INTRODUCTION

 

In the recent decade, the clinical usefulness of Sensory evoked potentials (SEPs) entered the operating room, allowing the intraoperative monitoring of the CNS and thus safeguarding CNS structures during high-risk surgeries¹. One of the most common causes of myelopathy and radiculopathy is the cervical disc herniation^{2, 4,5,11}.

Somatosensory Evoked Potentials test is a way to assess the sensory signals passing from the peripheral nervous system through the dorsal column of the spinal cord to the higher sensory cortex. In cervical disc prolapse, these ascending sensory pathways are compressed leading to abnormalities in SEPs. The spinal cord evoked potentials are corresponding to the activity in the posterior column pathways^1-10.

MRI cervical spine is the main investigation for anatomical diagnosis in degenerative cervical spine. It can show the number of discs causing compression as well as the level of this compression. It can also show the canal width, cord signal and grade of cord compression but it cannot give any informative regarding the cord or radix dysfunction^5,6.

 

PATIENTS AND METHODS

 

This is a Prospective study including 20 patients having cervical disc herniation. All patients had surgery in Cairo university hospitals. 13 were male (65%) and 7 were females. The age of males ranged from 45 to 65

with mean age 55 years. The age of females were between 40 to 60 with a mean of 50 years. All clinical findings are shown in (Table 1).

Patients included in this study had radiculopathy and/or myelopathy that could be attributed to cervical disc compression and were diagnosed by Magnetic resonance imaging (MRI). All patients with radiculopathy went under conservative treatment with no improvement of symptoms.

All patients were indicated for surgery according to radiological findings, together with persistent symptoms and signs not improved by conservative measurements.

The myelopathy is an indication for surgery after exclusion of other neurological causes. In addition, the persistent disabling radicular pain not relieved by conservative therapy, with disc prolapse compressing the cord documented by MRI is another indication^{2, 12,13}.

All patients were operated upon by anterior cervical discectomy with cage fusion. 10 patients (50%) had single level, 8 (40%) had double level and two (10%) had more than two levels. The two patients who had more than two levels were fixed by anterior cervical plate.

SEPs were recorded using Nihon Kohden® Neuropack  machine (MEB_9200K), Japan from both median nerves in all cases. The stimulating electrode was placed at the wrist. The recording electrodes were placed as follows: over both Erb,s points, fifth spinous process, both C3 and C4. Fz was used (according to the international 10-2- system) as a reference electrode and a ground electrode placed on the forearm between the stimulating and recording electrodes. The Erb’s point potential is N9, cervical potential is N13 and the cortical potential is N20. The median nerve was stimulated at the wrist and the intensity was to elicit visible twitches of the muscle. 200 responses were averaged for each SEP measurement. 

Assessment of SEPs for both median nerve, preoperatively and in first, third and sixth month after surgery, was done using the abnormality criteria of Spehlman¹⁴. These SEPs values are compared for statistical significance using the Friedman variation analysis. Wilcoxon signed Range test was applied to know when significant improvements occurred in parameters with statistical significance (Table 2).

 

RESULTS

 

Clinical assessment of the patients postoperatively in the first, third and sixth months showed that there was no persistent radicular pain, but three cases showed local shoulder tenderness due to a shoulder problem associated with the cervical problem and it was known before surgery. However, 10 cases (50%) still had radicular sensory loss, but sensory loss was improved in the remaining 10 cases (50%). Regarding the weakness, six cases (30%) still had radicular motor weakness, but improvement of motor power was noticed in 11 cases (55%) with radiculopathy and myelopathy in the postoperative findings.

There was no remarkable hypoactivity in the deep reflexes during the first month postoperatively like the preoperative period and no spasm in the paravertebral muscles.

Later on, after 3 months, 5 cases (25%) showed radicular motor weakness and 4 cases (20%) showed subjective sensory loss. and after six months, 2 cases (10%) had sensory loss and one case (5%) still had radicular motor weakness.

 

Preoperative and postoperative findings of SEPs:

SEPs finding were assessed for the median nerve, including both sides in bilateral cases, preoperatively and postoperatively at first, third and sixth month. All parameters were analyzed statistically using Friedman variance analysis to find out which findings were meaningful.

We observed that the interpeak latencies of cervical – cortical (N13 – N20), Erb – cervical (N9 – N13), Erb – cortical (N9 – N20), brain stem – cortical (N14 – N20), brain stem – N14 and cortical N20 were not statistically significant (p>0.05).

On the other hand we noticed that N14 latency originating from the nuclei of the dorsal column of the medulla spinalis was statistically meaningful (p<0.05).

Marked improvement was noticed between 3 and 6 months (p<0.05) and at 6 month (p<0.05). This was concluded by studying the findings post operatively in regards to the latency difference of N14 which appeared very near to the normal values.

This improvement in N14 latency means that the neurophysiological and functional improvement occurred in the posterior column function in the postoperative 6 months.

 

Table 1. Preoperative clinical findings.

 

Case	Symptoms duration	Pain	Sensory loss level	Muscle strength weakness	PMS	Atrophy	Reflex absence
1	8 month	Right	C4, C6	- Arm abduction - Wrist extensor ms group	+	-	Deltoid, triceps
2	One year	Left	C6	Wrist extensor ms group	+	-
3	15 month	Left	C7	Elbow extensor ms group	+	-
4	5 month	Right	C5, C6	Elbow flexors, wrist extensors	+	-
5	2 years	Left	C6	ـــــــــــــــــــــ	+	-
6	1,5 year	Right	C6, C7	Wrist extensors elbow extensors	+	-	Brachioradialis
7	3 years	Bil. SB	C6	All arm muscle group	+	-	Deltoid biceps, tri, brachioradialis
8	One year	Right	C7	Elbow extensors	+	-	Triceps
9	6 month	Bil. SB	C6, C8	Wrist extensors finger flexors	+	-	ـــــــــــــــــ
10	One year	Left	C6, C7	- Wrist extensors - Elbow extensors	+	-	brachioradialis
11	10 month	Bil. SB	Bil. or whole arm	Quadriparesis	+	-	Deltoid, biceps, triceps, brachioradialis
12	2 month	Left	C6	Wrist extensors	+	-	ـــــــــــــــــ
13	3 years	Left	C7	ـــــــــــــــــ	+	-	Triceps
14	4 months	Right	C5	Elbow flexors	+	-	ـــــــــــــــــ
15	2 years	Bil. SB	C5, C6	- Elbow flexors - Wrist extensors	+	-	ـــــــــــــــــ
16	10 years	Left	C5	ـــــــــــــــــ	+	-	ـــــــــــــــــ
17	11 months	Bil. SB	Bilaterally whole arm	Quadriparesis	+	-	deltoid, biceps triceps, brachioradialis
18	6 years	Left	C6, C7	- Wrist extensors - elbow extensors	+	-	Brachioradialis
19	7 months	Right	C4, C6	- Arm abduction - Wrist extensors	+	-	Deltoid, triceps
20	3 years	Left	C7	Elbow extensors	+	-	ـــــــــــــــــ

PMS Paravertebral muscles

Table 2. Mean values, pre and postoperatively (Wilcoxon signed Rank test).

 

Parameters of comparison	P-value
Postop. Month 1 N14 – perop. N14	0.553
Postop. Month Ш N14 – preop. N14	0.273
Postop. Month VI N14 – preop. N14	0.029*
Postop. Month Ш N14 – postop. Month 1 N14	0.917
Postop. Month VI N14 – postop. Month 1 N14	0.196
Postop. Month VI N14 – postop. Month Ш N14	0.029*

*Significant at P<0.05

 

 

DISCUSSION

 

Somatosensory evoked potentials started to be used in the spinal cord operations in 1970’s. Its usage in the preoperative, perioperative and postoperative period was in 1980’s¹⁰. It is the way to assess the transfer of  vibration and touch sensation from the peripheral nervous system to the central Nervous system through the sensory pathways of the Dorsal column^9,14-17. In 1980, Sharm showed that it is possible to distinguish the myelopathy and radiculopathy by analysis of SEPs abnormalities by stimulation techniques¹⁶.

This study specified that changes in N14 latency were the most statistically understandable and meaningful ones. By using Wilcoxon Signed Range test, it was observed that these changes were prominent at the first month 6 (p<0.05) and also between the 3rd and 6^th months after surgery (p<0.05). The changes of N14 latency were moving very close towards the normal values compared to the values before surgery. This means that the functional recovery of the medulla spinalis takes 3–6 months for improvement after operative decompression.

This could be explained by the origin of these waves (N13, N14) from the Nuclei of the Dorsal column and gray matter¹⁴, so the changes in their latencies could be used as a prognostic indicator  in the follow up after surgery for 3-6 months. 

In 1992, Restruccia et al. suggested that N13 and N14 latencies differences can be used as an evaluation way for the subclinically presented cervical patients. This means that these latencies differences were affected in the early stages of degeneration due to dorsal column dysfunction. In their study, they suggested that N13 potential changes were in proportion to the reflex abnormalities, and N20 latency delay was in proportion to symptoms of sensory loss^5-7. They suggested that the function of medulla spinalis gray matter is assessed by N13 wave and latency extension to N14 and N20 and that the medulla spinalis dorsal column dysfunction in cervical myelopathy was responsible for these findings^5-7.

The importance of SEPs was also documented by May et al. in 1996. They studied 26 cases with multiple cervical disc herniation. They monitored the SEPs in preoperative, perioperative and postoperative periods. They concluded that SEPs can be a factor for decision making in cervical disc surgery and also it has a prognostic importance in follow up⁽⁴⁾.

A study by Heiskari et al. on 11 cases with cervical myelopathy or radiculopathy using SEPs for assessment of compression of the roots or the medulla spinalis  concluded better results in postoperative period mostly due to less number of patients².

Other several studies discovered that abnormal latency of N14 potential in myelopathic patients is in direct proportion to the feeling loss regardless the radiological findings and it is a warning for dorsal column compression. In this study, the same findings was observed in cervical disc patients. It was found that N14 latency is important in following up the postoperative clinical recovery in cervical disc patients^1,5,6,7,18.

The somatosensory evoked potentials can be used in assessment of sensory loss and medulla spinalis dysfunction caused by compression of the cervical disc.

 

Conclusion

SEPs can be considered as an objective numerical parameter for the diagnosis of dorsal column dysfunction caused by compression. The SEPs changes after surgery are parallel to clinical recovery. Following up patients with SEPs in the postoperative period can be used to quantify the improvement.

 

[Disclosure: Authors report no conflict of interest]

 

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