Our patient harboured m.11778G > A, the most common mtDNA variant associated with LHON . He experienced the loss of vision typical for the disease at age 46 and developed myelopathy symptoms within 2 years thereafter. MR imaging of the central nervous system and CSF investigation showed normal results, but the findings in evoked potentials investigation were suggestive of posterior column dysfunction, either at cervical level or reflecting a more diffuse posterior column dysfunction.
There are previous reports of myelitis or myelopathy in patients with LHON [8,9,10,11,12]. In these cases, however, MR imaging of the spinal cord has been abnormal or signs of inflammation or autoimmune activation have been observed in the CSF. In most cases, the tone of the lower limb muscles has been reported to be increased. Autopsy studies of LHON patients have revealed spinal cord degeneration particularly in the posterior column and posterior spinal roots, but Leigh-like subacute infarction of the spinal cord has also been reported . Serum copper levels were not measured. Copper deficiency myelopathy would be an unlikely diagnosis as the patient was male, there was no anaemia or other cytopenia, and spinal cord MR imaging was normal . The patient was not tested for syphilis, which is today rare in Finland. There was no medical history of syphilis, and there was no pleocytosis in the CSF. Moreover, several features suggestive of tabes dorsalis, such as Argyll Robertson pupils, lancinating limb pain, and Charcot joints, were not present .
Despite clinical examination and MR imaging, the conclusive diagnosis or exclusion of myelopathy is sometimes difficult. Evoked potential studies are sensitive tools to detect even subtle central nervous system lesions. Standard SEP technique assesses mainly the function of the posterior column–lemniscal system . Abnormalities of evoked responses reflect the global damage of the evoked nervous pathway and are more sensitive than MR imaging to reveal spinal cord lesions in MS ; SEP investigation is also more sensitive than spinal cord MR imaging in detecting subacute combined degeneration of the spinal cord caused by vitamin B12 deficiency . In a previous study, abnormal findings in both MEP and SEP studies were common in patients with various types of mitochondrial disease, but no data on LHON patients were included .
LHON epidemiology has been previously studied in several countries, including Finland. The prevalence of LHON has been reported to be 2.0/100,000 in Finland  and 3.7/100,000 in the North East of England . In the Finnish study, the penetrance was 31% among men and 8% among women in families with the homoplasmic m.11778G > A mutation, but it was highly variable between families .
Even though myelopathy is uncommon in LHON and the reported cases remain sparse, the link between LHON and the development of myelopathy is probable. The development of myelopathy in LHON is plausibly related to the respiratory chain dysfunction in mitochondrial disease, and the dorsal column seems to be particularly vulnerable [9, 12]. Our findings suggest that evoked potential investigations may assist in confirming the diagnosis, when clinical features are in line with possible myelopathy but findings in CSF analysis and central nervous system imaging are normal.