What is nodular prurigo?
Nodular prurigo is a skin condition characterised by very itchy firm lumps. It is the most severe form of prurigo. It is not known why these lumps appear. It is also called ‘prurigo nodularis’.
Nodular prurigo is very difficult to treat effectively.
Clinical features of nodular prurigo
Nodular prurigo can occur at all ages but mainly in adults aged 20–60 years. Both sexes are equally affected.
The individual prurigo nodule is a firm lump, 1–3 cm in diameter, often with a raised warty surface. The early lesion may start as a smaller red itchy bump. Crusting and scaling may cover recently scratched lesions. Older lesions may be darker or paler than surrounding skin. The skin in between the nodules is often dry. The itch is often very intense, often for hours on end, leading to vigorous scratching and sometimes secondary infection.
Nodular prurigo lesions are usually grouped and numerous but may vary in number from 2–200. Nodular prurigo tends to be symmetrically distributed. They usually start on the lower arms and legs, and are worse on the outer aspects. The trunk, face and even palms can also be affected. Sometimes the prurigo nodules are most obvious on the cape area (neck, shoulders and upper arms).
New nodules appear from time to time, but existing nodules may regress spontaneously to leave scars. Nodular prurigo often runs a long course and can lead to significant stress and depression.
What is the cause of nodular prurigo?
The cause of nodular prurigo is unknown. It is uncertain whether scratching leads to the lumps, or if the lumps appear before they are scratched. The reason for the lumps, the inflammation and the increased activity and size of nerves in the skin is under investigation but remains unknown.
Up to 80% of patients have a personal or family history of atopic dermatitis, asthma or hay fever (compared to about 25% of the normal population).
Nodular prurigo may commence as an insect bite reaction or another form of dermatitis. It has been associated with internal disease including iron deficiency anaemia, chronic renal failure, gluten enteropathy, HIV infection and many other diverse conditions.
In some cases, nodular prurigo has been associated with brachioradial pruritus, which is due to compression or traction of spinal nerves. This theory may explain why local treatment is not always successful. It has been speculated that widespread nodular prurigo may also follow sensitisation of the spinal nerves and that the nodules appear because of scratching.
Investigations in nodular prurigo
A skin biopsy may be useful to confirm the diagnosis of nodular prurigo. Under the microscope, the skin is enormously thickened and may appear quite abnormal, sometimes resembling squamous cell skin cancer. The nerve fibres and nerve endings in the skin are markedly increased in size. The skin is inflamed and there is an increased number of neural mediators known to cause itching and nerve growth.
Direct immunofluorescence looking for antibody deposition in the skin is usually negative. Rarely, the blistering disease bullous pemphigoid can present as nodular prurigo. In this case, immunofluorescence reveals immunoglobulins in the basement membrane zone below the epidermis. The prurigo nodules can be present for weeks or months before any blisters appear.
It is important to identify underlying diseases that are associated with nodular prurigo; blood tests may include full blood count, liver, kidney and thyroid function tests. Patch testing may be worthwhile if contact allergy is considered possible.
Treatment of nodular prurigo
Unfortunately this is one of the more resistant conditions skin specialists are called upon to treat. Local treatments tried include:
- Emollients applied liberally and frequently to cool and soothe itchy skin – menthol or phenol may be added.
- Oral antihistamines taken at night to reduce itch and allow sleep.
- Ultrapotent topical steroid creams. To enhance their effect, apply under occlusion; cover with a plastic or hydrocolloid adhesive dressing and leave this in place for several days.
- Corticosteroid injections (triamcinolone acetonide 10 – 40 mg /ml) into thicker nodules.
- Coal tar ointment as steroid alternative.
- Calcipotriol ointment (topical vitamin D3) may be more effective than topical steroids in some cases.
- Capsaicin cream, which induces itching and burning until eventually the itch stops completely – it requires repeated applications four to six times daily.
- Cryotherapy, which may shrink the nodules and reduce their itch.
- Pulsed dye laser, which may reduce the vascularity of individual lesions.
Antibiotic ointment may be used on individual infected lesions, and oral antibiotics (usually flucloxacillin) are indicated for significant secondary infection.
Systemic treatments that may be helpful for more severe disease are listed below, in no particular order. Combination treatment is frequently recommended.
- Phototherapy (UVB and PUVA)
- Tricyclic anti-depressants such as amitriptyline or doxepin
- Anticonvulsants used for neuropathic pain and itch, such as gabapentin or pregabalin
- Naltrexone, an opiate antagonist (this counteracts the narcotic effect of morphine, heroin and similar drugs), which has been reported to reduce itching in some subjects.
- Oral steroids
- Thalidomide, which is reserved for very severe cases and may be difficult to access.
- Ciclosporin, which may reduce the lumps and the itching but its use is limited by side effects.
- Systemic retinoids such as acitretin or isotretinoin, which may shrink the nodules and reduce the severity of the itch.
Hope for the future
- Clinical trials of a drug called serlopitant for nodular prurigo are on-going. It is a neurokinin (NK)-1 receptor antagonist and shows promise in reducing itch in various skin disorders.