A halo nevus is one type of congenital melanocytic nevus, which is a proliferation of melanocytic cells formed during embryonic life, but after conception. It manifests in adolescence or earlier, in most cases.
It was first described in 1916 and was earlier called the Grunewald nevus. Other names include the Sutton nevus or leukoderma acuisitum centrifugum. It is a rare type, and is so called because of the paler periphery of the lesion in contrast to the hyperpigmented area at the center. The hypopigmentation is due to the autoimmune infiltration by lymphocytes, and forms the ‘halo’ of the lesion’s name.
Observations have showed that the halo nevi last for 10 years or more, but a large subgroup passes through various stages to finally regress completely. The process takes approximately 8 years on average. It is necessary to inform patients that this is what may be expected, so as to prevent the demand for excision in cases where it is not required.
It is hypothesized that the operation of the autoimmune response in the central part of the halo nevus causes destruction of the nevus cells. This is borne out by the fact that histopathological examination fails to show any nevus cells but does reveal a lymphocytic infiltrate.
All nevi which have a pale ring or halo around a dark center are not halo nevi. On the basis of their histopathological findings, they may be classified into four forms:
True halo nevi are thus extremely rare. In most cases, they are actually compound nevi, junctional nevi, or intradermal nevi. Among true halo nevi, they may show mild to moderate atypical changes in about half of all cases. Severe atypia is also seen in less than 10 percent of halo nevi. Thus, the halo phenomenon alone does not define a halo nevus. Rather, it occurs in association with a wide range of nevi.
A halo nevus shows a sequence of orderly changes to final regression, as follows:
The importance of surveillance of these nevi is that like other melanocytic nevi, these may also give rise to malignant melanoma in 5-10 percent of cases (taking all types together). Many of these nevi are also linked to neurological and skeletal abnormalities as well.
A halo nevus may be associated with melanoma in another distant part of the body. Therefore, the presence of a halo nevus should signal the need for a full body skin examination and ophthalmoscopic examination to rule out intraocular melanoma. Single halo nevi should undergo excision biopsy to exclude the presence of melanom.