Author's Reply.

Sir,

I read with interest the letter by Dr. Daniele Torchia with many valuable comments on my article ‘Pigmentary nevi on the face have unique patterns and implications: the concept of Blaschko's lines for pigmentary nevi’.[1]

He argued that actually, most nevi taken into consideration for the study were known to follow, the checkerboard/block-like pattern of cutaneous mosaicism and not followed Blaschko's lines.

To my experience, unlike epidermal nevi, incontinentia pigmenti or other non pigmentary nevi that often present as curvilinear lesions, most of the pigmentary nevi appear as patch. So their distribution and pattern hardly can be matched to Blaschko's line that is a hypothetical linear structure. In my article also, many of the pigmentary nevi had a sizeable width except few that appeared linear (curvilinear to be specific).

Thus, it was not very unusual to have an impression that these were actually distributed in checkerboard/block-like pattern. I find logic for his comment “it is very likely that the distribution pattern displayed in Sarma's study is, in fact, the block-like one.”

However, I feel, had Dr. Daniele Torchia read the article with little more attention, he could have easily appreciated my observation, which was new and unique.

Unlike Blaschko's line that has well defined patterns; ‘block’ or ‘checkerboard’ pattern is less defined. Thus, there is a tendency towards mentioning the later pattern for many patches of different shapes without having the classical distribution. I believe that categorizing the pattern of nevi in the current article under ‘block-like pattern’ would have simply neglected the actual pattern.

What I found and intended to depict that the marginal lines of these patches followed some patterns. Such patterns, despite having some noticeable but minor differences were easily appreciated as Blaschko's lines on the face. Each and every photograph and the manually drawn outline can be helpful to appreciate the similarity in distribution. What apparently appeared as a block in superficial examination actually had some pattern in their outline.

Thus, it was neither my intention to negate the theory of ‘checkerboard or block-like pattern of mosaicism’ in pigmentary nevi nor whether most of the pigmentary nevoid patches can be put under ‘checkerboard or block-like pattern of mosaicism’. I actually tried to define the pattern. What I found was the interrelationship between checkerboard or block-like pattern of mosaicism and Blaschko's lines. This was unique. Dr. Torchia probably missed this finding due to hurried reading and commented very superficially.

Considering the crucial role, Blaschko's line in understanding the mosaicism, particularly in understanding the development and migration of melanocyte, I have hypothesized the existence of ‘embryological pigmentary unit’ encircled by these modified Blaschko's lines that outlined each nevi. Thus innumerable number of units is possible depending upon the time of mutation.

I agree with Dr. Torchia, that face is a small area to correctly appreciate the pattern of nevi. However, I must say; only face can offer so excellent scope to visualize the complex pattern of mosaicism. Thus, face was divided into few ‘embryological pigmentary segments’ with distinct characteristics and differences between each segment.

Finally, I have mentioned the list of pigmentary nevi that have been described to follow Blaschko's line[2-7] in Table 1. I think, it will be helpful to understand that nevus lentiginosus linearis is not the only pigmentary nevi to follow Blaschko's line as mentioned by Dr. Torchia.

Table 1

Conditions presenting with pigmentary nevi a linear distribution or distribution along lines of Blaschko