NEUROFISIOLOGIA
Neurofisiología, Aplicación
en la práctica clínica
Dr. Gerardo Del Valle Carazo
Costa Rica
Generation
and Propagation of Action Potentials.
Waveform Analysis.
Dr. Jun Kimura
Kyoto University, Kyoto,
Japan
Principles
and Practice of Nerve Conduction Studies
Dr. Jun Kimura
Kyoto University, Kyoto,
Japan
Síndromes
de Atrapamiento
Dr. Norman Fuentes Víquez
Costa Rica
Multiofocal
Motor Neuropathies
Dr. Jun Kimura
Kyoto University, Kyoto,
Japan
Potenciales
Evocados Motores
Dr. Mauricio Sittenfeld
Appel
Profesor de Neurología
UCR
Costa Rica
Myopathic
Disorders
Dr. Jun Kimura
Kyoto University, Kyoto,
Japan
Cranial
Nerve Testing
Dr. Jun Kimura
Kyoto University, Kyoto, Japan
Neurofisiología,
Aplicación en la práctica clínica
Dr. Gerardo Del Valle
Carazo
Costa Rica
La neurofisiología
nos ha suministrado de una serie de poderosas herramientas diagnósticas
en nuestro quehacer diario; extensión o complemento de la historia
clínica y de la exploración física; con resultados
fácilmente obtenibles y reproducibles en la práctica rutinaria.
Nos permiten con bastante exactitud la clasificación y ubicación
anatómica de lesiones en las vías nerviosas centrales, periféricas.
unión neuromuscular y fibras musculares; lesiones que pueden o no
tener manifestaciones clínicas evidentes para el médico.
Son el resultado de agudas observaciones de diferentes autores a través
de la historia de la humanidad, que se han ido concatenando como un gigantesco
rompecabezas que aún no ha sido completado. Rápidamente
comentaremos algunos procedimientos rutinarios.
Electromiografía
Nos permite conocer
las características de los potenciales de acción del músculo
y de la unidad motora, útil para diferenciar entre el músculo
normal, las miopatías y el músculo que sufre o ha sufrido
denervación. La electromiografía convencional nos permite
evaluar la actividad que se produce al insertar la aguja. (Actividad de inserción),
la actividad en reposo (espontánea), la actividad voluntaria en el
Patrón de reclutamiento y Patrón de máximo esfuerzo.
Actividad en reposo: lo normal es el silencio o ausencia de actividad eléctrica;
patológica es la presencia de Positivos, Fibrilaciones Fasciculaciones,
Descargas repetitivas complejas como las Descargas miotónicas, etc.
La exploración de los músculos de una determinada región
anatómica, dependientes en su inervación de diferentes nervios
y raíces nerviosas, nos permite ubicar con exactitud una lesión
en las raíces, plexos o nervios periféricos.
Reclutamiento y
Patrón de Máximo Esfuerzo
Se evalúan
las características del reclutamiento de unidades motoras y del patrón
ante el esfuerzo máximo.
Reclutamiento disminuido,
con potenciales de gran amplitud, se observa en lesiones de tractos nerviosos
periféricos y reclutamiento súbito, con potenciales de muy
pobre amplitud se suele observar en la patología primaria del músculo
esquelético
Electromiografía
Cuantitativa
Evalúa las características
de los potenciales de unidad motora: amplitud, duración, número
de fases. potenciales de reinervación. El perfil Miopático
se caracteriza por potenciales predominantemente polifásicos, de
pobre amplitud y duración. El Neuropático por potenciales
de gran amplitud y duración, con aumento en el porcentaje de polifasia.
Electro neurografía
Nos permite investigar
la neuroconducción de los nervios sensitivos, motores o mixtos: Se
evalúa: Latencia distal, amplitud y duración de los potenciales
motores o sensitivos. Porcentaje de decremento de la amplitud ante
el estímulo proximal (bloqueo) y la velocidad de conducción
en los diferentes segmentos del nervio.
Neurografía
sensitiva antidrómica: El nervio sensitivo se estimula en la
región proximal y los electrodos de registro se sitúan en
la porción caudal. Neurografía sensitiva ortodrómica:
Los electrodos de registro se colocan en la porción proximal del
nervio y el estímulo se aplica en la región distal.
Neurografía
Motora: Se utilizan estímulos ligeramente supramaximales en varios
puntos del trayecto de un nervio motor y se obtienen registros en un músculo
inervado por el nervio explorado. Estos procedimientos nos permiten
clasificar la disfunción nerviosa en mielinopática o neuroaxonal.
Inching:
Permite la ubicación topográfica de una lesión con
exactitud.
Protocolo De Ondas
F: Un estímulo supramaximal es aplicado a un nervio motor, viaja
en forma antidrómica a la médula y estimula la neurona del
asta anterior. La motoneurona se despolariza y produce un potencial
tardío de latencia variable. Evalúa el funcionamiento
de la porción proximal al de la vía
periférico motora.
Reflejo H: El
estímulo inframaximal aplicado a un nervio mixto, viaja ortodrómicamente
por las fibras sensitivas, ingresa a la médula a través de
la raíz posterior y establece un arco reflejo con la neurona motora
del asta anterior, la cual se despolariza y produce un potencial tardío
de latencia constante. El estímulo es sub-umbral para las fibras
motoras. útil en la evaluación por hernia discal SI.
Estudio Simpático
Reflejo:
Investiga el sistema
Autonómico Simpático mediante la respuesta sudomotora al estímulo
eléctrico o auditivo. El estímulo eléctrico se
aplica en una mano o pie y la respuesta se obtiene en la porción
distal de los miembros,
Respuesta R-R:
Registro de las variaciones
de la frecuencia cardiaca en diferentes situaciones, respiración
normal, respiración profunda, maniobra de Valsalva, cambios súbitos
de posición, etc.
Examen de Decremento.
(T. de Jolly)
Se aplican trenes de
estímulos eléctricos supramaximales de una determinada frecuencia
a un nervio y se recogen los potenciales evocados en el músculo adecuado.
Evalúa el estado del factor de seguridad en la transmisión
neuromuscular. Util para detectar alteraciones funcionales de la unión
neuromuscular.
Potenciales Evocados
Tienen 1a habilidad
de detectar funciones sensoriales o motoras anormales, en presencia de una
historia clínica o exploración neurológica no satisfactoria.
En pacientes con disfunción de otros sistemas y sospecha de enfermedad
desmielinizante, nos pueden revelar la presencia de disfunción en
un sistema sensorial que no ha mostrado alteraciones clínicas.
Nos ayudan a definir la distribución anatómica de un proceso
patológico. Nos permiten monitorear objetivamente cambios en
el estado del paciente. Estos Test nos suministran una extensión
cuantitativa y sensible de la exploración neurológica.
Nos hablan de disfunción de una vía o sistema, No de la etiología
de la disfunción, sin embargo, las características de la disfunción
nos permiten elucubrar sobre la posible etiología.
¿Que es potencial
evocado?: Potencial evocado, es una manifestación eléctrica
tisular nerviosa, ante un estímulo externo. Han sido utilizados
desde la década de los 50s, pero no es hasta los 70s, en que se demuestra
su utilidad clínica. Tipos: somestésicos, visuales,
auditivos, cognitivos, motores. Los parámetros a evaluar: latencias
Absolutas, amplitud del potencial, latencias interpico y tiempo de conducción
central. Nos permiten clasificar la disfunción en neuroaxonal
o mielinopática.
Potenciales Evocados
Somestésícos:
El estímulo
de la sensibilidad profunda, provoca una onda de despolarización
que viaja a través del nervio periférico, asciende por las
columnas posteriores de la médula espinal, Al núcleo gracilis
y cuneatus, decusa a nivel del lenmisco medial en el tallo cerebral y alcanza
la corteza somestésica pasando por el núcleo ventral posterolateral
del tálamo y la cápsula interna. Las respuestas obtenidas
en los electrodos de registro son promediadas por el ordenador para anular
la actividad artefaccial, Se evalúa: morfología, amplitud,
latencias absolutas e interpico de los potenciales obtenidos.
Caso
Clínico. Varón de 19 años, sufrió un traumatismo
cérvico-occipital en una piscina, luego aquejó sensación
de adormecimiento del hemicuerpo izquierdo. Un Tac de cerebro realizado
a los Cuatro días fue normal. Los potenciales evocados somestésicos
mostraron ausencia de respuesta cortical derecha al estimular el tibial
izquierdo (trazo superior en rojo) y un potencial de pobre amplitud al estimular
el mediano izquierdo.
Una resonancia nuclear magnética del cerebro, mostró una
imagen compatible con un infarto isquémico del núcleo ventral
posterolateral del tálamo derecho.
Potenciales Evocados
Visuales
Estímulo mediante
un "dámero o tablero" de patrón cambiante o mediante un destelleo
de luz roja (goggles). Nos permiten evaluar las vías ópticas
retro retinianas.
Potenciales Evocados
Auditivos del Tallo Cerebral
Estimulo monoauricular
mediante "clics", mientras que el otro oído recibe un ruido enmascarador.
Se evalúan las primeras 5 ondas, las cuales representan al nervio
auditivo y las estructuras tronculares de la vía auditiva. Aplicaciones
clínicas: Detección de esclerosis múltiple, Neurinoma
acústico, Tumores del tallo cerebral: monitoreo transoperatorio.
Detección temprana de sordera en infantes. S. De muerte súbita
en infantes. Evaluación de muerte neurológica.
Blink Reflex:
Se estimula eléctricarriente
una rarna superficial del trigémino y se recogen los potenciales
M del orbicular de los párpados, inervado por el nervio facial.
P 300 y Tiempo de
Respuesta. (Potencial Evocado Cognitivo)
Útil para
evaluar pacientes con demencia; trastornos psicológicos o en tratamientos
con ciertos fármacos. El paciente debe concentrarse en discriminar
entre dos tipos de estímulos auditivos; Uno frecuente y el otro imprevisto.
Mide: % de aciertos del estímulo blanco. Tiempo de reacción.
Latencia de la positividad 300. No se conocen con certeza los generadores
de estos potenciales, pero parece relacionarse con múltiples generadores
localizados en el lóbulo parietal inferior, lóbulo frontal,
hipocampos, lóbulo temporal medial y otras estructuras límbicas.
Algunas drogas pueden alterar las respuestas.
Los potenciales cognitivos
se suman a los Test Neuropsicológicos, y a los estudios de imagen
del SNC (SPECT, TAC, RNM y PET) en el estudio y diagnóstico de procesos
demenciales.
Potenciales Evocados
Motores:
Se utiliza un estimulador
magnético (T.M.S.). Se estimula el cráneo, sobre el área
motora, en el trayecto de la vía piramidal a través de la
medula y los nervios periférico motores. Los potenciales se
recogen en un músculo apropiado. Evalúan la vía
piramidal.
Monitoreo trans
operatorio: Permite indicarle al neurocirujano, cuando debe ejecutar
o no un procedimiento, útiles en cirugía del ángulo
ponto cerebeloso, lesiones ocupantes medulares y cirugía de escoliosis.
Bibliografía:
Chiapa H. Evoked potentials
in Clinical Medicine. Third Edition. Lippincott - Raven, 1997
Philadelphia.
DeLisa J. Baran E.,
Spielholz N. Manual of nerve Conduction Velocity and Clinical Neurophysiology.
Raven press 1994 New York
Kimura J Electrodiagnosis
in Diseases of Nerve and muscle: principles and practice. FA Davis,
Philadelphia.
Llobera N. Manual De
Técnicas En Electrofisiología Clínica. Universitat
de les Illes Balcars 1995. Mallorca.
Sethi R., Thompson
L.L. The Electromyographer Handbook. Second Edition, Little, Brown
and company, Boston/Toronto.
Stálbert E,
Young R. Clinical Neurophysiology. Butterworths & CO 1981 London.
NEUROFISIOLOGÍA
Generation and Propagation of Action Potentials.
Waveform Analysis.
Dr. Jun Kimura
Kyoto University, Kyoto,
Japan
1) Analysis of Triphasic
Waveform
Analyzing waveforms
plays an important role in the assessment of nerve or muscle action potentials.
A sequence of potential changes arises as two sufficiently close wave fronts
travel in the volume conductor. This results in a positive-negative-positive
triphasic wave as the moving fronts of the leading and trailing dipoles,
representing depolarization and repolarization, approach, reach, and finally
pass beyond the point of the recording electrode. Thus, an orthodromic
sensory action potential from a deeply situated nerve gives rise to a triphasic
waveform in surface recording. The potentials originating in the region
near the electrode, however, lack the initial positivity, in the absence
of an approaching volley. A compound muscle action potential, therefore,
appears as a negative-positive diphasic waveform when recorded with the active
electrode near the end-plate region where the volley initiates. In
contrast, a pair of electrodes placed away from the activated muscle registers
a positive-negative diphasic potential indicating that the impulse approaches
but does not reach the recording site.
The number of triphasic
potentials generated by individual muscle fibers summate to give rise to
a motor unit potential recorded in electromyography. The waveform of
the recorded potential varies with the location of the recording tip relative
to the source of the muscle potential. Thus, the same motor unit shows
multiple profiles depending on the site of the exploring needle. Moving
the recording electrode short distances away from the muscle fibers results
in an obvious reduction in amplitude. Additionally. the duration of
the positive - to - negative rising phase, or rise time, becomes greater.
The rise time greater. The rise time gives an important clue in determining
proximity to the generator source. Amplitude may not serve for this
purpose. because it may decrease with smaller muscle fibers or lower fiber
density.
2) Near-Field and
Far-Field Potentials
The near field represents
recording of a potential as ¡t propagases under a pair of usually
closely, spaced electrodes placed directly over the path of the impulse.
A bipolar recording registers primarily. though not exclusively, the near
field from the axonal volley along the course of the nerve. In contrast,
the far field implies detection of a voltage step long before the signal
arrives at the recording site. usually by a pair of widely separated electrodes
located far from the traveling volleys. Original work on short-latency
auditory, evoked potentials suggested that synaptically activated neurons
in the brainstem gave rise to stationary peaks. Further work with
the human peripheral nerve has documented that stationary peaks can result
solely from the propagating impulse in the absence of synaptic discharge.
Hence stationary activities registered in far-field recording may represent
a fixed neural source such as synaptic discharges or alternatively, a nonpropagating
peak from an advancing front of axonal depolarization.
In short latency somatosensory
evoked potentials (SEP) of the median or tibial nerve a voltage step develops
between the two compartments when the moving volley encounters a sudden
geometric change at the border of the conducting medium. Here, each
volume conductor on the opposite side of the boundary, in effect, acts as
a lead connecting any points within the respective compartment to the voltage
source at the partition. Consequently, the potential difference between
the two compartments can be detected as a voltage step in far-field recording
with G1l on one side and G2 on the other side of tile boundary. The
designation, junctional or intercompartmental potential, differentiates
this type of stationary peaks from fixed neural generators and helps specify
the mechanism of the voltage step generated by the travelling impulse at
a specific location.
3) Detection of
Conductíon Block
In documenting
motor conduction block, the combination of clinical and electrophysiologic
finding,, usually circumvents the ambiguity of the criteria based purely
on waveform analysis. In the presence of conduction block, a shock
applied distally to the nerve lesion in question elicits a vigorous twitch
and a large distal amplitude despite disproportionately severe clinical
weakness associated with paucity of voluntarily activated motor unit potentials.
As an exception, the same finding also characterizes any weakness attributable
to upper motor neuron involvement or hysteria. In equivocal cases,
inability to distinguish focal pathological temporal dispersion from conduction
block poses no major practical problem because either finding usually suggests
demyelination, leading to an appropriate treatment. Tile absence of
F waves compliments conventional nerve conduction studies to document conduction
block in the proximal segment.
The use of insufficient
stimulus intensities at the proximal site erroneously reduces the proximal
amplitude. Likewise increased threshold for excitation in regenerated
or chronically demyelinated nerves may account for a reduced proximal response.
In these cases, stimulation of more proximal, unaffected nerve segment gives
rise to a normal response, indicating the passage of impulse across the
lesion site despite its abnormally elevated threshold for local excitation.
During the course of wallerian degeneration. the distal stump of the nerve
remains viable for several days at a time when its proximal part fails to
transmit the signal across the injury site.
Physiologic as well
as pathologic temporal dispersion can effectively reduce the area of diphasic
or triphasic evoked potentials. The loss of area under the waveform
seen in the absence of conduction block implies a duration-dependent phase
cancellation of unit discharges within the compound action potential.
Segmental studies provide the best means in detecting pathological non-linear
changes as opposed to physiological linear regression in amplitude and area
of compound action potential. An awareness of this possibility helps
analyze dispersed action potentials in identifying various patterns of neuropathic
processes.
NEUROFISIOLOGIA
Principles and Practice of Nerve Conduction Studies
Dr. Jun Kimura
Kyoto University,
Kyoto,
Japan
1) Short
and Long Pathways
In the evaluation of
a focal lesion, studies of a longer segment tends to lower the sensitivity
of the test because the inclusion of the unaffected segments in calculation
dilutes the effect of slowing at the site of lesion. In contrast,
studying a shorter segment helps isolate a localized abnormality and provides
better resolution of restricted lesions that may otherwise escape detection.
For example, patients with the carpal tunnel syndrome show a sharply localized
latency increase, averaging 0.8 ms across a 1 cm segment. This, compared
to a normal value ranging from 0. 16 to 0.21 ms, clearly indicates a focal
abnormality. An abrupt change in waveform of the recorded response
provides an additional, and perhaps more convincing, finding that nearly
always accompanies a latency increase across the site of compression.
In fact, waveform analysis often localizes a focal lesion unequivocally even
in the absence of an abnormal latency prolongation. This technique
suits not only in assessing a possible compressive lesion but also in characterizing
focal nature of some widespread abnormalities such as multifocal motor neuropathies.
Despite the traditional
use of conduction studies across a relatively short distal portion of the
peripheral nerves, a longer segment may provide a better result in assessing
a more diffuse or multi-segmental process such as polyneuropathies.
A longer path has an advantage in accumulating all the segmental abnormalities,
which individually might not show a clear deviation from the normal range.
Thus. in general, the longer the segment under study, the more evident the
conduction delay for a diffuse process. A number of neurophysiological
methods supplement the conventional techniques for the assessments of longer
pathways. The selection of such techniques necessarily reflects the
special orientation of each laboratory. Those of general interest
include the F wave and the H reflex.
Assume a nerve impulse
conducting at a rate of 0.2 ms/cm (50 m/s). A 20 percent delay for
a 1 0 cm segment is only 0.4 ms, whereas the same change for a 100 cm segment
amounts to 4.0 ms, an obvious increase for easy detection. Evaluating
a longer, as compared to shorter, segment also improves the overall accuracy
because the same absolute error leads to a smaller percentage of change
in measuring either the latency or the distance. In routine
practice, a surface measurement of a 10 cm nerve segment may yield an estimated
distance of 9.5 to 10.5 cm. A 1 cm difference constitutes a 10 percent
error, or a calculated conduction velocity between 5 0 m/s and 55 m/s.
The same 1 cm error in a 100 cm segment represents only 1 percent error,
or a conduction velocity between 50 m/s and 50.5 m/s. The same argument
holds in determining the effect of possible error in latency measurement.
Thus, studying a longer path offers a better sensitivity and accuracy as
well as improved reproducibility in serial studies.
2) Reproducibility
of Various Measures
We conducted a multicenter
analysis on intertrial variability of nerve conduction studies in preparation
for future drug assessments in diabetic polyneuropathy. All measurements
were repeated twice at a time interval of 1-4 weeks by the same examiners,
who underwent a hands-on workshop to standardize the method. In all,
32 centers participated in the study of 132 healthy subjects (63 men) and
65 centers in the evaluation of 172 patients with diabetic polyneuropathy
(99 men). The protocol consisted of 1) motor nerve conduction studies
of the left median and tibial nerves for measurement of amplitude. terminal
latency, and minimal F-wave latency, and calculation of motor conduction
velocity and F-wave conduction velocity and 2) antidromic sensory nerve conduction
studies of the left median and sural nerves for recording of amplitude and
distal latency, and calculation of sensory conduction velocities.
In both the healthy
subjects and patients with diabetic neuropathy, amplitude varied most, followed
by the terminal latency, and motor and sensory conduction velocites.
The minimal F-wave latency showed the least change, with the range of variability
of only 10 percent for the median nerve and 1 1 percent for the tibial nerve
in normals. The corresponding values were 12 percent and 14 percent.
respectively. in patients with diabetic polyneuropathy. These results
support the hypothesis that the minimal F-wave latency serves as the most
reliable measure of nerve conduction for a sequential study in the same
subjects. When evaluating individual patients against a normal range
established in a group of subjects, however, F-wave conduction velocity
suits better, because minimizes the effect of limb length. Alternatively.
some prefer the use of a nomogram plotting the latency against the height
as a simple, albeit indirect. measure of limb length.
3) Clinical Consideration
Our data indicate that
the length of the nerve segment under study dictates the accuracy and sensitivity
of measurement. Although studies of shorter or longer segment pose
technical merits and demerits, the choice seems to depend entirely on the
pattern of the conduction abnormalities. In summary, short distances
magnify focal conduction abnormalities despite increased measurement error,
and long distances, though insensitive to focal lesions, provide better yields
and reliability for a diffuse or multisegmental process. These findings
also underscore the importance of choosing nerve stimulation techniques best
suited for detecting the clinically supsected lesion. Thus, electrophysiologic
studies serve well only when conducted as an extension of the history and
physical examination, which provide an overall orientation for subsequent
physiologic evaluation.
NEUROFISIOLOGÍA
Síndromes de Atrapamiento
Dr. Norman
Fuentes Víquez
Costa Rica
Los síndromes
de atrapamiento constituyen un conjunto de enfermedades que consisten en
la compresión de un nervio en algún sitio de su trayecto.
Por tanto, las alteraciones que producen son similares.
Las manifestaciones
clínicas dependen del nervio afectado, del sitio comprimido, del grado
de lesión producida y de la velocidad con la que la misma se instauró.
Los síndromes
de atrapamiento son junto con las radiculopatías, la causa más
común de referencia para estudios de electrodiagnóstico.
Los estudios de conducción
nerviosa y la electromiografía con agujas son cada día más
precisas para establecer el diagnóstico y pronóstico de las
neuropatías por compresión debido a la experiencia acumulada
y a los avances tecnólogos. También se han descrito
nuevas técnicas que permiten estudiar más nervios.
Se describe en este
trabajo la sintomatología, diagnóstico diferencial y la técnica
de estudio de los nervios más comúnmente afectados como son
el mediano, ulnar, radial, peroneo y tibial. Además se mencionan
en forma breve neuropatías compresivas en otros nervios menos comúnmente
afectados.
La gran ayuda del electrodiagnóstico
como método de laboratorio permitirá cada día realizar
diagnóstico más temprano y preciso de estas enfermedades,
corregir la causa que los produce, solucionar los molestos síntomas
y discapacidad que producen en los pacientes.
NEUROFISIOLOGÍA
Multiofocal Motor Neuropathies
Dr. Jun Kimura
Kyoto University, Kyoto,
Japan
Multifocal motor neuropathy
(MNN), a variant of CIDP, deserves special mention to distinguish this potentially
treatable condition from amyotrophic lateral sclerosis (ALS) and other motor
neuron syndromes. These patients develop chronic asymmetric predominantly
motor neuropathy with multifocal conduction delay and persistent conduction
block. Although MNIN typically causes distal upper limb weakness and
atrophy, proximal muscles, biceps brachii in particular, may show hypertrophy
possibly associated with continuous motor unit activity. Similar to
earlier reported cases with sensory and motor involvements the long-lasting
conduction block suggests chronic demyelination as the pathological basis.
The patient often has normal or occasionally even increased stretch reflexes
with a normal or only slightly elevated CSF protein. Some patients
develop cranial nerve involvement and others, central demyelination.
These features make it difficult to diagnose the condition solely on the
basis of clinical examination.
Conduction blocks typically
involve unusual sites such as the median nerve in the forearm or brachial
plexus rather than the common sites of compression seen in multiple entrapment
neuropathies. Most patients have selective involvement of motor fibers
with normal sensory conduction through the sites of motor conduction block.
Both motor conduction block and abnormally increased threshold probably
reflect a chronic local demyelinating lesion, which for yet undetermined
reason becomes persistent without repair. Some patients with features
indistinguishable from ALS have multifocal motor nerve conduction abnormalities.
In one series 17 of 169 patients clinically diagnosed as motor neuron disease
had some abnormalities in motor nerve studies including 10 with conduction
block. Demonstration of motor conduction block at multiple sites differentiates
this potentially treatable clinical entity from the small subgroup of ALS
patients with only lower motor neuron involvement.
Electrophysiologic
studies must confirm the diagnosis before initiating therapeutic trials
using, for example, immunosuppressants such as cyclophosphamide. Several
authors have documented a successful treatment by intravenous immunoglobulin.
Outcomes of therapy by either immunosuppressants or immunoglobulin vary
considerably among different reported cases. Some patients improve
but do not return to normal, others stabilize, some require long term therapy
and still others become refractory to any from of treatment. Most
studies suggest more favorable results after cyclophosphamide or human immune
globulin therapy compared to prednisone or plasmapheresis.
In our series, two
cases of MNN had local conduction block involving motor but not sensory
fibers at the site of nerve swelling. A nerve biopsy, taken adjacent
to the enlargement in one patient revealed subperineurial edema and slight
thickening of the perineurium under low-power light micrographs. The
perivascular area at the center contained scattered large-diameter axons
almost devoid of myelin or with very thin myelin. These thinly-myelinated
axons usually had small onion bulbs. The presence of cytoplasmic processes
covered with basement membrane suggested their Schwann cell origin.
A nerve biopsy specimen from another patient also revealed perivascular area
containing scattered demyelinated axons surrounded by small onion bulbs.
Morphometric studies using high-power light micrographs showed a fiber density
of 6458 fibers/mm2, as compared to 7906 fibers/ mm2 in the control.
Axonal diameter and myelin thickness showed a linear relationship in the
normal subjects. By contrast, the patient had numerous large-diameter
axons with thinner myelin, although some normally-myelinated large axons
remained.
The underlying pathogenic
mechanism centers on elevated titers of anti-GMI antibodies found in a wide
variety of neuromuscular conditions, but more commonly in some lower motor
neuron disorders and MMN. Antibodies may have predilection to the
GMI component of motor fibers, which have a longer carbon chain than in
sensory fibers. Autoantibodies may exert their effect, in part, by
binding to GM 1 on the surface of motor neurons. Anti-GMI antibodies
may or may not cause motor dysfunction by binding to the nodal and paranodal
regions. Sera of patients with MNN but not with progressive spinal
muscular atrophy induced conduction block in rats tibial nerve despite similar
elevation of anti-GMI titers in both categories. These antibodies
however, may not have a causal relationship with MNN, as evidenced by many
patients without raised levels. Surface-bound antibodies directed
against major axoplasmic antigen may act interfering with remyelination
rather than causing demyelination. In some cases, nerve ischemia may
play a role in the pathogenesis.
In an extraordinary
case, a patient had duck embryo rabies vaccine three months before the onset
of her motor neuron disorder. She had multifocal conduction block, elevated
levels of anti-GMI IgM antibodies and deposits of IgM at nodes of Ranvier.
Aside from attacking motor neurons guided by the abundant GMI on the cell-surface,
anti-GMI antibodies may cause conduction block in peripheral nerves by binding
to the node of Ranvier. An autopsy study in another patient showed
findings consistent with both ALS and MMN. It is necessary to clarify the
exact pathogenesis underlying these findings to properly classify the motor
neuron disease and MNLN.
NEUROFISIOLOGÍA
Potenciales Evocados Motores
Dr.
Mauricio Sittenfeld Appel
Profesor de Neurología
UCR
Costa Rica
Los Potenciales Evocados
Motores mediante la estimulación magnética de la corteza motora,
médula, raíces, plexos y nervios periféricos son otro
de los métodos utilizados en la Neurofisiología Clínica
para la evaluación vía cortico-espinal y detección
temprana de alteraciones de la misma.
Este método
de estimulación de un campo magnético fue descubierto por
Faraday en 1831, pero no fue hasta los estudios de Baker y colaboradores
en la Universidad de Sheffield en 1975 que se estudio la posibilidad de
obtener latencias al estimular diferentes sitios del sistema nervioso central
y periférico, que tuvieran aplicación clínica.
Como cualquier método
neuro-electrofisiológico son una extensión del exploración
neurológica con una buena correlación clínica se vuelve
un instrumento altamente poderoso para no solo determinar la afectación
de la vía piramidal, sino también en la posible localización
de la misma (supracervical-cervical-dorsal-lumbar), cuando la clínica
no aporta toda la información necesaria para poder seleccionar el
estudio de imágenes en una región espécifica.
Este es un método
indoloro que lo que requiere es la aplicación de un magneto trans-cutáneamente
aplicación de electrodos de superficie en un músculo especifico,
en las cuatro extremidades Y conociendo el nivel radicular del mismo se
puede localizar el sitio de lesión, mediante la determinación
de latencias periféricas y centrales.
En esta charla se presentan
varios casos donde la estimulación magnética nos ha ayudado
en un diagnóstico temprano y en la toma otras decisiones de uso de
otros métodos diagnósticos, especialmente de imágenes.
Estos especialmente
nos han sido útiles en determinación temprana de la afectación
córtico-espinal de la Esclerosis Lateral Amiotrófica, sobre
todo cuando todavía clínicamente la hiper reflexia o los reflejos
patológicos son claros. También en las Mielopatías
Cervicales Espondilóticas, en la determinación y, localización
de la de tumores medulares con mayor afectación ventral. En
combinación con potenciales Evocados Multimodales son de gran utilidad
en Esclerosis Múltiple y también en reconocer la afectación
tanto cordonal posterior como cortico espinal lateral de la Degeneración
Combinada Subaguda de la Médula.
También recientemente
hemos utilizado la estimulación magnética como parte de la
preparación diagnostica de pacientes con epilepsia refractaria para
cirugía de epilepsia, con lesiones frontales o hemisféricas
extensas, donde clínicamente tienen en el hemisferio contra lateral
una mano espástica no cortical, logrando demostrar que el lado lesionado
contra lateral tiene latencias más prolongadas que el lado ipse-lateral
y como este lado controla de forma bilateral ambas extremidades.
NEUROFISIOLOGÍA
Utilidad de los potenciales evocados en pediatría
Dr. Roberto Brian
Gago
Hospital Nacional
de Niños
Costa Rica
La detección
de las anomalías morfológicas del Sistema Nervioso ha avanzado
importantemente con las modernas técnicas de imágenes.
Sin embargo, los trastornos neurológicos "funcionales" ameritan estudios
electrofisiológicos para su identificación. También
estas exploraciones pueden complementar el estudio de trastornos estructurales.
Los potenciales evocados auditivos. visuales o somestésicos nos permiten
evaluar esas vías neurológicas, a través de estímulos,
identificando su condición "madurativa" alteraciones mielinopáticas
o axonales incluso infraclínicas y aproximaciones topográficas
de una lesión o trastorno.
Estudios en serie pueden
ayudar a juzgar la evolución de ciertas enfermedades o pronóstico
en TCE o coma.
La ausencia de respuestas
corticales ayuda en la decisión de una hemisferectomía en
cirugía de epilepsia.
Los PES son de gran
utilidad en el monitoreo transoperatorio de la cirugía en relación
a la columna vertebral.
Los Potenciales
Evocados Auditivos del Tronco Cerebral (PEATC): Permeten detectar hipoacusias
desde el recién nacido o en niños con Retardo Mental.
Trastornos malformativos, vasculares o expansivos del puente o ángulo
pontocerebeloso podrían ser sospechados por los PEATC, así
como patología del VIII par.
Los Potenciales
Evocados Visuales (PEV): Nos permiten sospechar lesiones del nervio
o quiasma óptico así como documentar y evolucionar cegueras
corticales. Pueden ser de ayuda también en el diagnóstico
de Migraña.
Los Potenciales
Evocados Somestésicos (PES): Facilita la exploración de
la sensibilidad profunda. difícil por clínica en niños,
también, al evaluar los cordones posteriores, explora alteraciones
de la médula espinal. Los PES junto con la EMG y Neurografía
Motora, participan en el diagnóstico de neuropatías periféricas
(hereditarias, tóxicas, metabólicas, inflamatorias, traumáticas
o autoinmunes (como en el Síndrome de Guillain-Barré).
Los Potenciales
Motores (PM): Estudiados mediante estimulación magnética,
contribuyen en la exploración de toda la vía piramidal.
NEUROFISIOLOGÍA
Myopathic Disorders
Dr. Jun Kimura
Kyoto University, Kyoto,
Japan
Myopathies refer to
any disorder where primary pathology involves muscle tissue. Primary
diseases of muscle include genetically determined disorders and those of toxic
or inflammatory nature. Entities traditionally referred to as muscular
dystrophies have a clearly delineated mode of genetic transmission and a
progressive clinical course, whereas congenital myopathies show a less well-defined
pattern of inheritance and a benign clinical course. Some myopathies
also result from an inborn error of metabolism as part of a hereditary systemic
disorder. In addition, a wide variety of inflammatory processes such
as dermatomyositis and polymyositis affect the muscle. Dysmaturation
myopathy without specific histochemical or cytoarchitectural characteristics
accounts for many cases of hypotonia in infancy.
Although patients with
a myogenic disorder develop hypotonia as one of the essential features,
not all floppy infants have a primary muscle disease. In fact, disorders
of the motor unit constitute less than 10 percent of the identifiable causes
of weakness during infancy. A disease of the central nervous system
commonly produces so-called cerebral hypotonia. Other nonmyogenic
etiologies include spinal muscular atrophy, poliomyelitis, inflammatory
polyneuropathy, myasthenia gravis and botulism. Central sleep apnea
may complicate a variety of neuromuscular syndromes, sometimes appearing
as isolated symptoms of excessive daytime sleepiness. Myalgia may
herald the illness as a presenting symptom in some patients with a wide variety
of myogenic disorders.
Differential diagnosis
depends on the pattern of inheritance, the distribution of muscle weakness
and the time course of progression. Recessively inherited disorders
most often show loss of function: the homozygous or hemizygous patients
have only defective copies of the defective gene. producing little or no
functional protein. In contrast, dominantly inherited disorders most
often show change of function. the heterozygous, patients have both normal
and mutant copies of the gene, which produces an abnormal protein that causes
dysfunction of the cell. Categorization of inherited disorders simply
by their inheritance pattern thus affords some prediction concerning the
underlying biochemical defect.
The useful screening
tests include the determination of creatine kinase and erythrocyte sedimentation
rate. Electromyography and analysis of force help delineate the physiologic
mechanism of weakness and fatigue. Muscle biopsies provide histological
and histochemical confirmation. Some advocate needle biopsies over
the tradicional surgical techniques. In patients with clinical myopathic
disorders, biopsy reveals prominent myopathic features regardless of the
age of patients, although myopathy in the elderly tends to accompany neurogenic
changes. Additional studies of interest include computerized tomography
and magnetic resonance muscle imaging.
Electromyographic studies
contribute not only in differentiating myogenic from neurogenic paresis,
but also in delineating the distribution of abnormalities and categorizing
dystrophies and myopathies. The patterns classically associated with
myopathy may occasionally result from neurogenic involvement. This
confusing feature develops in late stages following complex changes of denervation
and reinnervation. Nerve conduction studies also mimic a neuropathic
process of the motor axons with a reduction in amplitude of compound muscle
action potentials and preservation of sensory nerve potentials. Neuromuscular
transmission studies show no abnormality in primary disorders of muscles.
NEUROFISIOLOGÍA
Cranial Nerve Testing
Dr. Jun Kimura
Kyoto University,
Kyoto, Japan
Isolated cranial nerve
palsies may result from lesions of the respective nerves along their extra-axial
courses or as the sole manifestation of brainstem lesions. Cranial
nerves most commonly assessed in an electromyographic laboratory include
the facial and accessory nerves. They both travel superficially to
allow easy access to electrical stimulation from the surface. They
also innervate the muscles readily approachable by needle or disk electrodes
for recording.
1) Facial Nerve
Bell's palsy affects
the facial nerve sporadically in an isolated incidence. Although the
exact etiology remains unknown accumulating evidence suggests that herpes
simplex virus type 1 (HSV-1) reactivation causes Bell's palsy in some, but
not all patients, giving a rational for antiviral therapy with acyclovir.
The same principles apply to the electromyographic examination of facial
and limb muscles. In the face, however, physiologically small motor
unit potentials may mimic fibrillation potentials, and signs of denervation
appear early in less than three weeks following injury presumably because
of the short nerve length. Serial electrodiagnostic studies help delineate
the course of the illness. The amplitude of the direct response elicited
by stimulation of the facial nerve provides the best means for prognosis
after the fourth to fifth day of onset. An amplitude greater than one
half the control value on the normal side indicates a good prognosis, although
late degeneration can still occur. Preservation or return of RI or
R2 of the blink reflex also serves as a reliable measure in predicting a
satisfactory recovery, providing a reasonable assurance that the remaining
axons will survive.
Diabetic patients who
develop a facial palsy also tend to have a more severe paresis and the evidence
of substancial denervation. Acoustic neuroma strategically located
at the cerebellopontine angle, may compress not only the facial nerve, but
also the trigeminal nerve and the pons, i.e., the efferent, afferent and
central arcs of the blink reflex. Peripheral facial palsy may herald
other symptoms of multiple sclerosis in young adults. In these cases,
blink reflex studies usually show an absent or delayed RI, indicating demyelination
of the central reflex arc, which includes the intrapontine portion of the
facial nerve. Myokymic discharges, although characteristic of this
disorder, may also appear in other conditions such as pontine gliom, and
subarachnoid hemorrhage.
2) Trigeminal Nerve
Trigeminal sensory
neuropathy characteristically evolves with unilateral or bilateral facial
numbness sometimes accompanied by pain, paresthesia and disturbed taste.
This type of neuropathy may accompany systemic sclerosis or mixed connective
tissue disease. Patients with trigeminal neuralgia have altered cutaneous
sensation not only in the affected but also unaffected adjacent divisions,
suggesting combined peripheral and central pathology. A mandibular
fracture may result in an isolated lesion of the mandibular nerve.
Demyelinating lesions affecting pontine trigeminal pathways may cause trigeminal
neuralgia in patients with multiple sclerosis. Exposure to trichloroethylene
causes a cranial neuropathy with peculiar predilection to the trigeminal
root damage. Facial numbness may herald other symptoms of an expanding
tumor involving the trigeminal nerve. Other causes of trigeminal nerve
lesion include perineural spread of carcinoma. The blink reflex help
establish abnormalities of the trigeminal nerve. Other techniques of
interest include conduction studies of the trigeminal motor nerve and of
the mandibular nerve.
3) Accessory Nerve
Pressure from a tumor
or surgical procedures of the posterior triangle can damage the spinal accessory
nerve. Other causes include stretch induced injury, cargo loading,
coronary artery bypass, carotid endarterectomy, and ligature injury during
surgical exploration. In trapezius palsies following injury of the
accessory nerve, the upper vertebral border of the scapula moves away from
the spinal vertebrae. With the lower angle of the scapula relatively
fixed by muscles supplied by the C-3 and C-4 roots through the cervical plexus,
the whole scapula slips downward and the inferior angle rotates internally,
or clockwise for the right, and counter clockwise for the left scapula as
viewed from the back. This type of winging tends to worsen by abduction
of the arm to the horizontal plane, that displaces the superior angle further
laterally. The paralysis of the sternocleidomastoid causes weakness
in rotating the face toward the opposite shoulder in proportion to the degree
of muscle atrophy. Bilateral involvement of the muscles makes the
flexion of the neck difficult. In a sequential study of patients with
trapezius palsy, nerve conduction changes revealed the evidence of spontaneous
regeneration after complete axonal degeneration.
4) Other Cranial
Nerves
Hypoglossal nerve palsy
may result from compression by kinking of the vertebral or aneurysm. or
as a complication in approximately 5 percent of endarterectomies.