Postural instability is one of the axial symptoms of PD and it is due to the loss of postural reflexes. Although it is not common in early stages it is one of the most common distressing factors for patients in later stages.
If you find postural instability early in the disease you should consider a different condition such as Progresive Supranuclear Palsy (PSP), where patients usually fall backwards, Multiple System Atrophy (MSA), vascular disease or other. Fear of falls tend to limit patient's normal activities and some studies have shown that the median survival of patients is reduced to about six years from the moment frequent falls start presenting.
If you are comfortable performing a pull testi, ability to maintain balance with this test is an important measure of disability. However, done improperly, it can give a false sense of balance or in the worst case, the patient and examiner fall. Please perform this test only if you have been instructed on the proper technique”.
Postural instability is the least treatable of the major motor features of PD.
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Wolters ECh. Parkinsonism and Related Disorders. Amsterdan: VU University Press; 2007.