MEDIAN N E R V E SOMATOSENSORY EVOKED POTENTIALS
S T U D I E S O N L A T E N C Y V A R I A B I L I T Y A S A F U N C T I O N O F S U B J E C T H E I G H T , L I M B L E N G T H A N D N E R V E C O N D U C T I O N V E L O C I T Y
JOAQUIM P. BRASIL-NETO *
SUMMARY — R e p o r t on t h e r e s u l t s of r e g r e s s i o n a n a l y s i s s t u d i e s c o n c e r n i n g m e d i a n n e r v e s o m a t o s e n s o r y evoked p o t e n t i a l s ( S E P s ) latencies, a s d e p e n d e n t v a r i a b l e s , a n d s u b j e c t h e i g h t , limb l e n g t h a n d n e r v e c o n d u c t i o n velocity (NCV), a s i n d e p e n d e n t v a r i a b l e s . T h e t e s t s w e r e p e r f o r m e d on 23 n o r m a l v o l u n t e e r s . A b s o l u t e S E P l a t e n c i e s could be p r e d i c t e d by a l i n e a r r e g r e s s i o n m o d e l w h e n t h e i n d e p e n d e n t v a r i a b l e w a s a r m l e n g t h ; w h e n i t w a s s u b j e c t h e i g h t , however, b o t h e x p o n e n t i a l a n d p o l y n o m i a l m o d e l s p r o v e d b e t t e r , t h e Latter s h o w i n g t h e b e s t coefficients of d e t e r m i n a t i o n , R 2. M u l t i p l e l i n e a r r e g r e s s i o n w i t h t w o i n d e p e n d e n t v a r i a b l e s ( a r m l e n g t h a n d NCV) w a s found t o b e b e t t e r t h a n s i m p l e l i n e a r r e g r e s s i o n for p r e d i c t i n g P / N 1 3 l a t e n c y . T h e r e g r e s s i o n l i n e for E P P / N 1 3 l a t e n c y o n h e i g h t w a s found t o b e a p o l y -nomial c u r v e ; a l t h o u g h t h e r e g r e s s i o n w a s found t o b e s i g n i f i c a n t b y t h e «F» t e s t ( a l p h a = 1%), t h e m o d e l h a d a l o w R 2 v a l u e (0.41). T h e s a m e a p p l i e s t o t h e P/N13-N19 i n t e r p e a k l a t e n c y r e g r e s s i o n curve, b u t t h e r e g r e s s i o n w a s s i g n i f i c a n t for a l p h a = 5 % i n t h a t case. A l t h o u g h i n t e r w a v e l a t e n c i e s a r e t h e m o s t useful p a r a m e t e r s for clinical i n t e r p r e t a t i o n o f m e d i a n S E P s , a b s o l u t e l a t e n c i e s m a y occasionally b e i m p o r t a n t , a n d s h o u l d b e c o r r e c t e d for body size; i n u n u s u a l l y t a l l s u b j e c t s , i t m i g h t b e useful t o d o u b l e c h e c k E P P / N 1 3 i n t e r -wave l a t e n c y p r o l o n g a t i o n b y e s t i m a t i n g t h e m a x i m u m e x p e c t e d P / N 1 3 l a t e n c y , u s i n g a model t h a t t a k e s i n t o a c c o u n t b o t h l i m b l e n g t h a n d NCV.
Potenciais evocadas sômato-sensitivos do nervo m e d i a n o : estudos de variabilidade das latên¬ cias em função da altura, comprimento do membro superior e velocidade de condução nervosa.
R E S U M O — D e s c r i ç ã o d o s r e s u l t a d o s d e e s t u d o s d e a n á l i s e d e r e g r e s s ã o i n t e r e s s a n d o a s la-t e n c i e s d o s p o la-t e n c i a i s evocados s ô m a la-t o - s e n s i la-t i v o s ( P E S S ) d o n e r v o m e d i a n o , como v a r i á v e i s d e p e n d e n t e s , e a a l t u r a do i n d i v í d u o , o c o m p r i m e n t o do m e m b r o s u p e r i o r e a v e l o c i d a d e de c o n d u ç ã o n e r v o s a (VCN), c o m o v a r i á v e i s i n d e p e n d e n t e s . Os t e s t e s f o r a m r e a l i z a d o s em 23 p e s s o a s n o r m a i s . A s l a t ê n c i a s a b s o l u t a s dos P E S S p o d i a m s e r p r e v i s t a s p o r u m m o d e l o d e r e g r e s s ã o l i n e a r q u a n d o a v a r i á v e l i n d e p e n d e n t e e r a o c o m p r i m e n t o d o b r a ç o ; q u a n d o t a l variável e r a a a l t u r a , p o r é m , t a n t o o m o d e l o e x p o n e n c i a l q u a n t o o p o l i n o m i a l m o s t r a v a m - s e m e l h o r e s , e s t e ú l t i m o a p r e s e n t a n d o os m e l h o r e s coeficientes de d e t e r m i n a ç ã o , R 2. A c u r v a d e r e g r e s s ã o d a s l a t ê n c i a s i n t e r p i c o s ( E P P / N 1 3 e P/N13N19) e m função d a a l t u r a r e v e l o u se do t i p o polinomial, com b a i x o v a l o r de R 2. E m b o r a as l a t ê n c i a s i n t e r p i c o s s e j a m os p a -r â m e t -r o s m a i s ú t e i s p a -r a a i n t e -r p -r e t a ç ã o c l í n i c a d o s P E S S d o n e -r v o m e d i a n o , a s l a t ê n c i a s a b s o l u t a s p o d e m e v e n t u a l m e n t e s e r i m p o r t a n t e s e d e v e m s e r c o r r i g i d a s p a r a a s d i m e n s õ e s c o r p o r a i s d o p a c i e n t e . São r e s s a l t a d a s a s s i t u a ç õ e s e m q u e t a i s c o r r e ç õ e s p o d e r i a m s e r p o -t e n c i a l m e n -t e ú -t e i s p a r a a u m e n -t a r a s e n s i b i l i d a d e d o -t e s -t e .
Potenciais evocadas sômato-sensitivos do nervo m e d i a n o : estudos de variabilidade das latên-cia* em função da altura, comprimento do membro superior e velocidade de condução nervosa. RESUMO — Descrição dos resultados de estudos de análise de regressão interessando as la-tênciias dos potenciais evocados sômato-sensitivos (PESS) do nervo mediano, como variáveis dependentes, e a altura do indivíduo, o comprimento do membro superior e a velocidade de condução nervosa (VCN), como variáveis independentes. Os t e s t e s foram realizados em 23 pessoas normais. As latências absolutas dos P E S S podiam ser previstas por um modelo de regressão linear quando a viariável independente era o comprimento do braço; quando tal variável era a altura, porém, tanto o modelo exponencial quanto o polinomial mostravam-se melhores, este último apresentando os melhores coeficientes de determinação, R 2. A curva de regressão das latências interpicos ( E P - P / N 1 3 e P/N13-N19) em função dia altura revelou-se do tipo polinomial, com baixo valor de R 2. Embora as latências interpicos revelou-sejam os pa-râmetros mais úteis para a interpretação clínica d o s P E S S do nervo mediano, as latênciias absolutas podem eventualmente ser importantes e devem ser corrigidas para as dimensões corporais do paciente. São ressaltadas as situações em que tais correções poderiam ser po-tencialmente úteis para aumentar a sensibilidade do teste.
The relationship between absolute somatosensory evoked potential ( S E P ) laten-cies and the distance separating stimulus site from waveform generator is obvious: the greater the distance, the longer the latency. The need for correcting absolute and interwave latencies for subject height has long been recognized for lower limb
H o s p i t a l for D i s e a s e s of t h e L o c o m o t o r S y s t e m - S A R A H , B r a s i l i a : * N e u r o l o g i s t , Clinical N e u r o p h y s i o l o g i s t .
S E P s ; for upper limb testing, however, the general a g r e e m e n t h a s been that the effect of differences in body size on the interpeak latencies is small enough that it can be d i s r e g a r d e d2
. Although t h e r e can be no doubt as to the fact t h a t interwave latencies are much more influenced by body size after lower limb stimulation than after upper limb testing, we have found evidence indicating care must be taken when interpreting S E P s from a subject w h o s e height is significantly different from the r a n g e found in the population from which median nerve S E P s normative values w e r e derived. Our normative studies carried out in a sample of 51 normal Brazilian s u b j e c t s1
indi-cated t h a t the highest acceptable E P - P / N 1 3 interwave latency should be 4.4 msec; this is significantly different ( p < 0 . 0 0 5 ) from the value found by C h i a p p a2
in Americans (5.2 m s e c ) . In other w o r d s , some of his normal subjects could have an abnormal latency, if tested according to our n o r m s . T h e most likely explanation for the diffe-rence is t h a t our subjects w e r e p r e s u m a b l y s h o r t e r and h a d s h o r t e r upper limbs than their N o r t h American c o u n t e r p a r t s .
In o r d e r to further clarify this problem we have t h o u r o u g h l y studied the rela-tionship between subject height, arm length, nerve conduction velocity and median nerve S E P latencies. We a r e not a w a r e of any other p a p e r s which have tried to a d d r e s s this issue in detail; Kritchevsky and W i e d e r h o l t4
MATERIAL* & M E T H O D S
M e d i a n n e r v e S E P s from 2 3 n o r m a l v o l u n t e e r s w e r e s t u d i e d . S t i m u l a t i o n o f t h e m e d i a * n e r v e i n t h e w r i s t w a s 'accomplished t h r o u g h u s e o f a n S C 6 T E C A s o m a t i c s t i m u l a t o r device, and d a t a w e r e p r o c e s s e d by a D A V 62 a v e r a g e r m o d u l e , b o t h c o n n e c t e d to a TE 42 electro-m y o g r a p h . S t i electro-m u l i w e r e s q u a r e p u l s e s of 0.2 electro-m s e c d u r a t i o n , d e l i v e r e d at a f r e q u e n c y of c p e r s e c o n d at t h e m i n i m u m i n t e n s i t y r e q u i r e d toi p r o d u c e a v i s i b l e m u s c u l a r t w i t c h . Three r e c o r d i n g c h a n n e l s w e r e e m p l o y e d : F z - E r b ' s point, F z - C V (fifth cervical v e r t e b r a ) a n d
Tabie 1 — Regression of median nerve SEP components on height (23 subjects).
r, Pearson's correlation coefficient; Rs, coefficient of determination; L} latency (msec); H, subject height (cm); *, p<^0.01.
Observation — The standard error of the estimate given by the polynomial model was 0.488 msec for EP; 0.479 msec for P/N13; 0.693 msec for N19; and 0.204 msec for EP-P/N13.
C O M M E N T S
T h i s s t u d y s h o w s t h a t t h e r e is a significant r e g r e s s i o n of median nerve S E P latencies both on subject height a n d limb length. T h e coefficients of determination a r e low, however, in the case of i n t e r p e a k latencies ( E P - P / i N 1 3 and P / N 1 3 - N 1 9 ) ; that is to say, these latencies have j u s t a small p r o p o r t i o n of their variability
depen-S t a t i s t i c a l a n a l y s i s i n c l u d e d : a s s e s s m e n t o f t h e d e g r e e o f l i n e a r c o r r e l a t i o n b e t w e e n S E P c o m p o n e n t s a n d s u b j e c t h e i g h t a n d / o r arm l e n g t h , w h i c h w a s a c c o m p l i s h e d t h r o u g h cal-c u l a t i o n o f P e r s o n ' s cal-c o r r e l a t i o n cal-c o e f f i cal-c i e n t ; cal-c a l cal-c u l a t i o n o f t h e r e g r e s s i o n line, t h e line t h r o u g h t h e g r a p h o f 'all s u b j e c t s ' E P p a r a m e t e r s t h a t m i n i m i z e d t h e s u m o f t h e s q u a r e s o f the d i s t a n c e s from t h e p o i n t s t o t h e line ( s e e F i g s . 1 a n d 2 ) ; c a l c u l a t i o n o f t h e coefficient o f d e t e r m i n a t i o n , R 2, w h i c h is t h e s u m of s q u a r e s e x p l a i n e d by t h e m o d e l d i v i d e d by t h e total s u m of s q u a r e s 3 a n d e x p r e s s e s t h e p r o p o r t i o n of v a r i a n c e in Y ( l a t e n c y ) e x p l a i n e d by the r e g r e s s i o n e q u a t i o n ( e . g . , f o r R 2 = 0.85, 85% of t h e v a r i a n c e in l a t e n c y is e x p l a i n e d by t h e m o d e l ) ; t h e s t a n d a r d error o f t h e e s t i m a t e w a s a l s o c a l c u l a t e d , and t h e u p p e r limit o f normal w a s s e t at 3 s t a n d a r d errors of t h e e s t i m a t e a b o v e t h e r e g r e s s i o n line.
S i n c e t h e l i n e a r m o d e l y i e l d e d l o w c o e f f i c i e n t s o f d e t e r m i n a t i o n for t h e r e g r e s s i o n o f m e d i a n S E P l a t e n c i e s o n h e i g h t , t h e e x p o n e n t i a l a n d p o l y n o m i a l m o d e l s w e r e a l s o t e s t e d , t h e l a t t e r b e i n g c h o s e n a s t h e m o s t s u i t a b l e o n e ; h o w e v e r , l i n e a r correlation o f S E P l a t e n c i e s w i t h l i m b l e n g t h s h o w e d e v e n h i g h e r R 2 v a l u e s ( s e e T a b l e s 1 a n d 2 ) .
T h e n o m e n c l a t u r e u s e d f o r S E P c o m p o n e n t s t h r o u g h o u t t h i s p a p e r i s t h a t d e s c r i b e d b y Chiappa (2). M o n t a g e s a n d c o m p o n e n t i d e n t i f i c a t i o n r o u t i n e l y u s e d a t our laboratory are s h o w n in F i g . 3.
R E S U L T S
Table 2 — Regression of median nerve SEP components on arm length (IS subjects): linear model.
r, Pearson's correlation coefficient; R2J coefficient of determination; L, latency (msec);
a, arm length (mm); SEE, standard error of estimate; *, p<^0.01.
Observations — (1) arm length was measured from the stimulation site in the wrist to the subcutaneous needle electrode at Erb's point; (2) multiple linear regression of P/N13 on arm length and nerve conduction velocity: L — 11.304 + 18.672 a —0.149v; v = nerve con-duction velocity in m/sec. Standard error of the estimate — 0.188 msec.
dent on body size. Absolute latencies, however, m a y easily be corrected for subject height, arm length and nerve conduction velocity. Although in most instances interwave latencies are more useful for clinical interpretation 2 w e have a l r e a d y pointed out the
potential a d v a n t a g e s of correcting absolute values for body size w h e n subjects of unusual height are t e s t e d ; in the case of an a b n o r m a l l y long E P - P / N 1 3 interpeak latency in an unusually tall subject, for example, one could doublecheck the a b n o r -mality by estimating the maximum acceptable latency for P / N 1 3 . If this is also prolonged, the o d d s a r e t h a t the long E P - P / N 1 3 interval is really pathological. In predicting P / N 1 3 value, one should use multiple linear regression, which t a k e s into account both a r m length and nerve conduction velocity. T h e latter is an easily obtai-nable by-product of median S E P t e s t i n g : all one h a s to do is divide arm length (in millimeters) by EP absolute latency (in milliseconds); the result is nerve conduction velocity, in m e t e r s per second. In our subjects, it r a n g e d from 56.9 to 65.9 m / s e c , the mean w a s 62.3 m / s e c , the s t a n d a r d deviation w a s 3.04. We have set the lower limit of normal as the m e a n minus 3 SD, which is 53.2 m / s e c .
T h e reason w h y we do not recommend the use of the E P - P / N 1 3 on height regression equation for estimating t h a t interpeak latency is the low coefficient of determination of the model. T h e s a m e applies to P / N 1 3 - N 1 9 interpeak latency.
In conclusion, this s t u d y s h o w s t h a t the effect of body size on median nerve S E P interpeak latencies is small and can be d i s r e g a r d e d most of the time. T h e exception would be in the event of an unusually tall or s h o r t subject being tested, or when a b n o r m a l S E P latencies coexist with low-normal nerve conduction velocity and it is necessary to check w h e t h e r t h e r e is central slowing in conduction as well. Correction of median nerve S E P latencies for body size ( h e i g h t a n d / o r arm l e n g t h ) and nerve conduction velocity is, therefore, a potentially useful diagnostic tool. Although it is not to be used on a routine basis, as in lower limb S E P s interpretation, its judicious use in selected cases may increase the diagnostic sensitivity of the test.
R E F E R E N C E S
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