Optical Coherence Tomography as a Biomarker in the Differential Diagnosis between Parkinson's Disease and Atypical Parkinsonian Syndromes: A Narrative Review  
[PERSON]  
1 2 nd Department of Neurology, AHEPA University Hospital, Aristotle University of Thessaloniki, 546 36 Thessaloniki, Greece; [EMAIL_ADDRESS] (S.K.); [EMAIL_ADDRESS] (D.P.); [EMAIL_ADDRESS] (T.A.)  
[PERSON]  
1 2 nd Department of Neurology, AHEPA University Hospital, Aristotle University of Thessaloniki, 546 36 Thessaloniki, Greece; [EMAIL_ADDRESS] (S.K.); [EMAIL_ADDRESS] (D.P.); [EMAIL_ADDRESS] (T.A.)  
[PERSON]  
2 2  
recently [11], a growing need has emerged for prompt and accurate differential diagnoses between PD and atypical Parkinsonian syndromes. Being able to identify the nature of each patient's disease is of great importance in terms of overall management and decision making [1].  
It is noteworthy that atypical Parkinsonian syndromes are accompanied by distinct clinical features: vertical spranuclear gaze palsy in PSP, autonomic nervous system dysfunction in MSA, alien limb phenomena in CBS, and fluctuating cognition in LBD being among them [1]. However, clinical information alone has not proved sufficient for valid differential diagnoses between PD and non-PD disorders. Thus, in a setting of Parkinsonism syndrome, biomarker-based approaches might be of added diagnostic value in an attempt to provide valuable insight into the further classification of Parkinsonism. To date, several biomarkers have been examined as adjunctive tools within the investigation process of Parkinsonism, yielding inconclusive results.  
It is well established that movement disorders as a result of central nervous system (CNS) degeneration are usually coupled with a varying degree of retinal degeneration, as well as clinically evident abnormalities of visual system [12; 13; 14]. Specific patterns of retinal alterations may be identified through the implementation of optical coherence tomography (OCT) techniques within this group of patients. Introduced in medical imaging in 1991, OCT enables the imaging of internal structures in biological tissues through measures of optical reflections within a two-dimensional map [15]. Thus, the detection of subtle pathological alterations within biological tissues, including the retina, is made possible [16]. Representing a transparent multi-layer structure with distinct morphological features, the retina may be efficiently assessed by OCT [15; 16]. OCT represents a non-invasive, readily available, reproducible, and cost-effective technique that enables in vivo visualization of the retina and consequently provides valuable information regarding subtle alterations of retinal morphology [17]. Similarly to basal ganglia, the retina is characterized by a high dopamine concentration, thus highlighting the potential role of OCT in detecting early dopamine dysfunction within the retina, and subsequently in the timely diagnosis and prognostication of Parkinsonism [18]. Revolutionizing retinal imaging since the 1990s [15], OCT may serve as a surrogate marker that potentially reflects the underlying brain degeneration process in a setting of Parkinsonism.  
Taking into consideration the urgent need for the efficient differential diagnosis of a Parkinsonian syndrome and the potential utility of OCT on distinguishing between PD and Parkinson-plus syndromes, the objective of our study was to review all the available literature published within the last decade dealing with OCT as a tool within patients presenting with Parkinsonism.