Neurology of Balance Brad Cole, MD

Gait instability is a common problem because so many areas of the central and peripheral nervous system must be working at near normal capacity in order to have steady balance. In addition, many non-neurologic etiologies contribute to instability. Complaints of an unsteady gait and easy falling are most common in the elderly and frequently the etiology is multifactorial with both neurologic and non-neurologic contributing factors. About 60% of all individuals over the age of 80 complain of imbalance which puts these individuals at risk for falls with resulting hip fractures, subdural hematomas and other injuries.

Two-thirds of the total body weight is centered in the upper body, which creates some degree of inherent instability. Normal balance from a neurologic perspective requires the following:

 Strength (UMN and LMN pathways, neuromuscular junction and muscle)  Coordination (cerebellar and basal ganglia pathways)  Vestibular processing  Sensation (proprioceptive pathways)  Vision  Frontal lobe motor programs

In this handout, we will review some of the most common causes of gait instability in each of the above categories. All of these specific conditions will be discussed in more detail in the sophomore neuroscience course.

1. Strength

Any UMN or LMN lesion, neuromuscular junction disorder, or muscle disease will impair balance because of weakness. The most common causes are:

 Myelopathy: This refers to spinal cord disease, usually at the cervical level and most often due to compression although there are a number of metabolic, infectious and other conditions that can also impair spinal cord function. The result is spastic weakness and other UMN findings below the level of the lesion. In the elderly, this is usually due to progressive cervical spondylosis with encroachment on the cervical cord. At the level of the lesion, there may be focal radicular findings. In part because the sacral fibers of the CST are located laterally, the main presentation of cervical myelopathy is leg weakness and spasticity. Typically both legs have spastic weakness since the entire spinal cord is involved, although if one side of the cord is involved more than the other, patients may have a Brown-Sequard syndrome. A is classically narrow based.

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 Multiple strokes: Patients who have had one or especially multiple strokes have gait instability that worsens with each stroke. Typically, patients have a hemiplegia and other focal deficits depending on the location of the strokes. Recall that a hemiplegic gait has a flexed arm and an extended leg and foot which necessitates circumduction when walking.

 Amyotrophic lateral sclerosis (ALS or Lou Gehrig’s disease) is a condition that results in degeneration of both the UMNs and the LMNs. Foot drop is a common presentation followed by progressive weakness. , atrophy, and a mixture of UMN and LMN weakness are found on examination. In addition, the UMNs and LMNs that supply the bulbar muscles are involved as well, resulting in progressive dysarthria and dysphagia. ALS is relentlessly progressive and more than half of patients die within 2-3 years of the onset of symptoms.

 Lumbosacral radiculopathy will result in gait instability due to focal weakness and sensory loss from the involved myotomes and dermatomes.

 Peroneal (fibular) neuropathy results in a foot drop and a .

 Inclusion body myositis (IBM) is a myopathy that primarily involves older men. There is selective pronounced quadriceps muscle weakness which causes the knees to buckle when walking. In addition, the finger flexor muscles are involved and patients complain of difficulty opening jars or other tasks that require finger flexion.

2. Coordination

Patients in this category do not have weakness, UMN or LMN findings, but rather have an inability to properly coordinate movement.

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 Degeneration of the substantia nigra pars compacta results in Parkinson’s disease (PD) which causes a shuffled gait with decreased step size. This is typically a later finding in PD (i.e. – other symptoms predominate early in the disease course). Patients have difficulty standing up from a chair and need to push off. There is also a diminished arm swing when walking and the feet may “freeze” to the ground at times. On examination, patients have rigidity and other specific findings. There are numerous conditions that have overlapping features with PD but yet have a different etiology. These are referred to as “parkinsonism” and typically have significant gait instability that is an early clinical feature (in contrast to Parkinson’s disease where it is a late feature).

 Any lesion or degenerative process that involves the cerebellum will result in . Gait instability is most pronounced when the vermis or anterior lobe is involved. Patients typically have heel to shin ataxia on examination. We have already discussed some of these conditions such as the effects of chronic alcoholism, medulloblastoma, cerebellar strokes, etc. The cerebellar gait is wide based. If the cerebellar hemisphere is involved, patients may also have finger to nose ataxia.

3. Vestibular processing

Peripheral vertigo is caused by any lesion of the vestibular apparatus in the inner ear or 8th nerve; central vertigo refers to vertigo from brain stem lesions. Both result in vertigo (a sensation of movement in the environment, or a feeling that “I am moving”) and gait instability. Lesions in the inner ear tend to have more intense vertigo than brain stem lesions because of the significant “mismatch” of information (i.e. when the lesion is in the brain stem, vestibular processing from both ears are impaired, resulting in less of a side-to-side mismatch).

 Benign paroxysmal positional vertigo (BPPV) is the most common cause of peripheral vertigo. The mechanism is dislodged otoconia (calcium carbonate crystals) from the utricule or saccule into a semicircular canal (SCC), most often the posterior SCC. This transforms the SCC from an organ that normally senses only angular acceleration into one that now senses linear acceleration as well. This mismatch of inputs from the right and left SCC is triggered by head positions that stimulate the involved SCC. Rolling over in bed, standing up to get out of bed, or lifting the head up are all common triggers and patients may have many attacks per day. Symptoms include very intense vertigo that lasts for seconds and is associated with nausea, vomiting, and nystagmus on examination. Although patients may not have the intense attacks of vertigo while walking, there is a sensation of gait instability and a feeling as if walking on a rocking boat. More details in the sophomore course with regards to diagnosis and treatment.

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 Acoustic schwannoma causes gait instability from a combination of 8th nerve dysfunction and potentially from compression of the cerebellum.

 Lateral medullary syndrome causes gait instability because of involvement of the inferior cerebellar peduncle and cerebellum (resulting in ipsilateral ataxia) but also from involvement of the vestibular nuclei which causes vertigo and nystagmus.

4. Sensation

A disturbance of proprioceptive information results in gait instability. Patients often have a stomping gait and have a loss of proprioception and vibration on examination.

 Vitamin B12 deficiency most profoundly involves the posterior columns although other areas of the nervous system may also be involved.

(PN) typically presents with numbness and tingling in the toes and later the fingers (stocking-glove). PNs that preferentially involve the large myelinated sensory fibers are more likely to cause gait instability since these are the fibers that convey proprioception. Many PNs also involve the motor fibers as well, resulting in distal LMN weakness which further contributes to gait instability.

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Recall that the Romberg test is used to assess gait instability. This test assumes that the patient has adequate strength to remain standing unassisted. By asking the patient to close their eyes, balance is then primarily dependent on proprioception and vestibular function – the patient will sway or begin to fall with either of these conditions.

5. Vision

Adequate vision is necessary to have normal balance. Any cause of visual loss, from ophthalmologic causes (cataracts, macular degeneration, etc.) to neurologic (optic neuritis) will contribute to easy falling. Thus, it is important to take a history of visual disturbances and to assess visual acuity, visual fields, and to perform an ophthalmoscopic examination.

6. Frontal lobe motor programs

The brain has motor programs that allow for normal walking. Any disturbance of these motor programs results in a gait apraxia. The general term “apraxia” refers to a failure in the execution of a learned motor task despite normal strength, coordination, and sensation.

 Alzheimer’s disease not only results in memory loss, but involves the entire brain. At autopsy or on brain MRI scans, for example, there is diffuse atrophy. Motor programs for all activities are negatively impacted and patients have multiple apraxias that range from difficulty dressing, teeth brushing, and a gait apraxia. Falling is usually a late feature of Alzheimer’s disease.

 Normal pressure hydrocephalus (NPH) is an exceptionally rare yet over-diagnosed condition (although an exceptionally common disease entity on national boards). NPH may result from a failure of the arachnoid granulations to reabsorb CSF, leading to a communicating hydrocephalus. The ventricles initially enlarge and push forward against the midline pathways that descend from the paracentral lobule that are important for control of the legs and the bladder. NPH has a very non-specific triad of symptoms that overlap with many other conditions including Alzheimer’s disease. In NPH, these occur in the following order:

o Gait apraxia – difficulty is primarily with gait initiation, where the feet initially seem almost glued to the floor (“”) but then improves with walking. The gait problem typically occurs first in NPH which helps to distinguish from Alzheimer’s disease in which gait instability is a later feature. o Dementia o Urinary incontinence

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Review books use the term “wet, wacky and wobbly”; if you are going to remember the correct order of progression, please learn to say “wobbly, wacky and wet”.

7. Non-neurologic etiologies

Chronic diseases associated with aging frequently results in mild gait disturbance. Some of the more common contributing factors include:

 Spine arthritis and other musculoskeletal/orthopedic conditions that involve the lower extremities  Orthostatic hypotension usually due to treatment of hypertension which can cause a lightheaded dizziness on standing and walking  Encephalopathy related to misuse/overuse of medications, infections and other metabolic conditions.  Pain of the low back, hip, knees or ankles  Cardiorespiratory problems interfere with walking through shortness of breath and fatigue

8. Psychogenic gait disorders

This refers to gait instability that cannot be explained by neurologic or other disease. This can be associated with non-genuine findings on examination and is associated with psychiatric disease, conversion reaction or malingering. The gait is unusual, often lurching erratically, holding onto walls, and staggering from side to side. This gait pattern is sometimes referred to by the antiquated term “astasia-abasia”. This gait pattern can be inconsistent and patients may later be observed to walk normally or to fluctuate in response to suggestion. There are many specific psychogenic gait patterns including “walking on ice” (small, cautious steps with fixed ankle joints), Quasimodo gait (dragging one leg), buckling of the legs without falling, etc.

9. Fear of falling

Fear of falling refers to elderly individuals who have fallen in the past and because of an intense fear of falling walk with a very cautious and slow gait, often holding onto furniture and walls. This can be severe to the point that the fear of falling becomes the most significant contributor to gait instability. Unlike psychogenic gait disorders, fear of falling does not suggest a psychogenic etiology.

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10. Multifactorial gait instability . This is probably the most common diagnosis that is made by neurologists who evaluate patients for gait instability since the elderly frequently have a combination of all the conditions listed above. The more of these that are present, the more likely the patient is to be unsteady. Here is a common scenario: An 80 year old man who has had Alzheimer’s disease for the last two years as well as a long history of diabetes, macular degeneration and arthritis presents with falling over the last 6 months. Even without the benefit of the examination, we could predict that the patient likely has the following contributing factors: gait apraxia from Alzheimer’s disease, peripheral neuropathy from diabetes, visual loss due to macular degeneration, and musculoskeletal factors related to arthritis. One of the main goals of taking a careful history and performing a detailed neurologic examination in this patient is to try and uncover something that may be treatable since the above conditions are not readily “fixable”.

Diagnosis and Treatment

When a patient is referred for gait instability, you need to have all of the neurologic and non- neurologic categories in mind as you take the history and perform the neurologic examination. Fortunately, the neurologic causes of gait instability (for the most part) have objective findings on examination.

Treatable causes are especially important to diagnosis, of course. Physical therapy and rehabilitation can work with patients to provide specific gait training and to evaluate for gait assistive devices.

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