Formatting docbook indexterm

Question

I am creating a manual and I use the index for listing the functions in alphabetical order. The functions are declared as follows

<itemizedlist>
    <listitem id="period">
        <indexterm><primary><varname>period</varname></primary></indexterm>
        <para>Constructor for <varname>period</varname></para>
        <para><varname>period(timestamptz, timestamptz, left_inc = true, right_inc = false): period</varname></para>
    </listitem>
    <listitem id="timestampset">
        <indexterm><primary><varname>timestampset</varname></primary></indexterm>
        <para>Constructor for <varname>timestampset</varname></para>
        <para><varname>timestampset(timestamptz[]): timestampset</varname></para>
    </listitem>
    [...]
</itemizedlist>

The problem is that the functions appear in the main text in monospaced font but this is not the case in the index despite the fact that the terms are enclosed into the varname element. Any idea how to achieve this ?