Corneal sub-basal nerve plexus density in Parkinson’s disease

Purpose : The objective of this case-control study was to examine the ocular surface in patients with moderately severe Parkinson’s disease, including confocal microscopy of the corneal sub-basal nerve plexus. Blink abnormalities, including reduced blink rate are frequently reported in Parkinson's disease. Additionally, non-motor involvement, including visual system abnormalities, is now recognised as an important feature of the disease.
Methods : Fifteen patients with moderately severe Parkinson’s disease (Hoehn and Yahr grade 3 or 4) and fifteen controls were recruited. Participants were excluded if they had any condition that could affect sub-basal nerve density. Ophthalmic assessment included slit-lamp examination, non-invasive tear break up time, tear stability and lipid layer thickness, tear quantity, central corneal aesthesiometry, in vivo corneal confocal microscopy (to measure corneal sub-basal nerve density) and blink rate assessment. One eye of each patient was included in statistical analysis. Autonomic dysfunction was assessed using the Survey of Autonomic Symptoms.
Results : Of the 15 Parkinson’s disease patients, ten were male and the mean age was 65.5 ± 8.6 years. Control participants were matched for age and gender. Four patients had a Hoehn and Yahr grading of 4, and 11 patients grade 3. The sub-basal nerve plexus density was reduced in patients with Parkinson’s disease compared with control participants (7.56 ± 2.4 mm/mm2 vs. 15.91 ± 2.6 mm/mm2, p<0.0001). There was no statistical difference in corneal sensitivity (p = 0.12), tear quantity (p =0.76) or tear stability (p = 0.10) between patients and controls. Lipid layer thickness was reduced in the Parkinson’s disease group (p = 0.02). There was a significant positive correlation between blink rate and sub-basal nerve density, although this was primarily due to the presence of an outlier. There was not a significant relationship between sub-basal nerve density and autonomic dysfunction.
Conclusions : This is the first study to report a reduction in corneal sub-basal nerve density in Parkinson’s disease, suggesting peripheral nervous system involvement. Parkinson’s disease has previously been thought to be a central nervous system disorder, although more recently there have been a number of reports providing evidence of peripheral neuropathy in the disease. The pathogenic mechanisms underlying peripheral neuropathy in Parkinson’s disease are not yet fully understood.