About 1,500 people are diagnosed a year but treatment options are limited.
A mutation to the SOD1 gene, which means the disease can be passed down through families, is behind two per cent of cases of a form of MND called amyotrophic lateral sclerosis.
The mutation means faulty proteins are produced. Instead of breaking down toxic products, they clump together in the brain and spinal cord and damage the neurons.
It is this genetic mutation that the drug tofersen has been shown to target, stopping the faulty proteins from being produced.
In trials, the drug lowered the levels of this protein in cerebrospinal fluid, and the signs in the blood of neuron damage.
But some local NHS services are opting not to offer it because of hospital capacity and its need to be delivered by a lumbar puncture – an injection into the spinal fluid – by skilled staff, once a month.
Some 44 patients are receiving the treatment, the MND Association said, but at least 20 identified patients have been refused.
Only eight of 24 specialist neurology centres that could offer the drug have chosen to do so.
In one case, a patient is travelling from Northern Ireland to Sheffield once a month to receive the treatment.
