Parkinson's disease (PD) is a neurodegenerative disease, which manifests due to the loss of dopamine cells in the substantia nigra area. Although it is argued that the most important motor dysfunction in PD is bradykinesia, other motor dysfunctions, such as resting tremor, increased muscle rigidity, and decreased postural reflex are also classic symptoms in PD (1). Traditionally, motor deficits are considered to be the key factors affecting postural stability in patients living with this chronic and progressive brain disease. However, impairments of peripheral sensory function may also occur in people with PD and impact the patients' ability to maintain equilibrium. Little has been done to look into the nature and extent of sensory impairments in people with PD. Recently, researchers have started focusing on the effects of impaired peripheral nerve function on a PD patient's ability to adjust postural balance (2–4). Compared with normal subjects, decline of peripheral sensory information is more common in PD individuals. Given peripheral sensory information plays such an essential part in postural balance, research seeking to provide insight into the influence of peripheral nerve function changes in PD patients is desperately needed.

Furthermore, previous reviews (5, 6) have described a link between peripheral nerve disorders and vitamin B₁₂ level, indicating that poor nerve function was associated with decreased vitamin B₁₂ level. The peripheral neuropathy resulting from vitamin B₁₂ deficiency may lead to deficits of postural balance in people with PD. However, few studies have focused on such associations in PD. Therefore, there is an urgent demand on evaluating the associations between peripheral nerve functions and vitamin B₁₂ level in people with PD. This study aimed to evaluate the differences of peripheral nerve function and serum biochemical indexes between people with PD and healthy individuals by using a series of physiological assessments.

All experimental procedures were approved by the Human Research Ethics Committee of Nanjing Brain Hospital, affiliated to Nanjing Medical University.

A total of 50 people (33 males and 17 females; mean age 66.4 ± 8.0 years) with PD and 50 controls (33 males and 17 females; mean age 64 ± 4.8 years) completed all assessments. No significant differences were observed in the demographic characteristics of both groups (Table 1). Based on the results of the clinical tests of disease severity, the PD patients were predominantly at an early stage of disease progression (Hoehn and Yahr score 1.5 ± 1.0) and had mild to moderate symptoms (UPDRS score = 38.7 ± 15.7). Among the 50 PD patients, only 6 patients were on carbidopa and 3 patients were on catechol-O-methyltransferase inhibitors. Levodopa equivalent daily dose and cumulative levodopa dose were calculated according to a previous study (7) (Table 2).

This current study investigated whether there were differences in peripheral nerve functions and serum vitamin B₁₂ levels between PD and control. Whether these changes were associated with balance was also examined. Overall, significant decline in peripheral nerve function was observed in PD and significant positive correlations were also noticed between peripheral nerve function and postural stability. Furthermore, low vitamin B₁₂ levels were associated with deficits of peripheral nerve function and poor balance in PD.

According to the results, it is consistent with previous studies (3, 18) that touch thresholds were increased and perceptions of sensory stimuli were reduced in the PD patients. This study revealed that all stimulation sites around the foot and ankle area demonstrated increased touch thresholds. Specifically, these positions corresponded to four main nerve distributions: lateral plantar, medial plantar, tibial, and sural nerve. Previous research (19) also pointed out that the plantar surface of the foot was spread with both slow and fast adapting cutaneous mechanoreceptors. According to monofilament stimulation, results showed that rapidly adapting receptors accounted for the main part, reaching almost 70% of the skin receptors. As for the PD group, these receptors were likely to be affected preferentially (20). It was noticed that impairments of peripheral sensation occurred in the people with PD; this finding was similar to results from other research that PD patients suffered from diminished proprioception and touch discrimination on the sole (3, 18). Furthermore, Nolano et al. (20) performed a series of skin biopsies for the lower extremities in PD, indicating that there was impaired peripheral sensory and cutaneous denervation. In our study, there was no significant decline in proprioception in people with PD, whereas other studies showed opposite results (3, 21). These contrasting results may be explained by differences in the limbs chosen to assess proprioceptive changes in people with PD. For example, most previous assessments of proprioception in PD patients have assessed the upper limb, whereas the current study reports on lower limb proprioception. It is well-known that the encoding of sensory information destined for the proprioceptive cortex is reliant on muscle spindles, Golgi tendon organs, and joint and cutaneous receptors. However, the upper and lower limbs are characterized by differences in muscle length, nerve conduction velocities, and pathway lengths between sensory receptor sites and the central nervous system (22). Furthermore, given the relatively greater importance of upper limb involvement in the performance of day-to-day tasks, the joints of the upper limb (e.g., elbows, wrists) may be more highly innervated and, hence, capable of sensing a smaller degree of angular change compared with joints of the lower limb (e.g., knee, ankle). Such differences in muscle length, neural path length, and density of innervation may explain the proprioceptive difference reported in this study and highlight the need for specificity when assessing proprioceptive changes in PD patients (23, 24).

In the current study, sensation thresholds of both warm- and cold-induced pain were increased, and even more PD patients failed to respond before the temperature reached the maximum limit. Given this situation, these thermally induced nociceptive thresholds were calculated as conservative estimates according to the maximum values. Previous research provided support for this study, indicating that there were reductions in mechanical pain perception and an increase in thermal thresholds in people with PD (20). Thermal, fast pain, and mechanical stimulations were controlled by Aδ-fibers, whereas transmission of slow pain sensation was controlled by C-fibers. Free ends of nerves were lost in these two types of fibers in PD, leading to deficits in signal transmission through dorsal column–medial lemniscus and spinothalamic tract pathways. As a result, PD patients failed to receive sensory stimulus of temperature and pain. In summary, our findings showed that worse performances were noticed in PD than the control during the whole temperature and pain assessments, collectively indicating deficits in the temperature and nociception sensation in PD.

As for nerve conduction assessments on the tibial and peroneal nerves, both velocity and F-wave frequency were decreased significantly in the PD group. This result was similar to findings in Toth's study (25), indicating that motor conduction velocities were slower in PD patients compared with controls. Toth considered peripheral neuropathy in PD was due to levodopa exposure and high methylmalonic acid levels.

With respect to M-wave amplitudes in the current study, similar muscle activation was noticed for both PD and control groups for the lower limbs. Compared with Abbruzzese et al. (26) who evaluated the peripheral nerve functions on the upper limbs, we assessed electrophysiological functions of the lower limbs. Different positions assessed in the current study may contribute to the differences noticed in the previous study. Although the findings on peroneal and tibial nerves that were in accordance with Abbruzzese's research were the latent phases of F-waves, neither study showed significant differences between the two groups. However, there were fewer PD patients than healthy individuals whose F-wave values were recorded in the current study. When stimulations were transmitted to the spinal cord and back again in PD patients, the breakdown may occur in the pathway. Thus, it cannot be ruled out that motor nerve had deficits on transmission in PD patients.

As for sensory nerve function, the findings in the sural nerve agreed with Toth et al. (25) whose study presented decreases in the velocity and prolonged latency in the PD group. This may also be explained by the biopsy results from Nolano's study (20) that demonstrated impaired sensory receptor functioning in PD, indicating there were potential degeneration of sensory nerve fiber which caused declines in nerve conduction velocity in people with PD. Biopsy results also indicated that there was loss of myelin in peripheral nerve endings and swelled nerve axons in PD patients. These findings also supported the possible mechanism that degeneration in layers of protein and fat around the peripheral nerve axons caused declines in nerve conduction velocity in the PD group (27).

It was also revealed that PD patients performed poorer than healthy individuals according to clinical balance assessments. Tinetti assessments have been widely used in evaluating balance in the aged group including PD individuals (28, 29). Sensory information from the cutaneous mechanoreceptors of the soles were decreased, which in turn may lead to decreased postural stability (30, 31).

Only PD patients demonstrated associations between some of the measurements of peripheral nerve and the clinical balance scores. When the process of information transmission was interrupted, sensations may not be transferred to the brain normally, which in turn impaired the posture adjustment. Compared with healthy individuals, lower scores on the clinical Tinetti assessments were recorded in PD patients, which were closely associated with declines on peripheral nerve functions, touch, temperature, and nociception sensation. These impairments may negatively influence the patients' capacity to safely perform and transition between tasks of daily life (e.g., sitting to standing, walking to turning) and provide insight into the seemingly cautious behaviors of patients' in more complex environments (32).

In this study, there were positive correlations between vitamin B₁₂ and peripheral nerve function and balance scores in PD. Previous research (33) indicated that decreases were evident in vitamin B₁₂ and folic acid in PD patients, who presented with neuropathy syndromes like numbness, pain, and sensory deficits in lower extremities, proving that vitamin B₁₂ deficiency affected thin-myelinated fibers or unmyelinated fibers (34). The decrease in methylation of myelin basic protein leads to myelin damage. Low vitamin B₁₂ also can affect homocysteine metabolism, which plays an essential role in neural axon and Schwann cell. Thus, high homocysteine is associated with low vitamin B₁₂, activating immuno-inflammatory responses, which in turn interferes with the metabolism of dopamine transporter in PD (35). It indicated that low vitamin B₁₂ levels is a critical factor concerning decreased peripheral nerve functions and impaired balance in PD patients. The associations between cumulative levodopa dose and vitamin B₁₂ level in the current study also indicated that vitamin B₁₂ levels were decreased with increasing exposure to levodopa in the PD group. This may be explained by the fact that accumulated levodopa can lead to higher homocysteine, which may pose a potential risk of inducing neuropathy (36, 37). In this study, no correlations were noticed between levodopa equivalent daily dose with other parameters. These findings may have been because most patients in this study were in the early stages of PD; hence, levodopa intakes were still at relatively low daily levels. The possible vitamin B₁₂ deficiency and neuropathy were not obvious until future accumulated. This finding was also in accordance with the results of this study, which indicated that vitamin B₁₂ levels were within the normal range for all subjects, as were the nerve conduction assessments.

Although this study has, for the first time, assessed the peripheral neurological function of PD patients and correlated the nerve function with outcomes derived from common clinical assessment of balance, there are a few limitations to acknowledge. First, due to the clinical design, this study is limited by the lack of an assessment of methylmalonic acid levels, which may be associated with low vitamin B₁₂. Similarly, given our methods, it is difficult to rule out the possibility that some participants presented with impaired glucose intolerance that was sufficient to cause neuropathy. Nevertheless, participants were screened for evidence of diabetes before their enrollment in the study, and all blood glucose levels were within the normal range. Future studies may seek to conduct an oral glucose tolerance test for all participants before their involvement to rule out its possible confounding effects. A further drawback is that we only assessed the dominant leg of each participant to maintain consistency between patients and controls. Finally, this study would have benefited from more patients and from the inclusion of biopsy data, both of which we plan to include in our future studies. These limitations may influence the nature of the results presented and should be noted when considering their implications to the wider PD population.

To summarize, this study revealed that peripheral nerve function was impaired in people with PD. These deficits were noticed in touch sensation, temperature and nociception, and nerve functions. Deficits of sensory input and low vitamin B₁₂ level may lead to declines in postural balance in PD. Given these deficits in sensory function, people with PD may benefit from the prescription of alternate therapies, such as exercise-based interventions (38, 39), to improve the patients' balance and reduce their falls risk.

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

The studies involving human participants were reviewed and approved by Human Research Ethics Committee of Nanjing Brain Hospital, Affiliated to Nanjing Medical University. The patients/participants provided their written informed consent to participate in this study.

FQ and YW conceived the study, designed the experiments, analyzed the data, and wrote the article. HC provided the preprocessed data. BL and HC carried out experiments. MD, HJ, and SL helped to analyze the data and experiments result. All authors contributed to article revision, read, and approved the submitted version.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.