair skin, eyes and hair.
• Sutton nevus: autoimmune diseases, such as vitiligo or thyroid disorders.
• Cherry nevus: being over 30 years old.
• Nevus of Ota: being of Asian or African origin.

Complications

Moles may become irritated or bleed when the surrounding skin stretches as they grow, but this is usually not significant. However, the main complication of a nevus is the development of melanoma (a rare but highly aggressive type of cancer), which may originate both in the skin and in the nervous system. In addition, it may lead to:

• Giant congenital melanocytic nevi: neurological complications, such as neurocutaneous melanocytosis, which is the growth of melanocytes in the central nervous system.
• Sutton nevus: decreased melanin in the surrounding hair follicles, causing patches of white hair to appear (poliosis).
• Cherry nevus: coagulation of the blood vessels that form it (thrombosis), causing a change in the colour of the angioma and, occasionally, pain.
• Nevus in the eye: if fluid leakage occurs, vision may become blurred or retinal detachment may occur.
• Nevus on the scalp: permanent destruction of the hair follicles (alopecia).

Which doctor treats nevus?

Nevus is diagnosed and treated in the specialty of Medical-surgical dermatology and Venereology.