Background Cognitive impairment in Parkinson’s disease (PD) adversely influences quality of life. There is currently no available biomarker to predict cognitive decline in PD. PD involves both the central and peripheral nervous system and especially small fiber damage occurs in PD. Corneal confocal microscopy (CCM) has been used as a non-invasive tool for quantifying small nerve fibre damage in PD. The present study investigated whether corneal nerve measures were associated with cognitive function in PD.
Methods Patients with PD were classified into those with normal cognitive function (PD-CN), mild cognitive impairment (PD-MCI), and dementia (PDD). Corneal nerve fiber density (CNFD), corneal nerve branch density (CNBD) and corneal nerve fiber length (CNFL) were quantified with CCM and compared with a control group.
Results Sixty-five PD patients (44.62% male; mean age 64.60±6.95 years; mean disease duration 4.63±2.53 years) and 30 controls (53.33% male; mean age 62.43±6.16 years) were studied. CNFD was decreased and CNBD was increased in PD patients compared to controls ( P <0.05). CNFD decreased progressively with decline in cognitive function in PD patients. CNBD and CNBD/CNFD ratio was higher in PD-CN compared to controls but decreased with worsening cognitive function in PD-MCI and PDD patients. CNFD correlated with the Montreal cognitive assessment (MoCA) score ( r =0.683, P <0.0001), unified Parkinson disease rating scale (UPDRS)-part III ( r =-0.481, P <0.0001) and total UPDRS scores ( r =-0.401, P <0.0001) in PD patients. CNFD, CNBD, CNFL were lower and CNBD/CNFD ratio was higher with increasing Hoehn and Yahr stage. There was no correlation between CNFD and Levodopa equivalent daily dose (LEDD) ( r =0.176, P =0.161). CNFD, CNBD and CNFL could discriminate between PD-MCI and PD-CN with an area under the curve (AUC) of 82.85%, 67.47%, and 78.74%, respectively. CNFD, CNBD and CNFL could discriminate between PDD and PD-CN with an AUC of 96.67%, 90.12% and 84.44%. A combination of all three CCM parameters further increased the AUC value.
Conclusions PD patients show evidence of corneal nerve loss compared with controls and corneal nerve parameters are associated with the severity of cognitive and motor dysfunction in PD. CCM could serve as an objective in vivo ophthalmic imaging technique to assess neurodegeneration in PD.