Why does skin itch with eczema?

Surprisingly, this is a very difficult question to answer as the current
scientific understanding of itch is really very poor. We do know
that certain small nerve fibres in the skin transmit ‘itch’ signals to
the spinal cord and then to the brain. These same fibres can also
transmit pain signals. There are certain centres in the brain that
receive these signals and then interpret them either as an itch
sensation or sometimes as pain. Two different types of nerve fibre
are involved, one being faster than the other. This explains why
itching can be made up of an early localised pricking sensation
followed by a diffuse itching or burning sensation.
Why people with eczema itch isn’t really known, but it may be
that the dry, inflamed skin of eczema fires off these nerve fibres,
causing the itch. There is, however, also some evidence that these
nerve fibres and the chemical signals (neurotransmitters) that they
contain may be abnormal in eczema. This could mean that the
abnormal itching sensation is the first problem, with the other skin
changes being ‘secondary’ – i.e. being the result of the damage
caused by scratching. A lot more research needs to be done into
the mechanisms of itch before we have a clearer picture.