MATURATIONAL FACTORS IN PEDIATRIC ELECTRODIAGNOSIS

The normative neurophysiologic data relating to the maturation of peripheral nerves and muscle in children has greatly expanded in the recent past (4-13). The reader is referred to the volume by Jones, Bolton, and Harper (11) for an excellent review of neurophysiologic norms in pediatric populations. Peripheral nerve myelination begins at about the 15th week of gestation and continues throughout the first 3 to 5 years after birth (14). Conduction velocities (CVs) are determined by myelination, diameter of the fiber, and internodal differences. Myelination occurs at the same rate, whether intrauterine or extrauterine. CVs are directly related to gestational and postconceptual age and are unrelated to birth weight (15,16). CVs increase in direct proportion to the increase in diameter of fibers during growth. A direct relationship exists between the diameter of the axon and the thickness of the myelin sheath. The diameter of the fibers at the time of birth has been shown to be one-half of that in the adult. No unusual acceleration of myelination occurs subsequent to birth (17). Peripheral fibers reach their maximum diameter at 2 to 5 years after birth (17,18). The nodes of Ranvier continue to remodel with peak internodal distances being reached at 5 years of age.

NERVE CONDUCTION STUDIES

In general, normal standard adult values for CVs are reached by ages 3 to 5. In infancy, upper and lower extremity CVs are similar under age 1. Subsequently, faster conductions are maintained in the upper extremities, and comparatively slower conductions in the lower extremities as with adults. Unique values for expected CVs are observed for specific peripheral nerves.

MOTOR NERVE CONDUCTION

Motor conduction velocities (MCVs) in infants are found to be one-half of adult values. In infants, conduction studies should be at least greater than 20 M/S. At birth, mean MCVs for the median, ulnar, and peroneal nerves are 27 M/S. The median nerve may lag in maturation of CV relative to the ulnar and peroneal nerves. Ulnar MCV values reach the lower adult range by age 3 (17). The slight difference between ulnar and median MCV values present in the first 3 years of life disappears in children by 4 to 5 years of age. Careful and consistent measurements are necessary to achieve reliable and valid data. Normative values or selected motor nerve conduction velocities (NCVs) are shown in Table 6.1.

DISTAL MOTOR LATENCY (DML)

Distal motor latencies (DMLs) show maturational changes between infancy and 3 to 5 years of age, similar to MCVs. Normative data for distal latencies have generally been more incomplete with ranges of distances provided (from stimulation to active electrode). While the stimulation distance should always be recorded in the electrodiagnostic report, the specific distal latency is rarely of critical importance in determining a diagnosis in pediatric electrodiagnosis, as distal peripheral entrapments are relatively uncommon. Rather, reported distal latencies, which are either unusually fast or unusually slow in the setting of otherwise normal MCVs, should raise a suspicion regarding technical problems and identification of appropriate waveforms.

The corrected DML may be used as an alternative for young children, by applying the formula of Slomic and colleagues found in Wagner and Buchthal (19):

Corrected DML = measured DML - [L - X/MCV],

where L = actual distance between stimulating cathode to the active recording electrode, and X = standard distance (4 cm for nerves of upper limbs and 5 cm for nerves of the lower limbs). Garcia and colleagues (13) have reported the most complete data to date on corrected DML in children (see Table 6.2). The corrected DML in the neonatal

TABLE 6.1 NORMAL MOTOR CONDUCTION VELOCITES (M/SEC)

NORMAL MOTOR CONDUCTION VELOCITES (M/SEC)

Data are presented as means ± standard deviation

*1 to 3 months; **3 to 6 months; ***7 to 14 years; ± 6 to 11 years

group is increased relative to other age groups, decreases over the first 12 months of life, remains unchanged between 12 months and 24 months, and slightly increases later. As most clinicians reading reports are not familiar with the corrected DML, an explanation of the calculation and normative interpretation should be included in the report, if this data is reported along with the actual DML and distance used.

TABLE 6.2 CORRECTED DISTAL MOTOR LATENCY (MSEC)

MEDIAN

ULNAR

PERONEAL

TIBIAL

<1 month

3.00 ± 0.18

2.80 ± 0.43

3.33 ± 0.54

3.21 ± 0.36

1-6 months

2.47 ± 0.18

2.20 ± 0.38

2.51 ± 0.22

2.62 ± 0.41

6-12 months

2.28 ± 0.21

1.98 ± 0.21

2.36 ± 0.31

2.55 ± 0.37

1 -2 years

2.34 ± 0.34

1.86 ± 0.16

2.35 ± 0.23

2.45 ± 0.46

2-4 years

2.34 ± 0.23

1.89 ± 0.17

2.57 ± 0.40

2.35 ± 0.24

4-6 years

2.56 ± 0.29

2.03 ± 0.25

3.02 ± 0.48

2.69 ± 0.46

Data are presented as means ± standard deviation

Corrected DML = measured DML - [L - X/MCV], where L = actual distance between stimulating cathode to the active recording electrode, and X = standard distance (4 cm for nerves of upper limbs and 5 cm for nerves of the lower limbs.

Source: Adapted from Ref. (13). Garcia A, Calleja J, Antolin FM, Berciano J. Peripheral motor and sensory nerve conduction studies in normal infants and children. Clin Neurophysiol. 2000;111 (3):513-520.

 
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