LETTERS / LETTRES
SUPERFICIAL ABDOMINAL REFLEX IS NOT SENSITIVE TO DIRECTION OF THE MOVING STIMULUS
ABSENCE DE RELATION ENTRE LA DIRECTION DU STIMULUS ET LA REPONSE AU REFLEXE CUTANE ABDOMINAL
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Keywords : Africa, Reflex, Superficial abdominal reflex, Afrique, Réflexe, Réflexe cutané abdominal
The superficial abdominal reflex is a skin – muscle reflex that is elicited by stroking the skin of the anterior abdominal wall [3, 4, 5]. Although it is generally advocated that the superficial abdominal reflex should be elicited by applying stimulus, usually with a blunt object, which moves from the lateral aspect of the abdominal wall towards the midline [4, 5], both directions for the stimulus, towards or away from the midline, are allowed in a text of neurological examination . Although direction or orientation of stimuli is important determinants of response in the visual  and the vestibular  systems, these have not been shown to be important in the somatosensory system. This study was done to determine if the direction of stroking the anterior abdominal wall, away from or towards the midline, determines the response of superficial abdominal reflex.
Subjects without neurological diseases, previous abdominal surgeries, lesions of thoracic nerves like herpes zoster, were recruited consecutively into the study after informed consent. Subjects were restricted to males to avoid the effects of previous distensions of the abdominal wall from pregnancies. Three dermatomes, T8, T10, T12, were examined on each half of the anterior abdominal wall. Visual grading of the contractions of the abdominal wall muscles, rather than neurophysiological methods, was considered appropriate for this study since most assessments of superficial abdominal reflex are done clinically. Two examiners elicited the superficial abdominal reflex of each subject, while two observers, who did not examine the subjects, graded the magnitude of contraction of the muscles on a scale of 0 – 3; 0 for no contraction, 1 for flicker of contraction, 2 for definite contraction, and 3 for contraction with deviation of the umbilicus to the side stimulated. Examiners were trained to recognize dermatomes of the anterior abdominal wall, and to apply uniform pressure over the same distance when stroking inwards or outwards, while the observers were trained to score uniformly. Stimulation was restricted to landmarks of dermatomes, and stimuli were not allowed to cross the midline or to contact the umbilicus. Five subjects were used to train both examiners and observers. After satisfactory agreement and reproducibility, subjects were recruited into the study. Each subject was examined by two examiners in each of two sessions that were separated by 24 hours because the superficial abdominal reflex fatigues easily. The scores of both observers were averaged to remove bias . To determine sample size for this study mean and standard deviations of scores for stroking towards or away from the midline of 10 subjects who were not part of the study were determined. To detect a difference of 0.5 between the mean scores of stroking towards or stroking away from the midline at of 0.05 and of 0.20, 65 subjects are required.
70 male subjects with mean age 26 years (range 22 – 34, median 25 SD 2), were studied. Mean scores of contractions of the abdominal wall muscles of all subjects are shown in stem and leaf plots of figures 1a and 1b. Mean score for stroking towards the midline was 2.4 (SD 0.75, median 2.6, range 0 – 3), while mean score for stroking away from the midline was 2.3 (SD 0.74, median 2.5, range 0 – 3). 95 % CI for the mean was 2.2 – 2.5 for stroking towards the midline, and 2.1 – 2.5 for stroking away from the midline.
The finding of this study shows no difference in the magnitude of contraction of abdominal muscles when the skin of anterior abdominal wall is stroked towards or away from the midline. This indicates that the direction of moving stimuli on the anterior abdominal wall does not contribute to the magnitude of contraction of the superficial abdominal muscles. This is not surprising since receptors of the somatosensory system are generally not sensitive to direction of stimuli .
Mechanical and electrical stimuli have been used to study the superficial abdominal reflex [1, 9]. An electrophysiological study  showed that electrical stimuli elicited two reflex discharges, a short latency monophasic wave and a long latency polyphasic wave, while mechanical stimuli which is similar to the bedside method, elicited only one long latency polyphasic wave. The two discharges of electrical stimulation, which has been likened to the R1 and R2 waves of the blink reflex, suggest that both monosynaptic pathway, which is probably spinal, and polysynaptic pathways, which probably involve the brainstem and cortex, are involved in the superficial abdominal reflex . Loss of superficial abdominal reflex in clinical situations when exaggerated muscle stretch reflex is present is probably due to involvement of corticospinal tracts in the pathway of the reflex .
In conclusion this study indicates that stimuli which move away from or towards the midline will elicit superficial abdominal reflex with similar magnitude.
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