The ET group saw greater inner retinal impairment independently associated with both longer disease duration and later disease onset, potentially linking greater neurodegenerative burden to late-onset ET.   Photo: Getty Images. Click image to enlarge.

It has only been in the past few decades that growing evidence has been able to change the notion that essential tremor (ET) is not benign but rather a monosymptomatic disease that leads to a progressive neurodegenerative condition. As one new study elucidates, ET represents a heterogeneous condition that may overlap with Parkinson’s disease (PD), even in early stages. Longstanding ET has exhibited a higher risk of PD development, too, especially with the tremor-dominant phenotype.

Consequently, the researchers of this study wanted to make differential diagnosis between the two conditions easier, since such overlap exists. OCT, which is reliable in assessment of neurodegeneration by proxy of the retina, was used here to compare various retinal structures in hope to determine differential diagnosis between ET and PD.

Macular layers and peripapillary retinal nerve fiber layer (RNFL) thickness was assessed among 42 ET eyes, 41 early PD eyes and 33 healthy control eyes. Structures of macular RNFL, ganglion cell layer, inner plexiform layer and inner nuclear layer were all thinner in PD eyes compared with ET, even more so compared with the controls. Considering the tremor-dominant phenotype subgroup, differences between ET and PD were more apparent, especially in the RNFL. ET patients exhibited a negative linear relationship with inner macular layers’ thicknesses with both age of onset and disease duration. Additionally, peripapillary temporal quadrant thinning was seen in ET eyes compared with the controls.

Unlike the one previous study that described retinal pattern in ET and controls using segmentation analysis, this study was the first to compare retinal thickness in ET and PD using complete segmentation of all retinal layers; the prior study only focused on inner retinal layers. However, the results of the previous study were consistent with this one, both finding thinning of the RNFL, ganglion cell layer and inner plexiform layer compared to controls.

The authors outline three other prior studies that compared retinal patterns in PD, ET and control patients that found retinal impairment in PD compared with ET. The first found global foveal thinning in PD patients compared with ET and control patients; the second saw lower parafoveal macular thickness and thinner peripapillary RNFL in PD compared to ET patients. Finally, the third found similar results to the present study, with macular RNFL thinning in PD compared with ET and reduced macular ganglion cell layer thickness in ET compared with controls.

Based on previous research and their current findings, the authors believe that, “taken together, our results provide an evidence of a patterned retinal degeneration involving inner macular layers with a progressively increasing severity in ET and early stages of PD.”

More generally, the authors posit that “the present investigation provides a comprehensive analysis of retinal segmentation in ET in comparison with early PD and HCs, possibly suggesting a continuum between these explored conditions.”