Discussion
The patient’s initial neurological syndrome was atrophy of the right leg. The first diagnosis was made between monomelic amyotrophy, spinal muscular atrophy, progressive muscular atrophy, and multifocal motor neuropathy. MMA is characterized by signs of spinal cord atrophy and vertebral canal stenosis, which was not detected on MRI, as well as the absence of sensitive disturbances [3]. SMA is characterized by symmetrical atrophy, when patient suffered amyotrophy and muscle weakness only in right leg for long time. EMG revealed no conduction blocks, and there was low titer for GM-1 and GM-2 antibodies, which made it possible to exclude MMN [4]. As the disease progressed, symptoms of upper motor neuron damage developed, such as brisk reflexes and pathological signs. EMG showed signs of damage to the motor neurons of the anterior horns of the spinal cord in the segments outside the affected right leg. Finally, in 2015 the combination of LMN and UMN clinical signs of one lumbar region, EMG LMN signs in lumbar and cervic regions, made it possible to diagnose clinically probable laboratory supported ALS according to the revised El-Escorial criteria concurrent with hydromyelia [2]. In 2019 the diagnosis was changed to possible ALS.
We have wondered whether the revealed central canal dilation has a clinical manifestation in this patient. Our clinical experience allows us to state that the dilation of the central canal with such small diameter without increasing over time cannot cause progressive severe paresis and atrophy of the lower limb [5].
The origin of the central canal dilation is unclear. The MRI showed no Chiari malformation or other extramedullary factors predisposing to syringomyelia. In addition, there was no history of spinal cord trauma, inflammatory processes, infarction or hemorrhage. We suppose that the presence of hydromyelia is a casual association. For example, Holly and Betzdorf (2002) argued, that slit-like syrinx cavities are remnants of the central canal and can be found in a small percentage of adults [6]. Patients with slit-like syrinx could be neurologically intact or just report the pain in different distributions. Petit-Lacour et al. (2000) demonstrated the incidence of asymptomatic slit-like syrinx could reach approximately 1.5% [7]. Follow-up imaging reveals no changes in cavity size.
However, we have found some case reports of neurodegenerative diseases concurrent with hydromyelia. In one case, Masciullo et al. (2016) described a patient with SPG56 – rare form of hereditary spastic paraplegia, caused by mutations in CYP2U1 and concurrent with hydromyelia [8]. The authors proposed that the presence of hydromyelia is not a casual association, but it may be part of the phenotype of SPG56. The authors supposed the hydromyelia in these cases is linked to mitochondrial dysfunction. It supported by the cases of hydromyelia described in patients with CMT2A and harboring mutations in MFN2 [9]. In the study Bombelli F. et al. have found an involvement of the CNS in the pathological process in these patients with CMT2A: 26% of patients develop CM abnormalities, 9% of patients develop hydromyelia. The authors also believe that the spinal cord cavity in these patients is not a coincidence, but is part of the phenotype. Both CYP2U1 and MFN2 encode enzymes regulating energy metabolism and mitochondrial dynamics. In recent ALS reviews mitochondrial dysfunction is indicated as one of the pathogenic links [10].