CCM is becoming increasingly important in the diagnosis and management of systemic diseases (such as diabetic peripheral neuropathy and autoimmune diseases) and ophthalmic diseases (such as corneal infection and corneal dystrophy) and is a new imaging technology that can be used to non-invasively observe corneal inflammation and sensory fibres in living tissues. To the best of our knowledge, there are few studies on the application of confocal corneal microscopy in MND. This study focused on immune inflammatory cells and nerve fibres in the inferior whorl area of the cornea. Langerhans cells are the main antigen-presenting cells of the cornea. Morphologically, long, large cells and small cells lacking cell dendrites indicate mature and immature phenotypes, respectively. Immature Langerhans cells can capture antigens, while mature Langerhans cells can sensitize naive T cells through the secretion of MHC molecules, interleukin 12, and costimulatory molecules, which are an important part of the immune system. Studies have shown that changes in corneal Langerhans cells are not only related to local inflammation of the eye but also affected by systemic inflammation. The corneal nerve fibres observed by CCM are mainly small sensory fibres of the trigeminal nerve. Studies have shown that the CCM nerve fibre length parameter is a reliable indicator for evaluating corneal nerve fibre damage and repair. Moreover, due to the highly complex pattern of nerves in the inferior whorl area, the main nerves cannot be distinguished from the branches. Therefore, the length of all nerve fibres in the whorl area can be quantified in the form of the IWL. The corneal nerve plexus not only has clinical significance for corneal diseases but also assists in the early assessment of the immune system, early detection of nervous system diseases and detection of late complications of certain systemic diseases, such as diabetes[15–17].
MND is a serious and fatal disease, and all treatable causes should be sought. At present, there are few studies on rheumatic immune disease combined with MND, but such cases are not uncommon in clinical practice. Some patients can improve after standard immunosuppressive therapy in a short time, and their prognosis is better. This study used ROC curves to evaluate the diagnostic value of the LCD and IWL in the inferior whorl area for identifying immune-related MND syndrome. ROC curve analysis is a method that determines sensitivity and specificity to evaluate the accuracy of diagnostic tests. Studies have found that the induction of a certain degree of inflammation not only promotes the regeneration of injured optic nerve axons but also supports the survival of retinal ganglion cells. The results of this study show that LCD>67.7 cells/mm2 generated a sensitivity of 79.5% and a specificity of 72.7%) and IWL>17.4 mm/mm2 generated a sensitivity of 69.2% and a specificity of 66.7%. The LCD had an obviously better diagnostic value than the IWL.
Our research seems to indicate that, compared with traditional rheumatic immune antibodies, Langerhans cells are more sensitive to immune treatment. The patients receiving immunotherapy were followed up for 2 weeks to 6 months, and the LCD was significantly reduced. Furthermore, we assessed the inferior whorl area (which has a unique pattern); it has been shown that it is a reliable marker for longitudinal evaluation of the subcorneal basal nerve plexus. At the same time, we rechecked the serum antibodies of 3 patients after immunotherapy, and we did not see the changes of antibodies.Rheumatic immune antibodies often remain unchanged in the short term. A study of 65 patients with systemic lupus erythematosus (SLE) showed that after standard treatment and 10 years of follow-up, the ANA positive rate only decreased from 95.6–78.6%. Even if rheumatic immune antibodies can change, long-term, standardized immunosuppressive treatment is often required. We did not find obvious changes in nerve fibres, indicating that the progression of neurodegeneration may have been under control.
ALS also has small fibre nerve damage. Bella et al performed skin biopsies on 51 ALS patients and quantified the density of intraepidermal nerve fibres (IENFs). The results showed that all patients had a reduced density of IEFNs, indicating that the neurodegenerative process of ALS affects small fibre nerves. Ferrari et al conducted CCM examination of 8 ALS patients and found that the length of corneal nerve fibres was reduced compared with that in the control group, and it was related to the degree of bulbar involvement. Our work found that the IWL of patients with MND and immune-related MND syndrome was lower than that of the control group (P<0.05), and it also suggested sensory small fibre nerve changes, which is consistent with the abovementioned literature reports. Furthermore, the IWL of immune-related MND syndrome was higher than that of ALS (P<0.05).We suppose that a certain degree of inflammatory factors may have a protective effect on nerves.A certain density of inflammatory cells can promote corneal nerve regeneration, while excessive inflammation may lead to loss of corneal innervation and subsequent neurotrophic keratopathy.
Immune mechanisms may be involved in the pathogenesis of ALS. In animal models, the specific deletion of the C9orf72 gene in mouse myeloid cells leads to lysosome accumulation, a hyperimmune response, and increased expression of the interleukins IL-6 and IL-1β, which changes the immune function of these cells and then causes neurodegeneration. Lu CH et al found that in ALS patients, the levels of TNF-α, IL-1β, IL-2, IL-8, IL-12, IL-4, IL-5, and IL-10 were significantly higher than those of the control group, suggesting that most inflammatory factors of the T cell immune response may be involved in the pathogenesis of ALS. The results of this study found that the LCD of ALS patients was higher than that of the control group (P<0.05), which provides additional evidence for the immune mechanism of ALS.
However, it must be noted that the number of research groups was not large enough. In addition, CCM can only assess the correlation between corneal nerve plexus pathology and disease, and the pathogenesis of the disease needs to be further studied.
In conclusion, this study demonstrated that CCM parameters, especially the LCD, are potential diagnostic tools for immune-related MND syndrome. The LCD is more sensitive to immunosuppressive agents. In addition, our study also provides some evidence for the immune mechanism and small fibre nerve damage of ALS.