INTRODUCTION

Hemoglobin E (HbE) is the most common abnormal hemoglobin in Southeast Asians, especially among the Khmer, Laotians, and Mon/Khmer speaking people, the Zhuang in Guangxi, People's Republic of China, and in India, Bangladesh, and Sri Lanka. The occurrence of HbE is most concentrated at the border of Thailand, Laos, and Cambodia, an area dubbed the HbE triangle. The gene frequency of HbE is between 0.05 and 0.10, reaching 0.5 in certain parts of Cambodia and the northeast of Thailand. It is estimated that 30 million Southeast Asians are heterozygous for HbE and 1 million are homozygous. The maintenance of so high a gene frequency indicates that the HbE variant somehow improves fitness. Correlation of HbE frequency with the incidence of malaria has been noted, suggesting the action of a balanced polymorphism. Studies of malaria and HbE are detailed in Chapter 26.

CLASSIFICATION OF HbE DISORDERS

In 1954, HbE became the fourth abnormal hemoglobin to be identified by electrophoresis, and a substitution of lysine for glutamic acid at position 26 of the β-globin chain was found in 1961. Many different syndromes are observed when HbE is variously combined with different α and β thalassemias and with other abnormal hemoglobins (Fig. 18.1). They can be classified into asymptomatic and symptomatic forms (Table 18.1).

Asymptomatic Forms

HbE Heterozygotes. HbE heterozygotes are clinically normal with minimal changes in blood counts and erythrocyte indices.