Postural Instability

Etiology

Posture balance control is a multifactorial activity that is dependent on the integration of motor, sensory, visual, cognitive, and vestibular neural networks. A disruption in any of these circuits can lead to postural instability. The specific etiology of postural instability is difficult to identify because it is a heterogeneous disorder, and there is much ground left to uncover regarding knowledge of the pathogenesis of the disease.[1]

In a healthy individual, balance control relies on visual, somatosensory, and vestibular information. The central nervous system aggregates sensory signals, and subsequently, a corrective motor response is produced. Slower visual processing speed, disturbances in visuospatial orientation, and decreased visual acuity, as seen in Parkinson disease, can create impediments in motor function and the ability to stabilize posture. Along with visual inputs, vestibular and proprioceptive cues are necessary to maintain an upright stance. Vestibular receptors detect deviations and accelerations in head orientation. In Parkinson disease, tilting reactions are impaired, which suggests that labyrinthine postural reactions are centrally integrated with the basal ganglia. Vestibular dysfunction, however, does not appear to fully explain postural deficits in patients with a mild to moderate degree of Parkinson disease.

Somatosensory system input for postural stability involves (1) the conscious perception of passive or active motion and direction of movements, and (2) the processing of proprioceptive information from muscle spindles and cutaneous and joint receptors. The basal ganglia are known to be involved in the integration of proprioceptive signals for the coordination of posture and movement. Proprioception is significant in controlling postural reactions induced by unexpected stimuli.[3]

Epidemiology

Postural instability is generally associated with Parkinson disease, which is the most common neurodegenerative disease following Alzheimer disease. The prevalence of Parkinson disease in the general population is 0.3% and 1% in people older than 65 years of age. Postural instability occurs in approximately 16% of Parkinson disease patients. Falls resulting from postural instability occur in approximately 60% of patients with Parkinson disease. Internationally, falls and fractures are the reason for about 75% of the total hospitalizations in patients with Parkinson disease.[1][4]

Pathophysiology

The pathogenesis of postural instability has not been completely elucidated, but it is known to involve the basal ganglia, which is an essential part of the brain involved in maintaining balance. Although the basal ganglia are hypodopaminergic in Parkinson disease, some patients with postural instability still remain unresponsive to dopamine replacement therapy. This supports that the pathology is non-dopaminergic. As the basal ganglia are perturbed in Parkinson disease, compensatory input from other cognitive, motor, and sensory portions of the brain acts to provide better orientation and stability.[1]

History and Physical

Taking a thorough history and completing a good physical examination is paramount to narrowing down the differential diagnosis for postural instability. Essential information to gain from patients who may have postural instability include associated symptoms and both modifiable and nonmodifiable factors. Modifiable factors include environmental factors, lifestyle changes, chemical exposures, and stress. Age is an example of a nonmodifiable risk factor.

Risk factors for Parkinson disease should be considered, including smoking cigarettes, drinking alcohol, coffee consumption, pesticide exposure, and living in rural areas. Some food agents such as docosahexaenoic acid (DHA) and vitamin D have been shown to have neuroprotective effects. Therefore, screening for vitamin D deficiency might help estimate postural instability risk in patients with Parkinson disease. Old age is a contributing factor for all the motor symptoms of Parkinson disease, including postural instability.[1]

Patients with postural instability may describe falling when rising from a chair or bed, or falling backward while sitting. It is important to ask about any frequent history of falls, the mechanism of falls and any expressed fearfulness about walking. A proper physical exam for postural instability includes the retropulsion test, which is widely used. In this test, a sudden jolt or pull is applied to the shoulders of the patient from behind as an external perturbation.

Patients with normal postural reflexes should be able to take no more than one step backward, indicating that balance is maintained, and postural reflexes are normal. Patients with postural instability will take two or more corrected steps backward, which is an abnormal result. There are variations to this test, and currently no agreed-upon method for scoring. Other clinical examinations for postural instability include the functional reach test (in which the patient keeps the feet in place and tries to reach as far forward as possible) and the timed up and go test.[2][5]

Evaluation

Postural instability is a clinical diagnosis made subjectively. Depending on the findings of the history and physical examination, appropriate laboratory testing and imaging can be ordered to discover any underlying disease processes contributing to the clinical manifestation.[1]

For patients without a clear diagnosis of postural instability following a history of physical examination, laboratory tests to consider for initial evaluation include complete blood count, rapid plasma reagin test, electrolytes, blood urea nitrogen, thyroid function test, glucose, creatinine, and vitamin B12 levels.[6]

Because of the association between postural instability and risk of falls, routine measurement of 25-OH vitamin D3 levels may be helpful and should be completed in all patients newly diagnosed with Parkinson disease. Given the risk of bone mineral loss and fractures, hyponutrition should be ruled out, and bone densitometry can be used to detect osteoporosis.[7]

Other diagnostic tests may be ordered to rule out neurological conditions based on the patient's gait pattern:

• If the patient presents with weakness and spasticity with gait (upper motor neuron signs), central lesions or spinal cord disease are possibilities, and brain or spine MRI can be ordered for further evaluation.[8]
• Foot drop may suggest a lower motor neuron disease such as lumbar spinal stenosis, mononeuropathy, or radiculopathy, and lumbosacral spine MRI or nerve conduction studies with electromyography can be ordered.[9]
• A frontal or apraxic gait in which the patient has difficulty initiating walking could suggest normal pressure hydrocephalus or neurodegenerative dementia, in which brain MRI may aid with evaluation.[10]
• A narrow-based or festinating (hurrying) pattern of the walk would suggest a Parkinsonian gait, in which MRI of the brain could be useful.[1]
• Functional gait disorder, in which walking does not conform to any of the typical patterns, is often seen with psychiatric disorders. This is usually a clinical diagnosis.[11]
• Patients with a vestibular gait pattern, in which patients may fall towards the side of the affected ear, can be evaluated in part with sensorineural vestibular testing.[12]

Treatment / Management

Physical Therapy: Evidence is increasing that balance exercises can improve postural stability and thus reduce the risk of falls.[13] It is important to note that while there is some evidence for the contribution of both muscle strength and power to balance ability in elderly adults, the evidence is weak for a cause and effect relationship between balance performance and muscle function.[14] Nevertheless, starting physical therapy early to prevent neuromuscular weakness and frailty is recommended. Physical therapy has demonstrated benefits in the short-term in patients with Parkinson disease suffering from postural instability and gait difficulty. Progressive resistance and aerobic exercises require long term compliance to have a sustained benefit to patients. Treatment modalities should be adjusted to fit the patient’s individual needs. Home therapy plans may be advantageous with respect to the quality of life, particularly for frail elderly patients. Further research is needed to evaluate the safety of home programs that are unsupervised.[15] (A1)

For postural instability associated with progressive supranuclear palsy, wheelchair use is often needed for ambulation and transfers. Physical and occupational therapy focuses on “learning to fall” techniques and “making the floor, your friend.” This can minimize the risk of injury and help patients maneuver even after falling.[16](B3)

Medication: While most of the motor features of Parkinson disease are responsive to dopaminergic treatment, postural instability typically does not respond well to these therapeutic measures. Because of this, it has been determined that lesions in non-dopaminergic systems possibly contribute to postural instability in Parkinson disease. Indeed, accumulating evidence does indicate that postural instability is at least in part related to non-dopaminergic lesions.[17] These lesions have become well-recognized in the advanced stages of Parkinson disease, where postural instability dominates the clinical presentation, often leading to frequent falls. Although it remains unclear which lesions are responsible, it has been proposed that cell loss in the locus coeruleus with a resultant deficiency in central norepinephrine is central in the pathophysiology of postural instability and balance impairment in Parkinson disease.[18]

Research has demonstrated an association between donepezil and reduced risk of falls, as well as methylphenidate and possible benefit against gait freezing. These results have not been replicated in large randomized control trials, however.[13] De-prescribing any benzodiazepines or anticholinergic drugs is a possible, simplistic measure that can benefit Parkinson disease patients with postural instability. In subsets of patients, both droxidopa and cholinesterase inhibitor drugs have been shown to reduce the number of falls in patients. More research is needed to further elucidate the future development of potential treatments aimed to correct postural instability.

Neurostimulation and Surgical Treatments: Transcranial direct current and vagal nerve stimulation have shown promise for management of postural instability with gait difficulty.[15] Deep brain stimulation focusing on the subthalamic nucleus and globus pallidus internus has been shown to provide a modest benefit for postural instability. New surgical targets are emerging but remain experimental.[13]

Differential Diagnosis

Postural instability has a limited diagnostic specificity, given that it can result from a variety of issues in afferent pathways, efferent pathways, musculoskeletal function, and central processing.[19] In addition to Parkinson disease, the differential diagnosis should include lower motor neuron syndromes, severe spasticity, normal pressure hydrocephalus, vestibular dysfunction, atypical parkinsonian disorders, and orthostatic hypotension.

Distinguishing postural instability from orthostatic hypotension takes into account symptomatology and changes in blood pressure with the latter. Dizziness from orthostatic hypotension is usually related directly to changes in position and is associated with fainting or lightheadedness. In contrast, dizziness from postural instability usually occurs when a patient reaches, bends, or turns. Many patients with frequent falls due to postural instability may have no subjective complaints of dizziness.[20]

Progressive supranuclear palsy (PSP) is the most common atypical parkinsonian disorder. Characteristic features of PSP include vertical supranuclear gaze palsy (patient unable to look down), postural instability with unexplained falls, dysarthria, dysphagia, and frontal cognitive dysfunction. Postural instability is the most frequent symptom presentation in PSP.[16] Both postural instability and falls are the most common initial symptoms of PSP, and the two are required for the diagnosis of the disease.[19] While postural instability is a late symptom of Parkinson disease and can sometimes indicate severe parkinsonism, it is generally an early symptom of PSP. As in Parkinson disease, postural instability in PSP does not respond well to levodopa.[2]

Patients with multiple system atrophy can often have asymmetrical parkinsonian bradykinesia, autonomic features, and gait disturbances that also occur in Parkinson disease. The disease onset, however, is important. Postural hypotension is usually present at diagnosis or in the early years after the presentation of multiple system atrophy, whereas in Parkinson disease, these happen later in the course of the disease.

Corticobasal degeneration presents with an asymmetrical akinetic–rigid syndrome that is similar to Parkinson disease. The positional or postural limb instability, however, is usually present upon initial evaluation.

Dementia with Lewy bodies may present with postural instability similar to Parkinson disease. However, visual hallucinations and fluctuations in a patient's attention and awareness are significant features in the diagnosis of this disease.[21]

Vertigo can also present with postural instability. However, these patients are more likely to complain of dizziness. Severe postural instability with frequent falling and other neurological signs such as ataxia, dysarthria, dysphagia, or diplopia suggests central vertigo (central nervous system etiology). Unidirectional instability preserved walking, and possible tinnitus or deafness would suggest peripheral vertigo (inner ear etiology).[22]

Prognosis

Postural instability is associated with poor prognosis. Frequent falling is correlated to postural instability and can result in broken bones and hospitalization. Dopaminergic treatment has not been shown to be effective. In patients with mild-moderate Parkinson disease, postural instability and gait difficulty is an important determinant of quality of life. It is a major contributor to disability in patients in Parkinson disease.[2][23]

Complications

Postural instability is an exceedingly debilitating symptom of Parkinson disease, as it is associated with an increased risk of falls and subsequent medical complications such as fractures, fear of falling, decreased mobility, social isolation self-restricted physical activity, and lessened quality of life.[24]