Optical coherence tomography as a tool to evaluate retinal changes in Parkinson's disease

Highlights

• We evaluated the retinal changes in Parkinson's disease (PD).

• Patients had reduced central macular thickness (CMT) compared to controls.

• Outer retinal layer was thinner in the patients.

• There was a negative correlation of RNFL thickness and CMT with severity of PD.

• These findings suggest a remote possibility of dopaminergic depletion in retina in PD.

Abstract

Background

Though Parkinson's disease (PD) is primarily a disease of the basal ganglia, recent evidence suggests that PD affects the retina.

Objective

The study was designed to evaluate the thickness of retinal nerve fiber layer (RNFL) and thickness and volume of the macula in PD and hence explore the utility of optical coherence tomography (OCT) in studying retinal changes in PD.

Methods

A prospective, hospital based evaluation of 30 patients with PD and 30 healthy controls was carried out. Various parameters such as RNFL, central macular thickness (CMT), central and total macular volumes (TMV) and retinal thickness were analyzed using OCT.

Results

(a) RNFL thickness was not significantly different between the patients and controls. A significant negative correlation was found between the RNFL thickness in the right nasal superior sector and the UPDRS motor score. (b) CMT was found to be significantly reduced in the right eye and a negative correlation with the UPDRS motor score was noted. (c) TMV was significantly greater in patients compared to the controls. (d) The outer retinal layer in the right nasal quadrant and the right inferior quadrants were found to be significantly thinner in patients with PD.

Conclusions

We did not find any significant abnormality in the RNFL thickness in patients with PD. Decreased CMT in patients with PD and a significant negative correlation of RNFL thickness and CMT with severity of PD suggest a remote possibility of dopaminergic depletion in the retina. However long term studies are warranted to validate our findings.

Introduction

Parkinson's disease (PD) is a neurodegenerative disorder characterized by tremor, rigidity, bradykinesia and postural instability [1]. In addition to motor symptoms, non-motor symptoms such as apathy, anxiety, depression, fatigue, memory disturbances, sensory impairment, sleep disorders and autonomic disturbances contribute to the morbidity [2]. Diminished visual acuity, color vision and contrast sensitivity are some of the visual disturbances described in PD [3], [4], [5]. Dopamine dysfunction in PD is seen not only in the basal ganglia but also in the retina especially in the horizontal, amacrine, bipolar and ganglion cells [6]. In an autopsy study of eight patients, Harnois et al., found that the dopamine content of retina is decreased in PD [7]. Most of the ganglion cell layer in the retina is found in the macula [8].

Optical coherence tomography (OCT) is a relatively new, non-invasive, noncontact trans-pupillary imaging technology which provides high resolution, cross sectional images of ocular and biological structures. OCT has been reported to be useful to assess a variety of ophthalmic conditions, including glaucoma, retinal diseases like diabetic retinopathy and retinal venous occlusion that can cause macular edema [9]. In patients with PD, OCT studies have yielded varying results. Some studies have found significant thinning of retinal nerve fiber layer (RNFL) in patients with PD [10], [11], [12], [13], [14], [15], [16], [17], [18]. Loss of RNFL in temporal quadrant was found in several studies [11], [12], [13], [14], [15], [16], [17]. This pattern of loss was similar to that found in Leber's hereditary optic neuropathy and Dominant optic atrophy, both of which are mitochondrial cytopathies associated with Complex-1 defect; La Morgia et al. proposed that mitochondrial defects may play an important role in the etiopathogenesis of PD [17].

On the contrary, in another recent study of 34 patients with PD and 17 healthy controls, Archibald et al. found no differences in the RNFL [19]. Four other studies also found no differences in the RNFL thickness between patients and controls [20], [21], [22], [23].

A significant reduction in central macular thickness (CMT) [14], [15], [24] and volume was found in patients with PD [10], [25]. Patients with PD were also found to have significantly thinner inner retinal layers (IRL) as compared to controls [10], [14], [15], [26], [27].

The objective of our study was to evaluate the thickness of RNFL and retina and macular volumes in patients with PD and hence explore the utility of OCT as a tool to detect retinal changes, if any, in PD.