Background: Pili annulati is an inherited hair shaft abnormality with a wide range of clinical expression.
Objective: We have examined closely three kindreds to reveal levels and character of expression of the phenotype and supplement current literature on the threshold for detection and aspects of hair shaft fragility.
Patients and methods: Eleven cases of pili annulati from three families were included in a clinical and morphological study. All cases were assessed clinically and by light and scanning electron microscopy (SEM) of hair shafts. In addition, transmission electron microscopy (TEM) (four patients) and amino acid analysis (three patients) were undertaken on clinically overt cases. Results Examination by light microscopy with a fluid mountant was more sensitive than clinical examination, increasing the detection rate by 120%. Microscopic examination revealed that the characteristic periodic bands become less frequent distally in the hair shaft. Microscopic features of weathering were found in two cases, adding pili annulati to the list of structural hair shaft dystrophies that may weaken hair and dispose to weathering. Amino acid analysis of the hair of three patients with pili annulati showed elevated lysine and decreased cystine content compared to 12 normal controls, consistent with the reduced threshold for weathering.
Conclusion: Careful light microscopy with fluid-mounted hair is needed to detect subjects mildly affected by pili annulati. Expression of the phenotype varies widely between individuals, between hairs and within hairs of the same individual, where ageing of the hair diminishes detectable features.