cias em função da altura, comprimento do membro superior e velocidade de ... regressão linear quando a variável independente era o comprimento do braço; ...
MEDIAN NERVE SOMATOSENSORY EVOKED POTENTIALS STUDIES
ON LATENCY VARIABILITY AS A FUNCTION OF SUBJECT L I M B L E N G T H AND N E R V E CONDUCTION VELOCITY
JOAQUIM
P.
BRASIL-NETO
HEIGHT,
*
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 nerve 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 conduction 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 performed 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 latencies could be p r e d i c t e d by a linear r e g r e s s i o n model 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 height, 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 models proved 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. Multiple linear 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 (arm l e n g t h a n d NCV) w a s found to be b e t t e r t h a n simple linear 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 latency. T h e r e g r e s s i o n l i n e for E P - P / N 1 3 latency on h e i g h t w a s found to be a polynomial 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 significant b y t h e «F» t e s t ( a l p h a = 1%), t h e model h a d a low R2 value (0.41). T h e s a m e a p p l i e s to t h e P/N13-N19 i n t e r p e a k latency regression curve, b u t t h e r e g r e s s i o n w a s significant 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 latencies 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 of m e d i a n S E P s , a b s o l u t e latencies m a y occasionally b e i m p o r t a n t , a n d should b e corrected for body size; in u n u s u a l l y tall s u b j e c t s , it m i g h t be useful to double-check E P - P / N 1 3 i n t e r wave latency p r o l o n g a t i o n by e s t i m a t i n g t h e m a x i m u m expected P / N 1 3 latency, u s i n g a model t h a t t a k e s into a c c o u n t b o t h limb l e n g t h a n d NCV. Potenciais evocadas sômato-sensitivos do nervo mediano: mediano: estudos estudos de de variabilidade variabilidade das das latên¬ latências em função da altura, comprimento do membro superior e velocidade de condução nervosa. cia* R E S U M O — Descrição dos resultados RESUMO r e s u l t a d o s de estudos e s t u d o s de análise de regressão r e g r e s s ã o interessando i n t e r e s s a n d o as latênciias dos ppotenciais tencies o t e n c i a i s evocados sômato-sensitivos ((PESS) P E S S ) do nervo m mediano, e d i a n o , como variáveis d e p e n d e n t e s , e a altura dependentes, a l t u r a do indivíduo, o comprimento c o m p r i m e n t o do membro m e m b r o superior s u p e r i o r e a velocidade de condução nnervosa e r v o s a (VCN), como variáveis iindependentes. n d e p e n d e n t e s . Os testes t e s t e s foram realizados em 23 pessoas normais. n o r m a i s . As A s latências latências absolutas a b s o l u t a s dos dos PPE E S SS podiam p o d i a m ser s e r previstas p r e v i s t a s por por um u m modelo de de r e g r e s s ã o linear regressão l i n e a r qquando u a n d o a variável viariável iindependente n d e p e n d e n t e eera r a o comprimento c o m p r i m e n t o do braço; b r a ç o ; quando q u a n d o tal tal variável era e r a a aaltura, l t u r a , pporém, o r é m , ttanto a n t o o modelo exponencial qquanto u a n t o o polinomial mostravam-se mostravam-se melhores, este último ú l t i m o apresentando a p r e s e n t a n d o os melhores m e l h o r e s coeficientes de determinação, d e t e r m i n a ç ã o , R 2. A curva curva de regressão r e g r e s s ã o das d a s latências l a t ê n c i a s interpicos interpicos (E E P - P / N 1 3 ee P/N13-N19) P/N13-N19) em em função função dia da altura a l t u r a revelouse do tipo polinomial, com baixo b a i x o valor de R 2. Embora E m b o r a as latências interpicos sejam os parâmetros m râmetros mais a i s úteis ú t e i s para p a r a a iinterpretação n t e r p r e t a ç ã o clínica ddos o s PESS P E S S do d o nervo n e r v o mediano, as a s latênciias latências a b s o l u t a s podem absolutas p o d e m eeventualmente v e n t u a l m e n t e ser s e r importantes i m p o r t a n t e s e devem ser corrigidas c o r r i g i d a s para p a r a as dimensões corporais do paciente. São ressaltadas r e s s a l t a d a s as situações em que q u e tais t a i s correções poderiam p o d e r i a m ser s e r pot e n c i a l m e n t e úteis para tencialmente p a r a aumentar a u m e n t a r a ssensibilidade e n s i b i l i d a d e do teste. teste.
The relationship between absolute somatosensory evoked potential ( S E P ) latencies 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 : Neurophysiologist. Dr.
Joaquim P.
Brasil-Neto
— SQS
303,
Bloco
I,
Apto.
506
-
* Neurologist,
70336 Braslia DF
-
Clinical
Brasil
S E P s ; for upper limb testing, however, the general agreement has 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 d . Although there can be no doubt as to the fact that 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 whose height is significantly different from the range found in the population from which median nerve S E P s normative values were derived. Our normative studies carried out in a sample of 51 normal Brazilian s u b j e c t s indicated that 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 a in Americans (5.2 msec). In other w o r d s , some of his normal subjects could have an abnormal latency, if tested according to our norms. T h e most likely explanation for the difference is that our subjects were presumably shorter and had shorter upper limbs than their North American counterparts. In order to further clarify this problem we have thouroughly studied the relationship between subject height, arm length, nerve conduction velocity and median nerve S E P latencies. We are 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 t have shown a linear correlation between arm length and P I 4 , a component which they believed to be of thalamic origin; they did not, however, atempt to study the correlation of other components to body size. Sauer and Schenck 5 pointed out that in hereditary motor and in sensory neuropathy type I the prolonged latency of the S E P s depends exclusively on the decreased conduction velocity of peripheral nerves, with normal afferent conduction within the cord; however, they did not report any method for correcting S E P latencies for decreased nerve conduction velocity. T h e aim of this study is, therefore, to find the best mathematical models to predict maximum acceptable S E P absolute latencies for a given subject, taking into account his height, arm length and peripheral conduction velocity. We feel this kind of information may be useful when testing unusually tall subjects, to prevent mistaking normal latencies for pathologically prolonged ones; conversely, this approach might be useful to increase diagnostic sensitivity when testing unusually short subjects, since their latencies may be within the normal range, although actually prolonged for their body size. 2
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2
4
MATERIAL* & M E T H O D S Median nerve S E P s from 23 normal 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 of the media* nerve in the w r i s t w a s 'accomplished t h r o u g h u s e of an SC 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 connected to a TE 42 electrom y o g r a p h . S t i m u l i w e r e s q u a r e p u l s e s of 0.2 m s e c duration, delivered at a frequency of c p e r second at t h e m i n i m u m i n t e n s i t y required 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 recording 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 vertebra) and Fzcontraliateral c o r t e x (C3' or C4'). F i l t e r setting! w a s 100-3,000 H z ; a n a l y s i s time w a s 50 m s e c pofet-stimulus d e l a y w a s 2.0 m s e c .
Statistical 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 of t h e d e g r e e of linear correlation b e t w e e n S E P c o m p o n e n t s and 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 calculation of P e r s o n ' s correlation coefficient; calculation of t h e r e g r e s s i o n line, t h e line through t h e g r a p h of 'all s u b j e c t s ' EP p a r a m e t e r s that m i n i m i z e d t h e s u m of t h e s q u a r e s of the d i s t a n c e s from the p o i n t s to t h e line (see F i g s . 1 and 2 ) ; calculation of t h e coefficient of 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 model divided by the total s u m of s q u a r e s 3 and e x p r e s s e s t h e proportion of variance 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 equation (e.g., for R 2 = 0.85, 85% of t h e variance in latency is e x p l a i n e d by the m o d e l ) ; the s t a n d a r d error of the e s t i m a t e w a s also calculated, and t h e upper limit of normal w a s s e t at 3 s t a n d a r d errors of the e s t i m a t e above the r e g r e s s i o n line. Since t h e linear m o d e l y i e l d e d l o w coefficients of d e t e r m i n a t i o n for the r e g r e s s i o n of m e d i a n S E P latencies on h e i g h t , the e x p o n e n t i a l and p o l y n o m i a l m o d e l s w e r e also tested, the latter b e i n g chosen as 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 , linear correlation of S E P latencies w i t h limb l e n g t h s h o w e d even h i g h e r R 2 v a l u e s ( s e e T a b l e s 1 and 2). T h e n o m e n c l a t u r e u s e d for 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 paper i s that described by Chiappa (2). M o n t a g e s and c o m p o n e n t identification r o u t i n e l y used at our laboratory are s h o w n in F i g . 3. RESULTS All t h e r e l e v a n t r e s u l t s of t h e s t a t i s t i c a l a n a l y s i s are g r o u p e d in T a b l e s 1 and 2; graphs s h o w i n g the r e g r e s s i o n l i n e s c a n be s e e n in F i g s . 1 and 2.
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
