identifying nerve involvement is the key to diagnosis of leprosy in the tuberculoid spectrum and this is often difficult , owing to sampling errors or complete destruction of nerves . leprosy is probably the oldest known disease to mankind and is widely prevalent in india . according to the latest data presented by the national leprosy eradication programme nlep , the annual new case detection rate is 10.48 per 100,000 population , that is , about 1.27 lakh new cases in 2010 - 11 . prevalence rates have gone up in some endemic areas and leprosy continues to remain a public health burden . diagnosis usually is not difficult in the lepromatous spectrum because of the abundance of bacilli in the lesions . in tuberculoid and indeterminate forms , diagnosis is more difficult . the granulomas overlap morphologically with other infectious and non - infectious lesions , notably tuberculosis , sarcoidosis and fungal infections , which are also common in our country . it is cutaneous nerve involvement by the inflammatory reaction that permits the differential diagnosis of leprosy from the other cutaneous granulomas . the pathologist often faces difficulties in visualizing nerve impairment , especially in inadequate biopsies . in sections stained with h and e , nerve remnants within granulomas are confused with epithelioid cells , fragments of arrector pili muscles , and small vessels . s-100 is an acidic calcium binding protein , an immunocytochemical marker of schwann cells of peripheral nerves . the utilization of s-100 protein as a nerve marker in sections of tuberculoid leprosy , sarcoidosis , lupus vulgaris , and fungal infections that share a granulomatous pattern could help in diagnosis and distinction of the above infections . there are older reports that have stressed on its utility but it is still not widely used in practice . this study seeks to add to the existing data , with particular regard to biopsies deemed  inadequate  . a total of 20 skin biopsies with histopathological diagnosis signed out as borderline tuberculoid leprosy ( btl ) ( as defined by ridley and jopling ) were included for the study . a total of 19 biopsies signed out as granulomatous inflammation - lupus vulgaris , tuberculous verrucosa cutis , sarcoidosis , granuloma annulare , granulomatous rosacea , and deep fungal infections were taken as controls . diagnosis was established based on histopathology , treatment and follow - up data , cultures and polymerase chain reaction for tuberculosis and fungal infections , in some cases . the original h and e sections , special stains for fungi ( periodic acid - schiff , gomori methenamine silver ) and mycobacteria ( ziehl - neelsen , modified fite - faraco ) were reviewed . immunohistochemistry was done by labeled streptavidin - biotin method using antibody to s-100 ( biogenex clone15e2e2 , purified bovine protein ) after heat antigen retrieval . h and e slides and s-100 immunostained slides were examined independently by four different investigators blinded to the original diagnosis . both h and e and s-100 stained slides were studied for the patterns of nerve involvement . they were categorized as : absent , intact , intact with perineural inflammation , infiltrated and/or fragmented . chi - square test was used to determine the significance of the various patterns in the two groups , with a p < 0.05 taken as significant . biopsies of 14 btl and 12 other granulomas were adequate , that is , showed subcutis . table 1 depicts the comparison of h and e and s-100 , including their sensitivity , in identifying nerve involvement . comparison of h and e staining and s-100 immunostaining in the identification of nerves in both adequate and inadequate biopsies of borderline tuberculoid leprosy(n=20 ) and other cutaneous granulomas(n=19 ) out of a total of 39 biopsies , h and e identified nerves in 17 , while s-100 identified nerves in 31 cases . s-100 proved to be more sensitive than h and e in both adequate and inadequate biopsies with overall sensitivity being 0.78 versus 0.41 . the patterns of nerve involvement in btl versus other granulomas are shown in table 2 . patterns of nerve involvement by s-100 staining in borderline tuberculoid leprosy and other cutaneous granulomas infiltrated and/or fragmented nerves were seen only in btl . in btl , s-100 identified nerve involvement in 15 cases , in comparison to h and e alone , which identified only 8 cases [ figure 1 ] the presence of normal nerves or nerves with perineural inflammation alone by s-100 favored a diagnosis other than leprosy [ figure 2 ] . absence of nerves , even in adequate biopsies , was seen in two cases each of btl and lupus vulgaris . ( a ) periadnexal granulomas in borderline tuberculoid leprosy with no identifiable nerves ( h and e , 100 ) ( b and c ) nerve fibers infiltrated by inflammatory cells and fragmented ( s-100 , 200 and 400 ) ( a and b ) perifollicular granulomas in rosacea with no identifiable nerves ( h and e , 100 , 200 ) ( c and d ) intact nerve fibres with perineural inflammation ( s-100 , 200 and 400 ) . this property is invaluable to a histopathologist making its microscopic diagnosis . in the lepromatous end of the spectrum , a confident diagnosis is possible owing to the abundance of acid fast bacilli , which may be detected even on h and e sections with a bluish hue . first of all , organisms are not demonstrated by special stains and even if present in low numbers , are elusive except to the most persistent and experienced eyes . nerve involvement alone is insufficient , as nerve encasement by granulomas may also be encountered in sarcoidosis and lupus vulgaris . in this respect , pathologists seek features like the elongated contour of granulomas as they follow neurovascular bundles of the adnexae , absence of caseation , epidermal changes and according to some , the absence of normal nerves in adequate biopsies as clues to leprosy diagnosis . the last criterion is somewhat debatable , since it is dependent on sampling and the size of tissue . morphologically , the presence of inflammatory cells or granulomas in the endoneurium are robust indicators . nerve remnants within granulomas are often confused with epithelioid cells , fragments of smooth muscle , or endothelial cells . s-100 is an immunohistochemical marker for schwann cells , melanocytes , langerhans cells , chondrocytes , and myoepithelial cells . it is a useful tool to define nerves , especially those within or in the vicinity of granulomas . fleury and bacchi were the first to use this technique in biopsies of clinically suspected btl , but with no histologic or bacteriologic proof . they affirmed its utility by demonstrating both nerve dissociation and presence of nerve twigs within granulomas . they noted that nerve fragmentation was the commonest pattern seen in 80% of cases , followed by total absence of nerves 16% . singh et al . , found that complete absence of nerve twigs in an adequate biopsy is the most reliable criterion for leprosy . the present study reaffirms the superiority of s-100 over h and e in identifying nerves , whether normal or diseased . even in biopsies considered inadequate owing to lack of subcutis , s-100 is twice as sensitive as routine staining . this is a very important and useful finding particularly for lesions on the face , where a deep biopsy may not be feasible . in this study , infiltration and/or fragmentation of nerves by inflammatory cells or granulomas by s-100 we noted the absence of nerves in adequate biopsies , touted by earlier studies as virtually diagnostic of leprosy , in both btl and other granulomas and it did not help us in making the distinction . lack of nerves could imply sampling error and is not conclusive of leprosy . in such a situation , the findings of normal nerves alone or perineural inflammation alone on s-100 staining were exclusively seen in other granulomas . in leprosy , these findings were seen focally , but always in association with nerve infiltration / fragmentation . on h and e staining , the presence of normal nerves in cutaneous granulomas is a snare leading to a false negative diagnosis . s-100 is very useful in this regard , since it demonstrates both involved and normal nerves . particularly , langerhans cells are seen within granulomas and are difficult to tell apart from completely fragmented nerves . in this study , we have not considered isolated s-100 positive cells as nerves . they had to be present in small clusters or in the contour of nerve twigs . there are reports of positive staining of mycetoma granules and m. leprae by s-100 . to summarize , s-100 is superior to h and e in identifying nerves , whether intact or damaged , even in biopsies lacking subcutis . infiltration with or without fragmentation of nerves is the only reliable indicator of leprosy . absence of nerves in adequate biopsies is not exclusive to leprosy and should not be regarded as decisive . the finding of normal nerves on h and e is a potential source for misdiagnosis as both fragmented and intact nerves are seen in leprosy . we therefore root for performing s-100 immunostain routinely in all biopsies with a clinical suspicion of btl . it is a simple , practical , and extremely valuable adjunct that facilitates early diagnosis and treatment of leprosy . this study reaffirms the role of s-100 immunostaining in identifying nerve involvement in bt leprosy.fragmented and/or infiltrated nerves are specific for leprosy.absence of nerves can not be taken as conclusive proof for leprosy.both intact and infiltrated nerves may be seen in a single biopsy . this study reaffirms the role of s-100 immunostaining in identifying nerve involvement in bt leprosy .
background : histopathologic diagnosis of borderline tuberculoid leprosy ( btl ) is fraught with hurdles . 
 it overlaps with other granulomas and documenting nerve involvement is the key to correct diagnosis . 
 this is difficult on h and e sections alone . 
 s-100 immunostaining may help in this regard.objectives:to study the patterns of nerve involvement in btl and other cutaneous granulomas using s-100 immunostain and compare its sensitivity with that of h and e staining , in both adequate and inadequate biopsies.materials and methods : a total of 20 cases of btl were reviewed . and 
 , 19 biopsies from other cutaneous granulomas were taken as controls . 
 s-100 immunostaining was done on paraffin sections . 
 the pattern of nerve involvement was graded as intact , infiltrated and/or fragmented , intact with perineural inflammation.results:of the 20 cases of btl , s-100 demonstrated infiltrated and/or fragmented nerves in 15 and absent nerves in 5 cases . 
 h and e stain identified neuritis in eight cases . 
 the sensitivity of s-100 and h and e is 0.78 and 0.41 . in the 19 controls , 
 s-100 identified normal nerves in 16 with 7 showing perineural inflammation only and their absence in 2 cases . 
 h and e identified normal nerves in nine cases . 
 the sensitivity of s-100 and h and e is 0.83 and 0.41 . in biopsies where subcutis was absent , 
 the sensitivity of s-100 in identifying nerve involvement is 0.66 compared with h and e 0.33.conclusion:s-100 staining is an efficient ancillary aid in distinguishing btl from other granulomas and is superior to h and e in identifying nerve involvement , even where subcutis is absent . 
 infiltration and/or fragmentation of nerves by s-100 is the only reliable marker of btl .