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January2005 Vol.42 Issue:      1 Table of Contents
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Prognostic Role of EEG and Multimodality Evoked Potentials in Neonatal Hypoxic - Ischemic Encephalopathy

N. Z.El Shazly1, H. T. Tomairek2, M. M. Bashir2
Departments of Clinical Neurophysiology1, Pediatrics2,Cairo University

ABSTRACT

Objective: (1) to study the predictive role EEG and EP and compare it to the developmental outcome in asphyxiated neonates. (2) to highlight the possibility of identifying the severity of handicap in relation to the degree of test abnormality. (3) to monitor the evolution of the EEG and EP at the age of 6 months (4) to compare the diagnostic reliability of each test . Methods: 31 full-term neonates with HIE and 15 age matched controls had EEG, median nerve somatosensory evoked potentials  (MNSEP) , posterior tibial nerve somatosensory evoked potentials (PTNSEP) as well as visual evoked potentials during the first month of life and a follow up testing at the age of 6 months. The three-point scoring system established by Scalais et al., 1998 for evoked potentials and the four-point scoring for EEG established by Pressler et al., 2001 were used to grade the different tests. Developmental evaluation and neurologic examination were carried out at the age of 6 months using Denver Developmental Screening test (DDST). Results: A statistically significant association was found between the EEG, MNSEP, fVEP and the developmental outcome. The sensitivity was 90.5%, 86.4%, and 81.8% respectively, the positive predictive power was 90.9%, 95.0%, 78.3% and the negative predictive power was 77.8%, 95.0%and 50% respectively. The grade of the test abnormality is significantly related to the DDST outcome. Conclusion:  This study confirms that the reliability of EEG and EP complementary diagnostic and of prognostic value tests reflecting the status of the CNS in neonates with HIE.

(Egypt J. Neurol. Psychiat. Neurosurg., 2005, 42(1): 137-147).

 





INTRODUCTION

 

Hypoxic ischemic encephalopathy (HIE) is one of the major causes of perinatal mortality and morbidity1, it accounts for the majority of non-progressive neurologic deficits in children2. Reliable prediction of neurologic outcome in infants not only allows appropriate advice for the patients but also identifies infants who would benefit from immediate neuro-protective treatment3; however prediction is very difficult in neonates with acute nervous system injury4. Many clinical scoring methods eg. Sarnat criteria, Levene modification, Apgar score, etc. may help predict outcome reliably however sedation and high dose antiepileptic drugs may falsify the clinical grade5. Imaging techniques eg. CT, Cranial ultrasound, & MRI have also provided information about the morphology of the central nervous system (CNS) however little or no information about the function is provided6. That kind of information can be obtained by neurophysiologic testing that not only identifies the functional loss but also the degree of abnormality and the prognosis.  The evolutionary changes that take place in the EEG during the first days in life and their relation to the outcome have been investigated by many authors. Severely abnormal EEG's have a worse prognosis than those showing milder forms of abnormalities2,5,7,8.

Scalais et al.9, studied the validity of multi-modality evoked potentials (EP) in prediction of developmental outcome in neonatal hypoxia and introduced a simple scoring system relating the degree of EP abnormality to the severity of the later handicap. 

The aim of this work is (1) To study the predictive role of EEG, somatosensory evoked potentials (SEP) and flash visual evoked potentials (fVEP) in relation to developmental outcome in asphyxiated neonates. (2) To highlight the possibility of identifying the severity of handicap in relation to the degree of test abnormality. (3) To monitor the prognosis of EEG and EP at the age of 6 months as well as (4) to compare the diagnostic validity of each test separately.

 

PATIENTS AND METHODS

 

Patients:

The study was carried on 31 asphyxiated neonates admitted in the neonatal ICU in the Cairo University Children's Hospital. They were compared to 15 age, weight   and sex matched healthy controls. They were studied as regards  peri-natal risk factors eg. maternal history of pre-eclampsia, diabetes, ante-partum hemorrhage etc…..,

Apgar score was carried out at 1 minute and 5 minutes. The gestational age was assessed using the Debuwitz score. Cord ph; blood gases, complete blood count, serum electrolytes, blood glucose etc… were carried out.   Infants who fulfilled more than 3 of the following criteria were included in the asphyxia group: i) Apgar score < 3 at 5 minutes, ii) ph < 7 or BE,< -12 in cord blood or in venous blood taken within 60 minute of birth or iii) those who needed positive pressure ventilation >3 minutes. Infants with malformations, systemic infections or metabolic derangements were excluded. Clinical and neurological examinations were repeated after 24 -36 hours and thereafter including consciousness, tone and reflexes. Neuro-imaging was done including cranial ultrasound, MRI or CT.

 

Neurophysiologic studies:

EEG & EP:

EEG & EP (fVEP& SEP) were carried out to the asphyxiated neonates and the control group. The infants were transported in a portable incubator to the neurophysiology unit.

(a)    EEG was carried out on a 14 channel Nihon Kohden machine. Electrodes were applied according to the 10-20 system. Examination was performed during natural sleep without sedatives. A special double distance montage was used (FP1-T3, T3-O1, FP2-T4, T4-O2, T3-C3, C3-C4, and C4-T4)

Follow up EEG examination was carried at the age of approximately 6 months.  A low dose of chloral hydrate (50 mg/kg) was some times used if necessary.

(b)   SEP and fVEP were carried on a 2 channel Nihon Kohden Neuropac 2. Cortical SEP's were recorded after stimulation of the median nerve (MN) and the posterior tibial nerve (PTN). The active electrode was placed on c3' (2 cm lateral and 2 cm behind cz) for the MN and cz' (2 cm behind cz) for the PTN according to the 10-20 system. The reference was applied on the fz point .The band pass was 1-200 Hz , sweep was set at 200 millisecond, stimulus intensity was about 20 milli-ampere, and the stimulus duration was 0.2 millisecond an frequency was 0.2 Hz. For fVEP, the active electrode was placed on the oz point and referred to the fz point, the sweep was 500 milliseconds, band pass was 1-100 Hz, and stimulation was performed using LED goggles at a rate of 1 Hz. The two eyes were tested simultaneously. For all tests the impedance was kept below 5 Kohm. Three trials for each testing were made to ensure reliability and reproducibility.

(c)           Wave identification:

SEP: N 20 was identified as the single most prominent reproducible negative peak after 15 msec. (Scalais et al, 1998). P35 was identified as the single most prominent reproducible positive peak after 25 msec.

fVEP: P100 was identified as the single most prominent reproducible positive peak after 80 msec.

 

Data scoring:

EEG grading modified after Pressler et al.5,

G0  Normal, GI slightly abnormal activity e.g., mild asymmetry or mild voltage depression. GII   discontinuous activity with inter-burst intervals (IBI) < 10 seconds, clear asymmetry or asynchrony, GIII     IBI 10-60 seconds, no sleep wake cycles, severe depression and GIV Background activity < 10UV or IBIB > 60 sec.                                                                                                                                

SEP and fVEP grading9

G0 normal, GI increased latency or abnormal morphology or non-reproducible response. GII missing components or low amplitude <2 UV or poorly reproducible waves.  GIII absence of identifiable components.

 

Follow up:

The Clinical and neurological outcome as well as the neurophysiologic studies were reassessed at the age of  6 months, the same was carried once again for all 31 infants as well as the control group. Denver developmental screening test (DDST) 1 0 was performed.  Infants were tested for personal-social development, fine motor adaptive tasks, and language and motor skills.

Cases were divided according to developmental outcome into normal and developmentally delayed. The delayed group was then re-classified into:

Mildly delayed: delay in one field, moderately delayed: delay in 2 fields, and Severely delayed: delay in more than 2 fields (modified after DDST).

 

Data management and statistical analysis:

The data were coded and entered on an IBM compatible personal computer using the statistical package SPSS version. 12.0. Data were summarized using the mean and standard deviation for quantitative data and percent for qualitative data.

Kappa which  measures the agreement between the evaluations of two raters when both are rating the same object as well as Somers'd which is a measure of association between two ordinal variables that ranges from -1 to 1 11 were used.

RESULTS

 

(i)            Clinical data:

The study population consisted of 31 asphyxiated infants, 6 females (19.4%) and 25 males (80.6%) and 15 normal controls, 6 females (40%) and 9 males (60%).

Clinical examination of the asphyxiated neonates showed: 12 infants (38.7%) had tonic fits, 9 (29%) had multifocal and 6 (19.4%) had clonic and 4 (12.9%) had subtle fits in the form of pedaling movements. Muscle tone was normal in 17 (54.8%) infants 4 (12.9%) were hypertonic and hypotonia was found in 10 (32.2%) infants. All infants had poor suckling and poor or absent Moro's reflex. Fifteen infants (48.4%) were drowsy and 16 (51.6%) were lethargic.  Neuroimaging showed 2 (6.5%) infants had basal ganglia lesions, 18 (58%) had brain oedema, 5 (16.1%) infants had cystic encephalomalacia and 6 (19.4%) infants had normal imaging. 

At the age of 6 months 9 (29.1%) infants had normal developmental tests and 22 (70.9%) showed developmental delay. Delayed infants were then classified as 5 (16.1%) having mild, 13 (41.9%) having moderate and 4 (12.9%) having severe delay.

 

(ii)           EEG and EP results:

1)            Descriptive analysis:

EEG and EP were carried out in the same session for all infants. The age range at first examination was 2- 36 days with a mean age of 16 days.

Table (1) shows the mean and standard deviation of N 20 and p100 latencies in the control group.

Nine asphyxiated infants had normal EEG, and 22 had background abnormalities. Cortical responses to MN and PTN stimulation were examined in the same session. Ten infants had normal MNSEP and 21 had different grades of abnormalities. PTN cortical responses were difficult to obtain except in 5 infants who had normal responses and 26 had different grades of abnormalities. On first time testing, 9 infants had normal VEP, 5 had GI, 7 had GII, and 10 had GIII abnormalities. The distribution of abnormalities within different grades is shown in table (2).

A follow up EEG and EP testing was carried-out at approximately 6 months of age. Table (3) shows the prognosis of the performed testing. The majority of the infants with G1 EEG and MNSEP had improved to G0 on follow up , however infants with initial GIII and GIV either remained within the same grade or showed some improvement to a less severe grade of abnormality . PTSEP showed variable changes on follow up. Responses tended to persist within the same grade or show little improvement in grade. As regards the VEP, the follow up tends to become normal even for GII and GIII abnormalities.

 

2)            Association with outcome:

As compared to the control group, a statistically highly significant association was found between the EEG, MNSEP, fVEP and outcome of DDST (p = 0.001for EEG and p < 0.00001 for MNSEP and, 0.001 for VEP).

Sensitivity was 90.9%, 86.4%, 81.8% for EEG, MNSEP and fVEP respectively. The positive predictive power of the tests was 90.9% 95.0%, 78.3 % for EEG, MNSEP and fVEP respectively while the negative predictive power was 77.8 %, 95.0%, 50.0% respectively. 

Somers 'd measure of raw predictive value  showed a trend of increase in the grade of EEG and EP with increase in the severity of handicap particularly for the ends of the relation i.e. developmentally normal children had normal or GI EEG and EP that improved to G0 on follow up  while severely handicapped infants had GIII and GIV results that persisted or showed little improvement on later examination. As for intermediate grades of handicap, the grades of abnormalities were variable (table 5). It is to be noted that on follow up GI abnormalities improve to G0 so that normal children have normal EEG, SEP and fVEP while severely handicapped children persist to have GII and GIII EP and GIII & GIV EEG (tables 4, 5). The positive predictive power of follow up tests was 100% for MNSEP, VEP, and EEG. As regards the PTNSEP the association   with outcome was statistically non significant (p = 0.095).

 

3)     Association between different diagnostic tests:

Kappa measure of agreement showed a symmetric relation between different diagnostic tests i.e. G0 EEG is associated with G0/I fVEP and SEP and GIII and GIV EEG is associated with GII and GIII fVEP and SEP, 8 (88.8%) out of 9 infants with G0 EEG had G0/I fVEP and 7 (77.7%) had G0/I MNSEP; 16 (84.2%) out of 19 infants with GIII & GIV EEG had GII / GIII fVEP and 14 (73.7%) had GII / GIII MNSEP


 

 

Table 1. Mean and Standard deviation of evoked response latencies in the normal control group.

 

Test

First 28 days

Age 6 months

Mean

SD

Mean

SD

    MNSEP

28.1

±2.6

19.2

±1.3

   PTNSEP

35.3

±2.7

22.8

±2.6

   VEP

117

±3.6

113.2

±2.9

  

Table 2. Distribution of asphyxiated Infants among the grades of the performed tests.

 

Test

G0

GI

GII

GIII

GIV

Total

EEG

 

 

 

 

 

 

Number

9

1

2

7

12

31

Percentage

29.00%

3.25%

6.45%

22.60%

38.70%

100%

MNSEP

 

 

 

 

 

 

Number

10

5

7

9

 

31

Percentage

32.30%

16.10%

22.60%

29.00%

 

100%

PTNSEP

 

 

 

 

 

 

Number

5

4

10

12

 

31

Percentage

16.10%

12.90%

32.30%

38.70%

 

100%

fVEP

 

 

 

 

 

 

Number

9

5

7

10

 

31

Percentage

29.002

16.10%

22.60%

32.30%

 

100%

 

 

Table 3. The evolution of EEG and EP at the age of 6 months.

 

 

Initial test

Follow up grade

Test grade

Number of infants

EEG

G0

GI

GII

GIII

GIV

GO

9

8

1

 

 

 

GI

1

1

 

 

 

 

GII

2

2

 

 

 

 

GIII

7

1

 

4

2

 

GIV

12

2

1

1

2

6

MNSEP

 

 

 

 

 

GO

10

10

 

 

 

 

GI

5

3

2

 

 

 

GII

7

2

5

 

 

 

GIII

9

1

3

5

 

 

PTNSEP

 

 

 

 

 

G0

 

 

 

 

 

 

GI

 

 

 

 

 

 

GII

 

 

 

 

 

 

GIII

 

 

 

 

 

 

fVEP

 

 

 

 

 

G0

9

9

 

 

 

 

GI

5

5

 

 

 

 

GII

7

5

 

 

2

 

GIII

10

2

 

5

3

 

 

 

Table 4. The association between developmental outcome and grade of EEG and EP abnormality within the first 28 days.

 

Developmental outcome

Test

Test grade

Total

G0

 

 

 

 

 

EEG

Number

Percentage

G0

GI

GII

GIII

GIV

 

 

7

77.8%

 

1

11.1%

 

 

 

1

11.1%

 

9

100%

MNSEP

No

Percentage

 

7

77.8%

 

2

22.2%

 

 

 

 

9

100%

PTNSEP

Number

Percentage

 

3

33.3%

 

3

33.3%

 

3

33.3%

 

 

 

9

100%

fVEP

Number.

Percentage

 

4

44.4%

 

4

44.4%

 

1

11.1%

 

 

 

9

100%

GI

 

 

 

EEG

Number

Percentage

 

 

 

 

2

40%

 

3

60%

 

5

100%

MNSEP

Number

Percentage

 

 

2

40%

 

1

20%

 

2

40%

 

 

5

100%

PTNSEP

Number

Percentage

 

 

 

2

40%

 

3

60%

 

 

5

100%

fVEP

Number

Percentage

 

2

40%

 

 

1

20%

 

2

40%

 

 

5

100%

GII

 

 

 

EEG

Number

Percentage

 

2

15.4%

 

 

2

15.4%

 

5

38.5%

 

4

30.7%

 

13

100%

MNSEP

No

Percentage

 

3

23.1%

 

1

7.7%

 

5

38.5%

 

4

30.7%

 

 

13

100%

PTNSEP

Number

Percentage

 

2

15.4%

 

1

7.7%

 

4

30.7%

 

6

46.2%

 

 

13

100%

fVEP

Number

Percentage

 

3

23.1%

 

1

7.7%

 

2

15.4%

 

7

53.8%

 

 

13

100%

GIII

 

 

 

EEG

No

Percentage

 

 

 

 

 

4

100%

 

4

100%

MNSEP

Number

Percentage

 

 

 

1

25.0%

 

3

75.0%

 

 

4

100%

PTNSEP

Number

Percentage

 

 

 

1

25.0%

 

3

75.0%

 

 

4

100%

fVEP

Number

Percentage

 

 

 

3

75.0%

 

1

25.0%

 

 

4

100%

Table 5. Association between the developmental outcome, EEG and EP at the age of 6 months.

 

Developmental outcome

Test

Test grade

 

Total

 

G0

 

 

 

 

EEG

Number

Percentage     

G0

GI

GII

GIII

GIV

 

9

100%

 

 

 

 

 

9

100%

MNSEP

Number

Percentage       

 

9

100%

 

 

 

 

 

9

100%

PTNSEP

Number

Percentage   

 

5

55.6%

 

3

33.3%

 

1

11.1%

 

 

9

100%

fVEP

Number

Percentage

 

9

100%

 

 

 

 

9

100%

GI

 

 

 

EEG 

No

Percentage   

 

1

20%

 

1

20%

 

 

3

60%

 

5

100%

MNSEP

Number

Percentage     

 

1

20%.

 

3

60%

 

1

20%

 

 

 

5

100%

PTNSEP

Number

Percentage    

 

2

40%

 

2

40%

 

 

1

20%

 

 

5

100%

fVEP

Number

Percentage

 

3

60%

 

 

2

40%

 

 

 

5

100%

GII

 

 

 

EEG 

Number

Percentage   

 

4

30.7%

 

1

7.7%

 

4

30.7%

 

1

7.7%

 

3

23.1%

 

13

100%

MNSEP  

No

Percentage    

 

5

38.5%

 

4

30.7%

 

2

15.4%

 

2

15.4%

 

 

13

100%

PTNSEP

Number

Percentage    

 

6

46.2%

 

1

7.7%

 

2

15.4%

 

4

30.7%

 

 

13

100%

fVEP

Number

Percentage

 

8

61.5%

 

 

3

23.1%

 

2

15.4%

 

 

13

100%

GIII

 

 

 

EEG 

No

Percentage   

 

 

 

1

25.0%

 

 

3

75.0%

 

4

100%

MNSEP 

Number

Percentage     

 

 

1

25%

 

 

3

75.0%

 

 

4

100%

PTNSEP

Number

Percentage     

 

 

 

1

25.0%

 

3

75.0%

 

 

4

100

fVEP

Number

Percentage

 

1

25.0%

 

 

 

3

75.0%

 

 

4

100%


DISCUSSION

 

The evaluation of neonatal asphyxia is problematic12. Diagnosis of HIE is mainly clinical however, prediction of neurologic sequlae depending on clinical criteria is difficult and may give false indications. Neurophysiologic testing was shown to be useful in helping to predict  the outcome in neonatal asphyxia. Previous studies examined either the EEG alone or evoked potentials as predictors of neurologic sequelae following neonatal asphyxia.  In this study both EEG and bimodality EP were carried out. The combined use of different neurophysiologic testing reflects the activity of different levels of the neuraxis e.g. the EEG and fVEP reflect the hemispheric activity while the SEP reflects the peripheral nerves, spinalcord, brain stem, thalamus and cortex. Combined testing provides a more global assessment of the nervous system.9

The study extends several points previously made about the role of EEG and EP in prognostication. In the literature, the sensitivity of EEG, SEP and fVEP ranges between 69%-89% for EEG2,7,13,14,15, 90-100% for SEP 16,17,18 and 77-91% for fVEP16,19. In our study sensitivity was almost comparable to previous studies, 90.9%, 86.4%, and 81.8% for EEG, MNSEP and fVEP respectively.

It is recommended that neurophysiologic testing should be carried out as soon as possible after the acute insult since on monitoring the progress of the EEG and the EP it was found that improvement occurs with time,  The degree and rate of improvement depend upon the degree of abnormality in the initial testing almost all G I abnormalities improve to G0 at the age of 6 months , and even severe forms of abnormalities (GIII & GIV) not only show improvement but may even become normal particularly for the fVEP (table 2). This corresponds to the findings of Whyte et al.20, who reported that on follow up mildly abnormal fVEP's (corresponding to GI in our study) were normal on subsequent examination. Corresponding findings were reported as regards the EEG5,7,21,8. Thus delay of the testing may falsify the prediction. The cause of such improvement may be explained by the fact that by time the pathologic reaction that accompanies the acute insult starts to resolve. This is accompanied by a concomitant improvement in the nervous tissue function which is in turn reflected in the spontaneous cellular activity (EEG) as well as the responses todifferent stimuli (EP)6.

However we recommend follow up testing at the age of 6 months as well. In agreement with previous studies9,15,16,20,21,23,24,25 we found that the persistence of abnormal EEG or EP response at 6 months reflects persistent pathology and indicates a bad prognosis while the improvement in grade of abnormality usually implies a more optimistic prognosis.

Our study emphasizes the interest of analyzing the degree of EEG and EP abnormality in relation to the degree of the expected handicap. The initial and follow up EEG and EP revealed a graded series of patterns varying from normal to severely abnormal EEGs and EP which had different prognostic significance depending on the degree of abnormality. Corresponding to Scalais et al.9, in this study we found a highly significant relation between the grade of the EEG and EP abnormality and the degree of expected handicap. G0 &I are associated with normal developmental outcome or a mild developmental delay on the other hand GIII and GIV abnormalities are accompanied by severe handicap. We also agree that the persistence of   the grade of abnormality or little improvement indicates severe disability on follow up. In the risk – scoring systems of fVEP and SEP abnormalities26 developed for quantitative evaluation of handicap after neonatal asphyxia, the authors found that the incidence and degree of abnormalities were related to the degree of handicap and that repeated observations were important; permanently high risk scores at repeated investigations were a serious prognostic sign. Similarly other studies emphasized that abnormalities that persist or worsened correlate with severe neurologic impairment whereas abnormalities which improve or normalize are associated with a more favorable prognosis20,22,23. On the other hand some studies  could not infer the severity of disability from the  degree of EP abnormality  this could be attributed to the fact that nearly all infants with abnormal outcome had severe handicap16 or because of the small size of the sample (10 asphyxiated infants) and the delayed first testing (first time examination within the first 6 months with a follow up at a mean age of 20 months), a time where most of the milder form of abnormalities within the acute stage have improved or even become normal, as shown in our study, and thus would give false indications 27. In the study by Nassar et al.28, a scoring system was used to categorize the fVEP19 and cases with GIV, V, VI abnormalities had abnormal outcome (GIV and V correspond to GII in our study and GVI corresponds to GIII.) However such a relation could not be verified for milder forms of abnormalities (GI, II, and GIII that correspond to G0, I in our study) furthermore no report about the severity of handicap and the grade of the fVEP was mentioned. The discrepancy could be attributed to the different scoring methods, and the shorter follow up (1 month).

Despite that a positive correlation was found between the bilaterally abnormal PTNSEP and the presence of CP at the age of 3 years29  yet in our study the difficulty in obtaining the PTNSEP and the low consistency of the waveform may explain the poor prognostic value of that test, moreover we examined the cortical response of the PTN unilaterally . Possibly a longer follow up is needed in our study.

In the literature, it was reported that upper limb SEP's were more sensitive than EEG in predicting short term outcome30 yet logistic regression analysis revealed that results of our  tests are homogeneous and parallel to one another hence we recommend the combined use of the EEG and EP in order to have a more global overview of the state of the nervous system as neurophysiologic tests are complementary to one another.

 

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الملخــص العربـــى

 

تعتبر حالات الاختناق الوليدي واحدة من أهم أسباب الإعاقة الذهنية و الحركية في الأطفال حديثى الولادة . يهدف هذا البحث الى دراسة إمكانية استخدام الرسم الكهربي للمخ والجهد المثارة الكهربية لتبين آثار الاختناق الوليدي علي المخ والتنبؤ بدرجة الإعاقة العصبية التى قد تظهر فيما بعد.

أجريت هذه الدراسة على٤٦ طفل من حديثى  الولادة مكتملى النمو وتم تقسيمهم إلى مجموعتين،  ۳۱ طفل مكتمل النمو يعانون من اختناق وليدى و 15 طفلا سليما حديث  الولادة مكتمل النمو. تم إخضاع هؤلاء الأطفال للفحص الأكلينيكى والعصبى  الدقيق وفحص الرسم الكهربى للمخ وجهد المثار الحسى للطرفين العلوى والسفلى والجهد المثار والبصرى فى الشهر الأول من العمر ومتابعة هؤلاء الأطفال إكلينيكيا وعصبيا وعن طريق الفحوصات الألكتروفسيولوجية حتى الشهر السادس من العمر ثم تم استخدام مقياس دنفر لقياس التطور لتقسيم الأطفال إلى معاقين وأصحاء ثم تقسيم المعاقين إلى درجات مختلفة من الإعاقة.

وقد أظهرت نتائج هذه الدراسة أن التغيرات غير الطبيعية التى تحدث فى الرسم الكهربى للمخ والجهد المثار الحسى للطرف العلوى والجهد المثار البصرى تتوافق مع نتائج مقياس دنفر وتزداد حدة هذه التغيرات كلما زادت درجة التأخر النمو الذهنى بينما لم يظهر الجهد المثار الحسى الأسفل هذا التوافق. ومن هنا نستنتج أن استخدام الجهود المثارة الكهربائية والرسم الكهربى للمخ وسائل حساسة لاستنتاج التطور الذهنى والحركى على المدى القصير.



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