Parkinson’s disease (PD), also known as tremor palsy, is a long-term, slowly progressive degenerative movement disorder of the central nervous system. The main clinical manifestations are resting tremor, myotonia, postural abnormalities and dyskinesia, often accompanied by non-motor symptoms such as olfactory disturbance, dementia, depression, anxiety, and tachypnea. The disease develops chronically and progressively and can involve multiple systems, dominated by degeneration of the nigrostriatal dopaminergic system and the presence of characteristic inclusions of residual neurons – pathological changes of Lewy bodies. Parkinson’s disease affects approximately 7 to 10 million people worldwide, and the incidence of PD in middle-aged and older adults aged >50 years will double in the coming decades. The early symptoms of PD are atypical, and patients are often diagnosed at the middle and late stages of the disease when symptoms appear, missing the best time for treatment and unable to effectively reverse the disease process, so early diagnosis plays an important role in the treatment and prognosis of Parkinson’s disease. However, there is a high rate of misdiagnosis based on early clinical manifestations. Combining clinical manifestations, biochemical markers, imaging manifestations and other auxiliary examinations can improve the accuracy of Parkinson’s disease diagnosis. Pre-motor manifestations: 1. Olfactory organ dysfunction It was first thought that PD was a purely motor disorder disease without sensory system disorders, but recent studies have shown that PD is accompanied by sensory system disorders, especially olfactory disorders, and data from previous studies have shown that 45% to 90% of PD patients have olfactory abnormalities. In a study of risk factors for neuronal degenerative diseases, it was found that olfactory impairment dysfunction increases the risk of Parkinson’s disease, and in immediate family members with Parkinson’s disease olfactory impairment was also found in the progression of Parkinson’s disease. Neuropathological studies have found that Lewy vesicles appear in the olfactory system before the substantia nigra. Autopsy of the human brain revealed neuronal loss and pathological changes in alpha-synuclein in the pear-shaped cortex, which is responsible for olfactory recognition. Tissue biopsies of olfactory epithelial cells from Parkinson’s patients and non-Parkinson’s patients with olfactory dysfunction suggest that olfactory dysfunction is more often due to damage to olfactory epithelial cells. Rodents have a relatively larger olfactory bulb, and in transgenic animal disease models, alpha-synuclein pathology is altered throughout the olfactory system and correlates with motor disease progression. However, there is no correlation between the degree of olfactory pathology and the course and severity of the disease, and most olfactory dysfunctions are asymptomatic, for which there is no good treatment yet. 2. Gastrointestinal dysfunction (1) Constipation Constipation is considered to be the most powerful and longest lasting evidence of premotor Parkinson’s disease, with 60% to 80% of Parkinson’s disease patients suffering from constipation. Little is still known about the pathological mechanisms by which constipation occurs in PD patients. One possible hypothesis is that the prolonged passage of food through the gastrointestinal tract during the preclinical phase of PD may lead to increased absorption of neurotoxins. Another is that Lewy vesicle lesions are found in the intestinal plexus of Parkinson’s patients, which may consequently lead to dopamine physiological dysfunction in the intestinal musculature. Lewy vesicles may affect the dorsal motor nucleus of the vagus nerve, causing constipation and autonomic dysfunction. (2) Salivation is a frequent phenomenon in 30% to 52% of PD patients, which is due to the slow swallowing of PD patients and the accumulation of saliva in the mouth and spillage. The salivation may be aggravated by the poor closure of the lips and the downward slumping of the head in some patients. The mechanism is cholinergic hyperactivity resulting in increased salivation, but some studies have also suggested dysphagia. Swallowing dysfunction may be one of the early symptoms of PD. Potulska et al. confirmed that delayed oropharyngeal and esophageal transit exists early in PD patients and worsens as the disease progresses, probably due to autonomic dysfunction of the vagus nerve. 3, sleep disorders (1) insomnia (insomnia) insomnia includes difficulty falling asleep, easy to wake up, sleep persistence disorder (e.g.: lamellar sleep), etc., which is a manifestation of early sleep abnormalities. The patient’s subjective lack of sleep perception leads to daytime fatigue, dizziness, and other symptoms. Objective criteria are polysomnography (PsG) confirming prolonged sleep latency, increased number of awakenings, waking time > 30 min, and reduced actual sleep time < 6.5 h per night. (2) Rapid eye movement sleep behavior disorder (RBD) RBD is a state of deep sleep caused by abnormal muscle paralysis during rapid eye movement sleep. A large proportion of RBD develops into PD, which indicates Lewy body lesions and neuronal dysfunction in the pontine medulla. The susceptibility to striatal neuron deficits is significantly higher in PD patients with RBD than in those without RBD symptoms, suggesting that early diagnosis of RBD may reduce the risk of further progression. The typical "Parkinson's personality" includes depression, anxiety, apathy, fatigue, and NMS; data show that about 35% to 45% of Parkinson's patients have depressive symptoms. Depression in PD patients is generally mild to moderate and the probability of suicidal ideation is low. The former is manifested by depressive symptoms that do not change even when the motor symptoms of Parkinson's disease improve significantly. Psychogenic, on the other hand, is a mood irritability and other emotions that arise from the fear that the disease will not be cured. It is now recognized that the thalamic nucleus basalis includes not only motor-related nuclei, but also structures related to mood and cognition. Patients can show early decreases in initiative and self-esteem. In contrast, cognitive deficits are mainly reflected in impairments in visuospatial processing and executive functions. Parkinson's disease with depression may be associated with damage to the limbic tract 5-hydroxytryptamine neurotransmission pathway and damage to noradrenergic and dopamine neurons. For cognitive impairment in Parkinson's disease, it has been suggested that it may be associated with the combined action of α-synuclein, tau and β-amyloid. Early manifestations of motor deficits The motor symptoms of PD usually appear insidiously, mostly over the age of 60, and the initial symptoms often go unnoticed by patients The traditional diagnosis of PD relies on one of the combination of bradykinesia with resting tremor, muscle rigidity and loss of postural reflexes. However, some patients have only resting tremor or muscle rigidity without motor bradykinesia. Considering the slow progression of PD, this group of patients should be diagnosed with PD and followed closely. Genetic testing In recent years, more and more PD causative genes are being discovered and it is recognized that genetic factors may play a role in the pathogenesis of PD. A small number (5-10%) of PD patients have been found to present with familial genetic traits. Several variants of the α-synuclein gene have been detected in familial PD patients, and these variants may contribute to the pathogenicity of α-synuclein and the formation of Lewy vesicles. Recently, single mutations in parkin, PINK-1, LRRK-2 and DJ-1 have been found to be associated with the pathogenesis of PD. LRRK-2 mutations are the most common form of mutation identified to date and exert toxic effects upon autophosphorylation or substrate phosphorylation, leading to the formation of PD; the positive detection rate of LRRK-2 mutations in PD families ranges from 5% to 40%. The positive detection rate of LRRK-2 mutations in PD families is 5% to 40%, and up to 2% in patients with sporadic PD. Therefore, detection of LRRK-2 mutations is an important part of screening for people at risk for PD. Although mutations in this gene have been found to be the cause of Parkinson's disease in only a very small number of patients with hereditary PD, it has greatly improved the understanding of the occurrence of Parkinson's disease and will certainly help in the search for new means of treatment for Parkinson's disease, which is equally important for most patients with non-hereditary Parkinson's disease. Currently, genetic factors in the pathogenesis of Parkinson's disease have become a hot topic of research among Parkinson's scholars and are being further investigated. Biological markers Many diseases can rely on the detection of specific molecular markers in the blood or other body fluids to aid in diagnosis. No neurodegenerative disease has yet been diagnosed with such tests, which are still at the experimental stage of research, but have great promise. Some studies have obtained indicators related to oxidative stress from blood or cerebrospinal fluid, including malondialdehyde, reactive oxygen species, 8-hydroxy-2-deoxyguanosine, platelets, pantodecalin, uric acid and glutathione, etc. Studies have shown significant differences between these indicators in PD patients and the normal population. Among them, 8-hydroxy-2-deoxyguanosine is more commonly used to assess oxidative stress. One trial examined and compared urinary 8-hydroxy-2-deoxyguanosine levels in 72 PD patients and normal subjects, and the results showed that the mean value of urinary 8-hydroxy-2-deoxyguanosine increased with the progression of PD and was not affected by the dose of dopamine, which suggested that urinary 8-hydroxy-2- deoxyguanosine is a potentially useful biological marker to assess the progression of PD. Detection of markers in cerebrospinal fluid avoids intra-systemic interference and accurately reflects disease progression in the central system. In addition, serum-based samples as well as cerebrospinal fluid can be collected continuously and can be easily followed longitudinally, which is valuable for predicting disease status. α-synuclein is decreased in the cerebrospinal fluid of PD patients and may be age-related [25]. α-synuclein oligomer to α-synuclein monomer ratio is important for differentiating the stage and rate of disease progression [26]. The level of β-amyloid in the cerebrospinal fluid of PD patients can predict their propensity to develop dementia, although extensive longitudinal clinical follow-up is still needed. Blood uric acid levels. Parkinson's disease, as a neurodegenerative disease, is characterized by the degeneration of dopaminergic neurons in the substantia nigra, and during the degeneration of neurons in Parkinson's disease, the antioxidant system in neuronal cells is impaired, oxidation levels are elevated, and free radicals are produced in large quantities; uric acid is the end product of purine base and nucleic acid metabolism, and as a natural antioxidant in the body, uric acid has the ability to scavenge oxygen radicals, other active radicals, and In Parkinson's disease, the decrease in blood uric acid level leads to the weakening of the toxic effect of oxygen radicals and nitrogen radicals, so the relative increase of oxygen radicals in Parkinson's disease, which leads to cellular function and structural changes, etc. The decrease in blood uric acid level is an important risk factor in the development of Parkinson's disease, which in turn provides a reference basis for the prevention and treatment of Parkinson's disease. Imaging changes 1. transcranial ultrasound imaging (TCS) transcranial Doppler ultrasound imaging (TCS) is a new ultrasound imaging technique for detecting brain parenchyma, which is performed through the intact skull using an auditory bone window. patients with PD show enhanced echogenic signal in the substantia nigra on transcranial Doppler ultrasound imaging. Echo signal in the substantia nigra is increased by 75% in approximately 90% of PD patients compared to controls of the same age, and the echogenic enhancement is more pronounced in the brain tissue contralateral to the involved limb, and the increase is more pronounced in patients with early onset than in those with late onset. This strong echogenicity is not related to the severity of the disease and does not change with the progression of the disease. It can be seen that this strong echogenicity in the substantia nigra is a feature of Parkinson's disease rather than a marker of Parkinson's disease staging or grading. 2. magnetic resonance imaging (MRI) and magnetic resonance spectroscopy imaging (MRS) MRI techniques are important in the diagnosis of Parkinson's disease.7TMRI can detect morphological changes in the substantia nigra in patients with PD [30]. Magnetization transfer MRI can detect melanin decline in PD patients. Diffusion tensor (DTI) MRI can detect a lack of directionality in the nigrostriatal currents in PD patients.MRI volume analysis revealed a significant reduction in cortical volume in PD patients compared to controls, although this reduction may also be related in part to poorer neurophysiological detection methods. New MRI techniques can assess iron content in the midbrain, which may help clarify the risk of developing neurodegenerative Parkinson's disease. MRS is a more recent analytical technique that is more commonly used to perform localization and quantification of metabolic and biochemical studies of living brain tissue. The most commonly used nucleotide lH signal allows the detection of N-acetylaspartate (NAA), choline (cho), creatine (Cr), lactate (hc) and other compounds in living brain tissue, and the calculation of common evaluation indicators such as NAA/Cr or Cho/Cr ratio, which can reflect neuronal and myelin integrity as well as evaluate the degree of their degenerative pathology. It has been demonstrated in animal models of PD that the degree of NAA decline in the substantia nigra densa and striatum is positively correlated with the degree of dopaminergic neuronal deficiency. More and more studies have concluded that MRS has a great role in the early diagnosis of PD, and MRS will become another new field in the early diagnosis of PD. 3.Dopamine function detection Electron emission computed tomography (PET) and single photon emission computed tomography (SPECT) have been widely used in recent years as auxiliary diagnostic tests for various clinical diseases. The main pathological change in PD is degenerative lesions of nigrostriatal dopaminergic neurons in the midbrain, resulting in a deficiency of the nigrostriatal dopaminergic transmitter system. Within the human nigrostriatal dopaminergic transmitter system there are a series of characteristic metabolic enzymes, transporter proteins and receptors that are involved in the synthesis, storage, release, reuptake and biological effects of dopamine. In PD patients, characteristic changes in the metabolic enzymes, transporter proteins and receptors of these neurotransmitters can be distinguished from Parkinson's-like disorders. After radioactive tracers that can bind specifically to these metabolic enzymes, proteins, receptors, etc. are injected into the subject, PET and SPECT perfusion techniques are applied for imaging and measurement to show changes in the distribution, quantity, and other indicators of the metabolites bound by these tracers in vivo for the purpose of diagnosing the disease and assessing its severity. The PET imaging currently used in the field of PD diagnosis includes imaging of dopaminergic system and non-dopaminergic system, and the types of applied ligands are various, including dopamine transporter imaging, type II vesicular monoamine transporter imaging, and dopamine D2 receptor imaging; non-dopaminergic system imaging, including glucose metabolism imaging, microglia imaging, etc. Dopamine transporter imaging can be used to evaluate the functional status of dopaminergic nerve fiber endings in the presynaptic striatum and is currently the most sensitive marker of PD. Jennings et al [37] applied 123I-labeled 2β-methyl-3β-(4-iodophenyl)-tropane (β-CIT) as the imaging agent and studied 35 patients with suspected PD by SPECT imaging to assess The results were 92% diagnostic sensitivity, 100% specificity, and 100% positive predictive value for the differential diagnosis of PD, using PET/SPECT in combination with specific radionuclide tracers. The application of PET/SPECT combined with specific radionuclide tracers can show the characteristic metabolic changes of PD in the in vivo state, which helps to diagnose PD at an early stage and has good application value for staging and efficacy evaluation of PD.