Trichothiodystrophy and fragile hair: the distinction between diagnostic signs and diagnostic labels in childhood hair disease

Summary Background  Hair typically becomes fragile when there are structural abnormalities and/or a reduction in the sulphur‐containing amino acids cystine or methionine. This finding in the setting of a neuroectodermal complaint is usually labelled trichothiodystrophy (TTD). The spectrum of features within this diagnostic grouping tests the validity of using sulphur‐deficient hair as a central characteristic. Objectives  To determine what diagnoses were found within a group of subjects with fragile hair and whether low cystine or methionine were relevant central characteristics. Methods  We examined cases referred to us from 12 U.K. centres for hair microscopy over 10 years where hair fragility or clinical characteristics raised the possibility of TTD. All samples underwent amino acid analysis. This was achieved through cation exchange chromatography coupled with spectrophotometric quantification. Results  Twenty‐five patients (11 male, 14 female) with a mean age of 11 years (0·3–37) were evaluated. Nineteen patients had features of hair damage. Of these, five patients had abnormalities on microscopy only and four patients had microscopic changes and tiger‐tail pattern but normal amino acid content. The remaining 10 patients had reduced cystine content, two of whom also had low methionine. All but one had the tiger‐tail pattern. Among the wide range of phenotypes there were only three cases matching a diagnosis of TTD. Conclusions  Our data suggest that clinically apparent fragile hair in childhood is only rarely associated with a diagnosis of TTD. The tiger‐tail change is sensitive but not wholly specific to TTD. We propose that the term trichothiodystrophy be limited in its use to define sulphur‐deficient hair rather than as a diagnostic term in a heterogeneous and incoherent multisystem disorder, where sulphur‐deficient hair is one feature.