People with small fiber neuropathy cannot feel the sensation of hot or cold because the disease targets the thermal nerves. The main function of thermal nerves is to convey the sensation of hotness or coldness to the brain so that the brain can register it and send signals to other centers of the body.
This is how thermal nerves work. When a person touches a hot object, the thermal nerves near the skin receive the heat sensation. They carry the impression of hotness to the different centers of the brain. The cerebrum center of the brain turns the processes the impression of hotness to the sensation. The motor center sends a signal to the hands to move away from the hot object.
When the thermal nerves are damaged, they are not able to carry the impression of hotness to the brain. The brain doesn’t receive the hotness impression and so the brain will not be able to give signal to the hands to get off of the hot surface. This is the reason why in the cases of neuropathy in feet, patients have already burned feet and they don’t know about it because they didn’t feel any hot temperature.
The same happens with cold temperature. The reason why people cannot feet hot or cold sensation when they have small fiber neuropathy is because there is a damage to the thermal nerves. The cause of the damage is still being studied up to now.