CLINICAL STUDIES / ETUDES CLINIQUES
LOW INCIDENCE OF EXTENSOR PLANTAR REFLEX IN NEWBORNS IN AN INDIGENOUS AFRICAN POPULATION
FAIBLE INCIDENCE DU REFLEXE CUTANE PLANTAIRE CHEZ L'AFRICAIN
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Keywords : plantar reflex, Babinski, African, neonate, Afrique, nouveau-ne, examen neurologique, reflexes, reflexes cutane plantaire
The plantar reflex, which is one of the most frequently tested reflexes in clinical neurology(11), is usually elicited to assess the integrity of corticospinal pathways(11). In healthy adults the plantar reflex is usually flexor, but it usually becomes extensor following lesions of the corticospinal pathways(14). In healthy newborns, however, the plantar reflex has been reported to be extensor in most studies(15), but a study reported higher occurrence of flexor plantar reflex in normal newborns(6). Although it has been proposed that the pattern of plantar reflex in newborns depends on whether the sole was stimulated with noxious or non-noxious stimuli(1), this is not accepted generally (14).
The presence of extensor plantar reflex in newborns, which usually converts to flexor during the first year of life(4) is attributed to immaturity of the corticospinal tract(11). However, a study shows high occurrence of flexor plantar reflex in infants in an African population(7). It has been reported that occurrence of extensor plantar reflex is low in Africans with lesions of the corticospinal tracts(13). These observations suggest that the pattern of plantar reflex in African newborns may be different from what has been observed in Caucasians. This study was conducted to determine the incidence of extensor plantar reflex in newborns in an indigenous population in Africa.
MATERIALS AND METHODS
This study was conducted in a large maternity hospital, which is located at the centre of Ibadan, Nigeria, a large town on latitude 07° 23. 882 N and longitude 003° 54. 564 E. This maternity hospital serves the indigenous population of over 3 millions in Ibadan, which is one of the largest towns in West Africa. Most women who attend this hospital do not belong to the high socio-economic group, who are more likely to attend private clinics or tertiary hospitals. Although some deliveries still take place in home settings by traditional birth attendants in Ibadan, the number is too small to bias selection of subjects into this study significantly.
Approval for the study was obtained from local ethical committee, and informed consent was obtained from the mothers. The study population was defined as all normal deliveries in this large public maternity hospital, which adequately represent normal babies in the indigenous population that the hospital serves. Normal deliveries were defined as babies born at term by vaginal delivery from pregnancies free of complications, deliveries free of foetal distress, Apgar score of at least 8 at 1 min after birth, and birth weight at least 2.5 kg. All babies born to mothers with hypertension, diabetes mellitus, pre-eclampsia, and other medical complications of pregnancy were excluded, as well as caesarean deliveries, and prolonged labour.
Two senior doctors in the paediatric unit of this maternity hospital were trained to examine the plantar reflex reproducibly, and with minimal inter-observer variation using the blunt end of a matchstick. If the response was equivocal the examiner was allowed to repeat twice. All the babies were examined between 24 to 48 hours after birth. The babies were examined supine with the knee held in extension. The lateral aspect of the sole of the feet was stimulated with firm, but not noxious stimuli, beginning at the heel and sweeping medially at the level of metatarsal, but avoiding the base of the toes as described in a standard neurological examination text(3).
The two examiners trained with five subjects, whilst they observed each other elicit the reflex to ensure uniformity. After the method was understood, 10 subjects were examined repeatedly twice by each examiner to determine between and within observer agreement. After satisfactory kappa of 1.0 for intra-observer agreement, and inter-observer agreement of 0.8, 50 subjects were examined in the pilot phase of the study to determine sample size. In these subjects, the plantar reflex was extensor in 13 (26 %), but flexor in 37 (74 %). The number of subjects required to detect incidence of 26 % with 95 % CI from 22 % to 30 % at alpha level of 0.05 and beta level of 0.20 is 461. Subjects were recruited consecutively.
The cumulative incidence of extensor plantar reflex was calculated. The expected number of newborns with extensor plantar reflex in this population if occurrence of extensor plantar reflex was 75 %, the average incidence that was reported in some studies outside of Africa(9), was calculated. The indirect standardised incidence ratio of observed and expected number of cases was calculated.
There were 461 normal newborns with mean Apgar score of 8 (SD 0.1, range 8 -10, median 8) at one minute, mean birth weight of 3.1 kg (SD 0.4, range 2.5 – 4.6, median 3.0), mean birth length of 48 cm (SD 3, range 34 – 55, median 48), and mean occipito-frontal circumference of 34 cm (SD 2, range 30 – 49, median 34).
The plantar reflex was extensor in 85 newborns, flexor in 374 newborns, but absent in two newborns. Observed cumulative incidence of extensor plantar reflex was 18 % (95 % CI 15 – 22). The expected number of newborns with extensor plantar reflex in this population, if occurrence of extensor plantar reflex was 75 %, was 346. The standardised incidence ratio was 25 % (95 % CI 22 – 27).
This study shows low incidence of extensor plantar reflex in newborns in this indigenous African population. The standardised incidence ratio, which shows that occurrence of extensor plantar reflex in this population is about a quarter of what is observed in Caucasians, indicate that flexor plantar reflex predominate in healthy newborns in this population. Although this finding cannot be generalised to all communities in Africa, it supports anecdotal reports(13).
Both noxious and non-noxious stimuli have been used to elicit the plantar reflex, which is a skin-muscle reflex(15), in different studies. It has been suggested(1) that the patterns of plantar reflex in studies depend on whether noxious or non-noxious stimuli was used to elicit the plantar reflex. Although a study which used noxious stimuli reported predominantly extensor plantar reflex in newborns(8), while a study which used non-noxious stimuli reported predominantly flexor plantar reflex in newborns(6), predominantly extensor plantar reflex has been reported in a study which used non-noxious stimuli(10). It is noteworthy that non-noxious stimuli, like Gordon, Schaefer and Gonda(3), can be used to elicit extensor plantar reflex in subjects with lesions of the corticospinal tracts. While there are qualitative and quantitative differences in the stimuli which different investigators use to elicit the plantar reflex, it is unlikely that differences in methodology alone will account for the high occurrence of flexor plantar reflex in this study.
Anecdotal observations that the pattern of plantar reflex is different in Africans have been supported by some studies(7,12) The physio-pathological basis for the low occurrence of extensor plantar reflex in Africans(13), in clinical situations when extensor plantar reflex is expected, is not known. Although it has been suggested that high occurrence of flexor plantar reflex in African newborns may be due to early maturation of the corticospinal pathways(5), this has not been confirmed. The suggestion that low occurrence of extensor plantar reflex in Africans with lesions of the corticospinal tract is due to barefoot walking(13)which may have damaged the receptors of the soles of the feet, has also not been confirmed.
In conclusion this study shows high occurrence of extensor plantar reflex in healthy newborns in an indigenous African population. Although the physiological basis of this observation is not clear, it is possible, as earlier suggested(5), that the corticospinal tracts mature earlier in Africans.