Chapter 11

Evaluation of the effects of ethanol on static and dynamic gait

Mallinson AI, Longridge NS, Morley RE

J Otolaryngol Head Neck Surg 2008 Dec;37(6):856-9.

ABSTRACT

Objective: We used two balance assessment devices, Computerized Dynamic Posturography (CDP) and Swaystar transducers to detect subtle balance system deficits in nine subjects who had ingested minimal amounts of alcohol.

Design: Nine subjects were evaluated with both modalities before, and repetitively after, ingesting a small amount of alcohol.

Methods: We measured condition 5 (sway referenced platform; eyes closed) on CDP and tandem walking with eyes closed while wearing Swaystar to see if either test could detect a balance deficit.

Main Outcome Measures: We measured total sway amplitude with eyes closed in pitch and roll planes during tandem walking with Swaystar, and static balance scores of CDP sensory organization testing condition 5 before and after alcohol ingestion at 20 min intervals.

Results: Although there was no detectable deficit measurable by CDP, eight of our nine subjects showed increased dynamic sway as measured by Swaystar, after alcohol ingestion. Total sway was significantly greater (p = .05) after alcohol ingestion.

Conclusion: It is important to assess dynamic, rather than static, equilibrium as it may have potential in detecting very subtle balance deficits.

KEYWORDS: ethanol, gait, imbalance, posturography, Swaystar

The complexities of human balance make it difficult to assess subtle balance and gait disorders in an accurate and efficient manner in the clinical setting. Subjective evaluation using rombergism is useful as a gross screening test, but many patients with minimal but still legitimate complaints (or who have compensated well for a vestibular lesion) can pass quick office assessments such as tandem Romberg or tandem walking tests.(1)

Computerized Dynamic Posturography (CDP) is a modern method of measuring static balance by quantifying sway. Office evaluation techniques of dynamic sway (e.g., assessment of tandem walking) usually consist of observation of sway amplitude by standing behind a patient and observing sway in the roll plane (side-to-side sway). It is difficult in the clinical setting to make simultaneous observations about dynamic gait in both pitch and roll planes.

Swaystar (Balance International Innovations, Basel, Switzerland) is a lightweight, easy-to-use, belt-mounted set of transducers that enables quantification not only of sway amplitude but also of trunk angular velocities during dynamic gait in both pitch (anterior-posterior, or A-P) and roll (side-to-side) planes. Information at the trunk is important since the first response to a perturbation of balance occurs laterally in the trunk.(2)

Many people are familiar with the impairing effects of small doses of ethanol on dynamic gait and the general postural stability that such doses can cause.(3) The direct effects of ethanol on the vestibular system serve to reduce the sensitivity of the peripheral end-organ.(4) However, ethanol also reduces the gain of the vestibulo-ocular reflex (VOR) and causes central inhibition.(5) These effects on the VOR result in a sensation of apparent concomitant motion (ACM) of stationary visual stimuli correlated with head movement. This symptom is suggestive of a reduced gain of the VOR. (6)

Even minimal alcohol-induced deficits may potentially result in loss of balance under crucial conditions (e.g., on a roof, on a ladder, on a precipice). Ledin and Odkvist measured these deficits and found that the most sensitive measurement techniques were test conditions that excluded visual input.(3) It may be that, under conditions of alcohol impairment, visual input is used as a compensation device (similar to compensation mechanisms developed by patients with vestibular lesions).

As ethanol suppresses the VOR, we wondered if it might also suppress the vestibulospinal reflexes (VSRs). The imbalance observed during intoxication is well documented. Fregly showed that the balance capabilities of patients with bilateral vestibular loss are degraded very little, if at all, by ethanol intoxication. (7) His interpretation of this finding was that ethanol-related ataxia results from a direct suppressive effect on the VSR and not on the central nervous system. It has also been suggested by Tianwu and colleagues that one of the reasons for postural instability after acute ethanol intoxication may be reduced vestibular function.(8)

Patients referred to our tertiary/quaternary care balance clinic often voice complaints of vague imbalance, which they sometimes characterize as “like I’ve had a couple of drinks.” Because we often hear this volunteered during history taking, and because of the apparent suppressive effects that ethanol has on both the VOR and the VSR, we postulated that even minimal (in some cases, asymptomatic) amounts of ethanol might induce postural instability, resulting in some detectable alteration of dynamic gait. Often our patients’ assessments show only slight (if any) abnormalities, and we wondered if it might be possible with CDP and/or Swaystar to detect these slight balance deficits in healthy subjects under the influence of a minimal dose of ethanol.

It has been shown that, with eyes closed, body sway is most pronounced under the influence of ethanol in men aged 40 to 49 years, but there were no other differences between individuals under the age of 40 years.(9) Because we wanted to eliminate any age effects, we studied healthy individuals under the age of 35 years.

METHODS

Both Vancouver General Hospital and University of British Columbia ethics committee approvals were obtained for this study. Five males and four females between the ages of 25 and 34 years were recruited. All were healthy and free of previous orthopedic trauma to the pelvis or lower extremities. Of note (and known before the experiment was begun) is the fact that one subject had a past history of mild head injury 3 years previously.

All subjects underwent CDP sensory organization testing (SOT) using CDP (Equitest). The details of the principles(10) and techniques,(11) of balance measurement using CDP are well outlined in these cited references for the interested reader.

Assessment in all subjects also consisted of tandem walking 10 steps, with eyes closed, while wearing Swaystar. In this test, no guidance was given to any subject about cadence, except to “take 10 tandem steps with your eyes closed, and do as well as you can.” The time taken to complete 10 steps was recorded.

Both CDP and tandem walking with eyes closed while wearing Swaystar was carried out prior to alcohol consumption (time 0). Immediately after baseline assessment, each subject consumed “three drinks” (89 ml or 3 oz.) of 40% alcohol on an empty stomach within a 5-minute period. Assessment was repeated at 20, 40, and 60 minutes, measured from when ethanol consumption began. At each time interval, we extracted the median of the three CDP SOT condition 5 (sway referenced platform; eyes closed) scores. We also extracted two Swaystar measurements for analysis:

• maximum sway amplitude of tandem walking with eyes closed in roll plane
• maximum sway amplitude of tandem walking with eyes closed in pitch plane

Swaystar computes maximal angular trunk sway in degrees, and we summed the total sway in each plane (pitch and roll) to compute a total sway amplitude measurement for each subject.

We estimated blood alcohol levels attained in each subject. The total amount of pure alcohol was calculated by using the amount and strength of alcohol consumed and multiplying by the specific gravity of ethanol. Estimated blood alcohol level was derived using the weight of the patient and the known constant of amount of body water per kilogram. Projected estimates for blood alcohol concentration (BAC) in our subjects are reported in Table 1.

The greatest effect of ethanol appears at about 50 minutes when one examines ACM.(3) (This is caused by a direct effect of ethanol on the VOR.) We examined our data at both 40 and 60 minutes post-alcohol ingestion and used the larger of the 40-minute and 60-minute condition 5 CDP median scores and the larger of the Swaystar total sway amplitude scores in each subject.

Table 1. Sway Amplitudes and CDP Condition 5 Scores Before and After Alcohol Ingestion

BAC-blood alcohol concentration

CDP-computerized dynamic posturography;

SOT-sensory organization testing.

*Total sway amplitude difference is significant at .05.

RESULTS

All subjects reported a slight subjective sensation as a result of their alcohol ingestion. Only one of our subjects (subject 4) was projected to have exceeded legal intoxication (see Table 1). The results across all nine subjects showed a significant (p < .05) increase in total sway using a one-tailed paired t-test. CDP SOT condition 5 scores at 40 and 60 minutes showed no significant change from baseline in any subject.

DISCUSSION

Our purpose in this experiment was to determine if ingestion of a small quantity of alcohol induces minimal vestibular impairment in normal subjects that might be detectable with either CDP or Swaystar.

Our study shows that it may be possible to detect subtle dynamic imbalance brought on by alcohol-induced vestibular impairment. We used an estimated blood level calculator to assess the degree of impairment of our subjects. Although we did not monitor actual blood alcohol level and did not use a breathalyzer, we used one of many scientifically derived formulas to estimate BAC. These formulas enable researchers to estimate BAC in a range of subjects and are accepted as accurate estimates of intoxication.(12) Our subjects were only minimally impaired according to BAC estimates.

SOT condition 5 is one of the two CDP conditions that maximally stresses the vestibular system, and two previous studies using CDP to measure the effects of alcohol(3,8) used condition 5 as they both found that sway was maximized by the total absence of vision. For those reasons, we used the median of three CDP SOT scores on condition 5. SOT condition 5 turned out to be unhelpful in detecting ethanol-induced unsteadiness in our subjects.

Allum and colleagues showed that, in patients with balance deficits, sway amplitudes increase in both pitch and roll planes.(2) We summed the Swaystar tandem walking sway amplitude scores in both planes to sensitize the assessment. In several of the trials, subjects took a step sideways. Although this is noticeable to an observer, it was not apparent on our recordings as no angular trunk movement is associated with a side step. We regard this as a caveat when assessing balance using Swaystar as gait abnormalities are detected only if they involve rotational (as opposed to translational) movements of the trunk.

Ethanol doses of 100 mL of whiskey (only slightly more than our doses) affect the VOR; these doses have been shown to induce positional nystagmus in 30 minutes,(13) and this is assumed to be due to a variable rate of diffusion of ethanol into the cupula (semicircular canal) and surrounding endolymph. Swaystar measured changes in VSR in our subjects by measuring increased sway amplitude, but we were able to do this only by summing pitch plane and roll plane sway. Perhaps this also reflects the multifactorial nature of posture maintenance (i.e., sometimes pitch plane sway is increased, sometimes roll plane is increased, and sometimes total sway is increased).

CONCLUSIONS

The effects of alcohol on balance probably arise as a result of a multifactorial influence on balance maintenance. As discussed, alcohol serves to sedate the vestibular signal,(4-6,8) but it also sedates centrally. Central effects occur at the level of the vestibulospinal system, but sedation is also cortical, and this may serve to steady a subject at low doses.(14) Subjective strategies to maintain balance may also differ from one subject to another.

Perhaps individual balance maintenance decisions may be executed at some central control level by our alcohol-impaired subjects. Our head-injured patient (subject 8) performed very poorly. It is unclear what, if any, effects his head injury had on his performance, but it could be speculated that either he had some impairment in his ability to make such decisions or vestibular damage (peripheral or central) was unmasked in him (“decompensated”) by the alcohol.

The present study showed that eight of our nine subjects showed increased dynamic sway using Swaystar, which we could not detect on CDP after 89 mL of ethanol. The difference was significant at p < .05. One subject had a threefold increase in sway measured by Swaystar (i.e., during dynamic walking) but no change in her posturography. Perhaps this illustrates the difference between unperturbed stance and ambulation. It was necessary to add sway amplitudes together since performance differences in pitch planes and roll planes were not significantly different. This may support the multifactorial nature of change in gait under different circumstances. Swaystar enables us to assess dynamic rather than static equilibrium when looking for subtle clinical deficits.

REFERENCES

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