Of 326 articles retrieved, 37 studies (consisting of 536 patients with tinea capitis) were found eligible for quantitative analysis: 16 original studies, 7 case series and 14 case reports. A PRISMA flow diagram is presented in Fig. 1.
The frequency, sensitivity, specificity, and positive and negative predictive values of the most characteristic trichoscopic findings for tinea capitis are presented in Table 1.
The Most Characteristic Trichoscopic Findings of Tinea Capitis
Comma hairs (Fig. 2) are short, C-shaped hairs, that are homogeneous in pigmentation and thickness . They were first described by Slowinska et al. in 2008 . Comma hairs are formed due to subsequent cracking and bending of a hair shaft filled with hyphae . The frequency of comma hairs varied between 13% and 100% (mean value: 51%) of patients with tinea capitis [6, 9, 12,13,14,15,16,17,18,19,20,21,22,23,24]. They were also occasionally detected in patients with alopecia areata and trichotillomania .
Corkscrew hairs (Fig. 3) are multiple twisted and coiled hairs with corkscrew-like structure . They were first described by Hughes et al.  in 2011 as a specific form of comma hairs in black-skinned patients (with African hair types) or a specific trichoscopic finding of tinea capitis caused by Trichophyton soudanense. However, corkscrew hairs were also described in white-skinned patients [25,26,27] with straight hair . They were observed in endothrix tinea capitis caused by Trichophyton tonsurans [22, 26,27,28,29,30,31] and Trichophyton violaceum [25, 32, 33]. Moreover, corkscrew hairs were detected in ectothrix-type fungal infection caused by Trichophyton verrucosum , Microsporum canis and Microsporum audouinii . The incidence of corkscrew hairs varied between 14% and 100% (mean value: 32%) of patients with tinea capitis [6, 9, 12,13,14,15,16,17,18,19,20,21,22,23,24]. Corkscrew hairs were described as a specific trichoscopic feature of tinea capitis. However, they may also be observed in ectodermal dysplasias [34, 35].
Morse Code-like Hairs
Morse code-like hairs (Fig. 3), also known as bar code-like hairs, represent hairs with multiple thin white bands across the hair shaft . The term was introduced in 2011 by Rudnicka et al. . Morse code-like hairs are formed due to the accumulation of spores around the hair shaft that cause a transverse perforation of the hair shaft. They were only described in patients with ectothrix-type fungal infection with an incidence rate between 12% and 56% (mean value: 22%) [9, 22, 36,37,38,39,40].
Zigzag hairs, first described by Rudnicka et al. , are bent hairs with multiple sharp angles. Their formation results from incomplete, transverse fractures along the hair shaft . Zigzag hairs were only described in patients with ectothrix-type fungal infection with an incidence rate between 5% and 49% (mean value: 21%) [6, 9, 13, 15, 18,19,20,21, 24]. They were also reported in patients with alopecia areata .
Bent hairs are characterized by bending of the hair shaft with homogeneous thickness and pigmentation . In contrast to comma hairs, no hair shaft shortening is observed . Very few studies reported bent hairs in tinea capitis, with an incidence rate between 4% and 71% (mean value: 27%) [6, 19, 20, 42]. They were only observed in patients with ectothrix-type fungal infection [37, 43].
Block Hairs and i-Hairs
Block hairs are very short hairs with a transverse horizontal distal end . i-Hairs are block hairs with an accented dark distal end . The terms were introduced by Rudnicka et al. . There were very few studies reporting block hairs and i-hairs in tinea capitis, with an incidence rate of 4–50% (mean value: 10%) and 4–33% (mean value: 10%) [13, 18,19,20,21, 24], respectively. They were also detected in patients with alopecia areata and trichotillomania .
Other Trichoscopic Findings of Tinea Capitis
Other commonly observed, but not characteristic, trichoscopic findings of tinea capitis include broken hairs, black dots (Fig. 2), and perifollicular and interfollicular scaling.
Broken hairs and black dots were observed in both endothrix and ectothrix tinea capitis with an incidence rate of 8–100% (mean value: 57%) [6, 9, 12,13,14,15,16,17,18,19,20,21,22,23, 42] and 17–80% (mean value: 34%) [6, 12,13,14,15,16,17,18,19,20, 42], respectively. In 6–95% (mean value: 59%) [6, 16, 19,20,21,22] and 53–100% (mean value: 89%) [6, 14, 16, 19, 20, 24, 42] of patients with tinea capitis perifollicular and diffuse scaling were detected.
Trichoscopic Differences between Microsporum and Trichophyton Tinea Capitis
In the present analysis Morse code-like hairs, zigzag hairs, bent hairs and diffuse scaling were only present in Microsporum tinea capitis (8/29, 28%; 6/29, 21%; 4/29, 14% and 4/29, 14%, respectively) (p < 0.001, p < 0.01, p < 0.05 and p < 0.05, respectively). Conversely, corkscrew hairs were more commonly observed in Trichophyton compared to Microsporum tinea capitis (21/38, 55% vs 3/29, 10%) (p < 0.001). No significant difference was found in the frequency of comma hairs, black dots, broken hairs, perifollicular scaling, block hairs and i-hairs between Microsporum and Trichophyton tinea capitis.
Detailed data are presented in Table 2.
Trichoscopy in the Monitoring of Treatment Efficacy
In a study conducted by Campos et al.  the disappearance of short broken hairs, corkscrew hairs and zigzag hairs was reported on the 4th week follow-up visit after starting the treatment. At week 8, the decrease of comma hairs, black dots, perifollicular scaling and diffuse scaling was additionally observed. On the 12th week follow-up visit after starting the treatment, dystrophic hairs were not present. However, perifollicular scaling and diffuse scaling were detected. Souissi et al.  also reported the disappearance of Morse code-like hairs with the presence of perifollicular scaling and diffuse desquamation on the 4th week follow-up visit. At this time, the fungal culture was negative. Moreover, Richarz et al.  presented a case of a patient with disappearance of corkscrew hairs on a follow-up visit after 4 weeks of therapy, with positive direct potassium hydroxide examination and negative fungal culture.