Background:
In the absence of visual feedback, humans depend upon proprioceptive information for reaching movements and coordination. Use of sensory information in order to assist movement is impaired in patients with early Parkinson's disease (PD). It has been postulated that patients with PD compensate for this kinaesthetic deficit by relying on visual information.
Case presentation:
We report two farmers who first noticed symptoms of PD when working on the farm in situations requiring processing of the proprioceptive/kinaesthetic information in order to execute motor output in the absence of visual cues. A 68-year-old right-handed farmer had a 5-year history of left hand awkwardness. He first noticed the problem while performing artificial insemination in cattle using the recto-vaginal technique. He was diagnosed with PD 15 months after his initial symptoms and responded well to a combination of carbidopa-levodopa and ropinirole but has not returned to performing artificial insemination since. Clinical examination revealed asymmetric parkinsonism with normal sensation on gross neurological examination (including proprioception). A 60-year-old right-handed farmer had a 6-year history of difficulty manoeuvring his right hand whilst turning the calf during calving only when he did not have direct sight of his hand. 18 months later he developed right hand tremor and bradykinesia. He was diagnosed with PD 2 years following these initial symptoms. He had a good response to a combination of carbidopa-levodopa, rasagiline and ropinirole. He switched to using his left hand during calf delivery but is not as dextrous as previously. Clinical examination revealed parkinsonism, more marked in the right hand and normal sensation in all modalities.
Conclusions:
Although task-specific motor impairment could explain the symptoms of these patients, it is noteworthy that loss of visual feedback was central to both of these presentations. Given that early kinaesthetic deficits are known to be present in patients with PD, we postulate that removing visual feedback can expose such deficits in early PD, even when not detectable on routine examination. These two patients suggest that sensorimotor control can be impaired early in PD and may be the first symptom. Once the visual compensatory mechanism is not available, it becomes difficult or impossible to perform complex hand movements.