Scalp dysaesthesia and lichen simplex chronicus: diagnostic and therapeutic update with literature review

Summary Scalp dysaesthesia, considered a variant of the cutaneous dysaesthesia syndrome, is characterized by chronic sensory symptoms, including pruritus, pain, burning and stinging in a well‐defined location, without objective findings. Its aetiology is not well elucidated and treatment options are limited, thus it can be challenging and frustrating for both patient and physician. It can be associated with lichen simplex chronicus. In this paper, we review the literature on the pathogenetic factors, diagnostic methods and therapeutic options in the management of scalp dysaesthesia. Dissociation, cervical spine disease and muscle tension seem to be the most important pathogenetic factors. Trichoscopy, reflectance confocal microscopy and biopsy are all helpful for the diagnosis of the disease. Therapies include high‐potency topical or intralesional corticosteroids, capsaicin and topical anaesthetics, sedative antihistamines, tricyclic antidepressants, transcutaneous electric nerve stimulation, botulinum toxin and vitamin B12.


Introduction
Scalp dysaesthesia (SD) is considered a variant of the cutaneous dysaesthesia syndrome, and is characterized by chronic sensory symptoms, including pruritus, pain, burning and stinging in a well-defined location, without objective findings. 1,2 The quality of life of patients suffering from SD may be severely affected. There is a marked female predominance, which is remarkable, with a peak age incidence of 35-50 years.
Because the symptoms are localized to the skin, patients often present to dermatologists, and it is therefore important that dermatologists are familiar with this condition and its underlying causes.

Pathogenesis
In terms of pathogenesis, SD has been linked to an underlying psychopathological background or to chronic pain syndrome. Localized radicular neuropathy and abnormal nerve conduction have been shown to be related to SD. Cervical spine involvement, 3,4 in which a hyperexcitable state may interfere with pathways descending from an inhibitory centre responsible for pain and pruritus modulation, has also been shown to be associated with SD. While both pain and itch are induced by chemical messengers (histamine, tissue proteases and prostaglandins), there is evidence indicating that a unique subpopulation of unmyelinated C nerve fibres are directly activated by pruritus-inducing stimuli. 5 Other potential triggers of SD are stress, hairstyle, heat and seasonality. Psychological stress 6 may directly affect the skin via psychoneuroimmunological reactions due to cutaneous reactivity and sympathetic activation coordinated by the hypothalamic-pituitary-adrenal (HPA) axis. The HPA axis plays a central role in the regulation of epidermal barrier function, cutaneous immune function and cutaneous adnexal and dermal functions, maintaining local and systemic homeostasis. 6 Psychological stress in patients with SD may be also aggravated by disorders such as cervical spine disease (mostly a degenerative disc disease) and muscle tension due to incorrect posture, resulting in nerve compression, which is correlated with the dermatomal distribution of pruritus. 3,4 Consequently, the patient's medical history, habits, social background and environmental factors should be thoroughly investigated to assist in correct diagnosis and appropriate management of this frustrating condition. [7][8][9] Differential diagnoses Owing to its symptoms, SD is often misinterpreted in the early stages as seborrhoeic dermatitis or red scalp syndrome, with subsequent delayed diagnosis and inadequate treatment. As SD can be intensively pruritic, it can result in subsequent development of lichen simplex chronicus (LSC), which appears as single or multiple, clearly demarcated, thickened and hyperpigmented plaques, with scaling and alopecia due to hair breakage. 10,11 However, LSC mostly involves the forearms, scrotum and shins, whereas scalp localization is rare. 12 Clinical examination usually reveals single or multiple, irregular, polycyclic or oval-shaped, welldemarcated, thickened and hyperpigmented plaques. Presence of scales and alopecia due to hair breakage are not uncommon. LSC is the result of chronic and repetitive scratching. Scarring is rare, but can be observed in elderly patients.
LSC may be misdiagnosed as trichotillomania, tinea capitis or alopecia areata, but trichoscopy is a useful aid to the differential diagnosis. The typical trichoscopic signs of LSC and their correlation with the corresponding pathological features were described in 2017 by Rakowska et al., 13 and include short hair shafts with split ends that emerge in a group of 2-4 hairs from a single follicular opening, acquiring the aspect of a broom (broom hair), and short hair shafts of the same length with broken tips due to trichorrhexis. In LSC, the skin is also usually thickened and hyperpigmented (postinflammatory hyperpigmentation). Trichoscopy allows discrimination between LSC and trichotillomania, in which the short broken hairs are of different lengths, and between LSC and alopecia areata, as the latter is characterized by black dots, dystrophic hair and exclamation-mark hairs 14 (Fig. 1). The short hairs of trichorrhexis nodosa must be differentiated from the Morse code-like hairs typically seen in tinea capitis, in which the multiple, whitish, transverse interruptions along the shaft are due to fungal penetration. 15 Histopathologically, the trichoscopic signs of LSC correspond to hyperkeratosis of the infundibular ostium, with hair shafts split into two parts by a layer of red blood cells, a feature known as the 'hamburger sign'. 16,17 However, this can also be observed in trichotillomania. Typical of LCS is the formation, at the level of the infundibulum, of jagged acanthotic projections that, together with the hair canal in the middle, resemble a 'gear wheel' on horizontal sections. 16,17 In the uppermost sections, the epidermis shows thick layers of orthokeratosis and hyperkeratosis. The follicular architecture and the terminal to vellus hair ratio are preserved, with a normal number of terminal follicles, but with a decrease in size and number of sebaceous glands. 16,17 In a recent paper, we also described the role of reflectance confocal microscopy (RCM) in LSC to facilitate the differential diagnosis between scarring and nonscarring alopecias, biopsy site selection and treatment monitoring. 18 In some cases RCM may be particularly informative, allowing diagnosis without the need of a biopsy.

Management
Management of SD, with or without LSC, is extremely challenging and often frustrating for both patient and clinicians. The goal of treatment is to interrupt the vicious cycle of itching. 19 In the scenario of a neurological trigger of any kind, a neurological consultation is advisable before starting treatment. Supportive counselling, comprising many different psychological interventions may be needed in severe cases. In combination with medical therapy, simple exercises of stretching, once or twice a day, may help to improve cervical nerve compression through the restoration of normal cervical homeostasis. 20 High-potency topical (with or without occlusion) or intralesional corticosteroids are usually effective, but their prolonged use as maintenance treatment is limited by their well-known adverse effects (AEs).
Topical treatments such as capsaicin in concentrations ranging from 0.025% to 0.1% (with 0.075% reported as optimal), tacrolimus 0.1%, salicylic acid 3-5%, lidocaine and compounds including lidocaine, such as TALK (topical amitriptyline 5%, lidocaine 5% and ketamine 10%) have with slow dose escalation for better tolerance, have been used with satisfactory results. 21,22 Creams and foams are generally better tolerated than lotions. At least twice-daily application is needed but it may take 4-6 weeks for effectiveness.
Systemic treatments include sedative antihistamines as supportive treatment, as they are not sufficient as monotherapy. Low doses of oral pregabalin 23 (up to 300 mg/day), gabapentin (up to 3000 mg/day), mirtazapine (15 mg/day), amitriptyline (10-25 mg/day), doxepin (up to 280 mg/day) 24 and naltrexone (3-5 mg/day) are all possible options, but AEs, especially sedation, hypotension and anticholinergic effects, may limit their use. A list of the possible treatment options is summarized in Table 1.
In addition, transcutaneous electric nerve stimulation 25 and botulinum toxin 26 have been reported to effectively reduce itch, but they are generally reserved for severe or recalcitrant cases and require the

Learning points
• SD is a condition of chronic pain, burning/ stinging sensation and/or pruritus of the scalp, in the absence of related skin or systemic disease. • A neurological and psychological aetiopathogenesis must be investigated, thus referral to a specialist is essential.
• SD may result in LSC, which appears as single or multiple, well-demarcated, thickened and hyperpigmented plaques, with scaling and alopecia due to hair breakage. • Trichoscopy is fundamental to the diagnosis of LSC, and histopathological confirmation may also be needed.
• Therapy of SD is based on high-potency topical or intralesional corticosteroids, capsaicin and topical anaesthetics, sedative antihistamines, tricyclic antidepressants, transcutaneous electric nerve stimulation, botulinum toxin and vitamin B12.