Microscopic  Image

Prurigo nodularis is an uncommon lesion that presents as extremely pruritic nodules with a  wide anatomical distribution but particularly on extensor aspects of limbs.

Often lesions are multiple but solitary nodules can occasionally occur.

Lesions tend to be symmetrical and are associated with excoriations.DermAtlas Images

Females are more often affected than males.

Many authors consider this condition as part of the spectrum of lichen simplex.

Atypical mycobacteria may be a contributing factor in prurigo nodularis.

Microscopic features: 

Histopathology reveals psoriasiform hyperplasia or pseudo- epitheliomatous hyperplasia  ;

Marked hyperkeratosis with hypergranulosis and focal parakeratosis ;

Occasional spongiosis and exocytosis of mononuclear cells;

Scale crust formation in association with changes of excoriation (fibrin deposition and cellular debris) ;

Prominent hyperplasia of hair follicles;

Mild fibosis of papillary and superficial dermis and telangiectasia ;

Mild to moderate perivascular mononuclear inflammatory cell infiltrate composed of lymphocytes , histiocytes and plasma cells ;

Nerve hyperplasia as reported in the past is not usually a feature.

Differential diagnosis:  Diagnosis is not difficult if the biopsy is big enough. In small biopsies differential diagnosis include Keratoacanthoma  and other causes of pseudo- epitheliomatous hyperplasia.

Note:   Pseudoepitheliomatous (pseudocarcinomatous) hyperplasia :  This is a benign pathological reaction pattern, histologically  characterized by bulbous thickening of squamous epithelium (follicular infundibula and acrosyringium). There is abrupt transition  beween lesion and adjacent epidermis. The cells have abundant often glassy cytoplasm .
Pseudoepitheliomatous hyperplasia is seen in the  following conditions:
Chronic irritation ;  skin trauma ;  dermal inflammatory processes like chromomycosis ; aspergillosis ;  pyoderma, Prurigo nodularis ; Granuloma fissuratum and in associatiation with chondrodermatitis nodularis helicis ;  Spitz naevus , melanoma,  overlying granular cell tumour and cutaneous T cell tumour .

Normal Histology of Skin Glossary    Gross examination of the skin specimen Reporting of biopsies taken for Inflammatory Skin Diseases Lichenoid (Interface)Tissue Reaction Pattern Psoriasiform Reaction Pattern Vesiculobullous Reaction Pattern Spongiform Reaction Pattern Vasculopathic Reaction Pattern Lichen planus-like lesions Bullous Pemphigoid Dermatitis Herpetiformis Hailey-Hailey Disease Erythema Nodosum Folliculitis Impetigo Furuncle(Boil) Carbuncle Toxic Shock Syndrome Drug related cutaneous lesions Cutaneous lesion in graft-versus host disease Verruciform Xanthoma Xanthelasma