Parkinson’s Disease (PD) is predominantly caused by cell death of dopaminergic neurons in the substantia nigra, part of the basal ganglia circuitry, although deterioration of other neuronal structures and pathways has also been reported (Dauer, 2003).
In PD the main clinical complaints are related to motor control like freezing, rigidity, bradykinesia, or resting tremor. Other Parkinson-related problems are in facial expressions, speech, mood, and cognition. Additionally, postural and balance control problems are reported (Dauer, 2003). The connection between peripheral nerve function and postural control has little been explored in this population (Qiu, 2020).
Qiu et al. from the Nanjing Brain Hospital in China, set to assess peripheral nerve function in PD patients.
Fifty patients with PD and fifty age and gender matched healthy controls were recruited. Measures taken for comparison were: venous folic acid and B12 levels. The Tinetti scale was used to assess postural balance and gait. Surface electroneurography was used to test peripheral motor and sensory nerve function of the dominant leg. Proprioception of the lower extremity was tested five times with variation of three positions of the knee while seated. Touch was tested on ten different locations on the foot using Semmes-Weinstein monofilaments.
Peripheral sensory small fiber nerve functioning was assessed using Quantitative Sensory Testing (QST) on the dorsum of the dominant foot. For temperature sensation Medoc’s TSA-II was used with the method of limits, to assess cold sensation and warm sensation threshold. While nociception was tested with the limits method for heat-induced pain and cold-induced pain.
How Parkinson’s Disease patients differ in peripheral nerve function
Even though PD patients were mostly in the early stage of their disease (average Hoehn and Yahr score 1.5 ± 1.0), and with mild-to-moderate PD symptoms, they had several measures deviating from the controls. Both B12 and folic acid levels were significantly lower in PD patients although within normal range. Similarly, nerve conduction studies of both motor and sensory nerves were significantly slower in PD compared to controls. Postural control and touch sensation were also reduced in PD.
Interestingly, in cold sensation and warm sensation thresholds there were significant differences between PD patients and controls – showing a relative insensitivity to thermal sensations for PD patients. This was not significant in heat-induced pain and cold-induced pain thresholds. Touch also affected, resulting in a significant decrease in sensation for PD.
What does thermal sensation got to do with balance?
Correlation analysis showed that PD patients with better sensitivity to cold, also had better balance scores. For thermal pain thresholds, increased thresholds (lower sensitivity to thermal pain) were associated with decreased balance in PD, but not in healthy controls.
This study showed deficits in thermal perception and thermal nociception in patients with PD. These deficits were accompanied by dysfunction of other sensory and peripheral motor nerve functions. Although on face value thermal thresholds don’t appear to be related to balance and gait, thermal functioning seemed to be connected to these important essential skills used in activities of daily living like balance and gait.