Aminopyridines, a family of compounds to which Fampridine belongs, works by lowering the requirements for axons (the electrical cabling of nerves) by blocking a specific group of proteins on their surface called voltage-gated potassium channels. This makes it more likely for an electrical impulse or action potential to be transmitted across a demyelinated segment of an axon. Although this will improve motor function the effect on sensory and other pathways may make some problems worse, for example exacerbation of pain and increase in the frequency and severity of MS-related positive symptoms (pins & needles, muscle spasms and seizures).
Aminopyridines will almost certainly increase the energy requirements of damaged, vulnerable, demyelinated axons as they will require more energy for repolarization the process by which they get ready to transmit another electrical signal. There is now good evidence that increasing the energy requirements of axons may result in further axonal injury and loss. Essentially this is the theory underlying the use of sodium channel blockers, such as phenytoin or lamotrigine, as neuroprotective compounds in MS; by reducing transmission you reduce the energy requirements and hence protect vulnerable axons.
For these reasons I am wary about the long-term use of Fampridine in MS-related motor fatigue. I am worried that Fampridine may speed up the rate of disability progression. I stand to be proved incorrect on this; we will only find this out by doing clinical trials and following up patients with progressive MS on these medications for long periods of time using standardised methods.
I have posted Hugh Bostock’s videos, from Queen Square, on conduction in a normal nerve and a demyelinated nerve, on YouTube, to illustrate how slow and difficult it is to transmit an electrical impulse down a demyelinated nerve. When you watch the videos please try and imagine how much more effort/energy is required for conduction in demyelinated axons; aminopyridines are the chemical whips that keeps these axons firing.