Nerve conduction study in young children suffering from cerebral palsy

INTRODUCTION

Cerebral palsy (CP) is the common cause of motor disability in childhood. It occurs in about 2.1/1,000 live births.^[1] William Little first described CP in the 1840s as ‘a group of non-progressive but often altering motor impairment syndromes secondary to lesions or anomalies of the brain arising in the early stages of its development’.^[2] It is characterised by abnormal muscle tone, posture and movement, thereby limiting the motor activity of the affected children.^[1] The clinical presentation may change as the central nervous system’s growth, development and maturation progress.^[1] The motor disorders of CP are often accompanied by disturbances of sensation, perception and secondary musculoskeletal problems.^[3]

Early detection of muscle tone in CP is important as the management of diseases, physiotherapy and occupational therapy solely depend on the degree of muscle tone. Identifying the degree of spasticity/muscle tone in younger CP children is still a challenging problem in the early stages of diseases. Examining the deep tendon reflexes is subjective rather than quantitated assessment.

Most of the previous reports on CP are concerned with aetiology and treatment.^[1,4-6] Few studies deal with quantitative evaluation of reflex and voluntary activity in children with spasticity CP and state that increased/abnormal reflexes are related to functional impairment in spastic CP.^[7] Some electrophysiological studies were performed on the newborn with birth asphyxia but not in cases of CP and reported suppressed electrophysiological parameters.^[8,9] A study in older children with spastic CP reported positive correlation between spasticity assessed by the modified Ashworth scale and Hoffman reflex (H-reflex).^[10] The excitability properties of nerves in adult CP patients were reported to be abnormal. The altered motoneurons excitability anticipates altered axons conduction properties.^[5] The effect of altered excitability of neurons on their nerve conduction properties remains to be explored in patients suffering from CP.

It has been known that the study of peripheral nerve pathophysiology is limited to nerve conduction. The effect of excitability of neurons on conduction is also important to understand the diseases. The electrophysiological examination, that is, H-reflex, may help assess the excitability status of neurons in this situation. H-reflex is the electrical analogue of the monosynaptic stretch reflex. It helps to assess the velocity-dependent increase in muscle stretch reflexes and spinal cord circuits quantitatively.^[11] The electrophysiological parameter, for example, maximum amplitudes of reflexly excitable motor neurons (Hmax), maximum amplitude of motor response (Mmax) and Hmax/Mmax ratio in % represents the proportion of reflexly excitable motor neurons.^[11] H-reflex latency (HRL) measures the total traversing time of the nerve impulse along the monosynaptic spinal reflex arc, whereas motor response latency (MRL) measures the time taken by the muscle to initiate a compound muscle action potential (CMAP) after direct stimulation of distal motor segment of the nerve. The H-reflex conduction velocity (HRCV) assesses the conduction in the proximal segment of the reflex arc, and motor nerve conduction velocity (MNCV) assesses the conduction in the distal part of the motor neuron^.[12] Therefore, above parameters help to assess the excitability and conductivity of the nerves.

This study was conducted to assess the alteration of electrophysiological parameters of nerve conduction velocity study in CP and age-matched normal children.

MATERIALS AND METHODS

The present cross-sectional study was performed on children suffering from CP. The period of study was from November 2018 to October 2020. The study protocol was duly approved by the Ethical Committee of the Institute of Medical Sciences, Banaras Hindu University, Varanasi, India, with IRB number Dean/2018/EC/852, ECR/Bhu/Inst/UP/2017/RR-20.

RESULTS

In the present study, [Figures 1 and 2] depicts the labelled original recording of H-reflex parameters and MNCV, respectively.

Close

Figure 1:

Above recordings shows Hoffman reflex (H-reflex) and M response (Motor response) in right lower limb (posterior tibial nerve-soleus muscle) of normal and cerebral palsy (CP) children. Horizontal axis denotes time (ms) and vertical axis represents amplitude (mV). Top panel (a) – Normal children; tracing shows: M response and H-reflex waves; AB: M response latency, AC: H-reflex latency. Middle panel (b) – spastic CP children. Bottom panel (c) – hypotonic CP children.

Sample recording of H-reflex parameters in normal and CP children

Export to PPT

Close

Figure 2:

Above recording shows MNCV by stimulating the posterior tibial nerve at two different points along its course, the popliteal fossa and behind the medial malleolus and position of active electrode over the belly of the abductor digiti minimi muscle on the medial aspect of the foot. Top panel (a) – normal children, middle panel (b) – spastic cerebral palsy (CP) children, bottom panel (c) – hypotonic CP children, MNCV: Motor nerve conduction velocity.

Sample recording of MNCV in normal and CP children

Export to PPT

H-reflex was elicited in all the normal children. The values of H reflex parameters and MNCV in normal children were within the normal limit of their respective age group [Table 1].

In spastic CP children, the values of HRL (0.013 ± 0.004 s, P < 0.05) were significantly lower in comparison to normal children, whereas the values of Hmax (3.96 ± 2.43 mV, P < 0.05) and Hmax/Mmax ratio (282.63 ± 132.29%, P < 0.05) were significantly higher as compared to normal children [Table 1]. Other electrophysiological parameters, HRCV (63.75 ± 11.34 m/s) and MNCV (61.714 ± 10.43 m/s), were similar to the control group. Correspondingly, the values of HRCV were similar to MNCV [Table 1].

In hypotonic CP children, the values of HRL (0.007 ± 0.004 s, P < 0.05) were significantly lower as compared to normal children [Table 2]. Other electrophysiological parameters such as Hmax (1.083 ± 0.38 mV), Mmax (3.01 ± 1.92 mV), MRL (0.003 ± 0.001 s) and Hmax/Mmax ratio (46.09 ± 24.08%) were similar to normal children. HRCV (63.78 ± 14.13 m/s) and MNCV (56.71 ± 13.65 m/s) were also similar to normal children. However, HRCV was significantly (P < 0.05) higher than MNCV in this group [Table 2].

DISCUSSION

In the present study, H-reflex was elicited in all CP and healthy children [Figure 1]. Hmax and Hmax/Mmax ratios were significantly higher in spastic CP children compared to the control group, whereas these parameters were not significantly different from normal children in hypotonic CP [Tables 1 and 2]. HRL was significantly shorter in all CP children. Proximal conduction velocity, that is, HRCV was significantly higher than distal motor conduction velocity, that is, MNCV in hypotonic CP children [Table 2].

Perinatal hypoxia and other prenatal causes were considered the major cause of CP.^[16] Injury or deformity at an earlier brain development stage, mainly of the basal ganglia and cerebellum, has been reported as an important cause of hypotonic CP.^[4,6] Further, generalised muscular hypotonia observed in hypotonic CP may not result from a primary disorder of muscle and peripheral nerves.^[6,15] Reports suggested that pyramidal tract disorders are observed in spastic CP. Furthermore, disrupted descending inputs may affect peripheral nerves and muscle fibres resulting in alteration of functions of concerned organ.^[5] In our observations, H-reflex was elicited in both types of CP children. Thus, all the components of H-reflex arc were anatomically intact and functional despite brain injury in patients with CP.

The present study observed a significant increase in Hmax and Hmax/Mmax ratio in spastic CP children [Table 1]. The increase in Hmax and H/M ratio suggested that the spinal motor neuron excitability was higher in the CP patients. Hagglund and Wagner studied the development of spasticity with age using the Ashworth scale. They reported that muscle tone in CP children gradually increased up to 4 years of age and then gradually decreased till 12 years of age. The same tendency is seen in all CP subtypes.^[17] In our study, the age group of patients was from 12 months to 24 months. Thus, above study supported our observation. Another study reported that perinatal damage to the corticospinal pathway secondarily leads to the disrupted development of spinal motor centres or malfunction of the spinal interneuron due to altered corticospinal input.^[18] These supra spinal lesions of the pyramidal tract occur before the central nervous system maturation is complete.^[18] Thus, in CP patients, disrupted descending inputs may lead to secondary changes in the spinal cord, peripheral nerves and muscle fibres, causing increased excitability in the spinal motor neuronal pool and tone of muscles and contributing to increased Hmax and Hmax/Mmax ratio in spastic CP.

In hypotonic CP, H-reflex parameters, that is, Hmax and H/M ratio were within the normal range, indicating no alteration in excitability in the spinal motor neuron pool. The electrophysiological investigation, that is, H-reflex in hypotonic CP children has not been reported in literature. Studies on other hypotonic disorders such as paralytic poliomyelitis and spinal cord injuries have reported reduced H-reflex parameters, which attributed to damage of the motor neurons in the affected side of the spinal cord.^[19] Prakash et al. and Kumar et al. studied electrophysiological parameters on birth hypoxia and foetal distress in newborns.^[8,9] They reported that motor neuron excitability of peripheral nerve of the spinal cord was affected by hypoxia in the early life of infancy, and there was transiently depressed spinal reflex excitability in the first 72 h after birth and it returned to normalcy^.[8,9] Another study on spinal cord injury reported that loss/suppression of the H-reflex is not immediate but a gradual process involving adaptation throughout the neuromuscular system.^[20] These reports supported our findings that H-reflex parameters were not much altered in our study. These finds may be due to mild damage to the brain or due to the age of children. Further, hypotonia may not be marked until the child gets older.

In the present study, HRL was significantly shorter; however, HRCV and MNCV were within the normal range according to their respective age in all CP children [Tables 1 and 2]. HRCV was higher than MNCV in all CP children in our study. However, in hypotonic CP, the HRCV was significantly more than MNCV.

HRCV indicates the conduction in the proximal segment of the reflex arc. It involves both sensory and motor segment of a spinal monosynaptic reflex arc and is used to assess the degree of myelination of the nervous system.^[12] However, MNCV indicates the conduction in motor nerves and mainly depends on the thickness of the myelin sheath and the remodelling of nodes of Ranvier.^[14] Only a few reports were available in the literature investigating the peripheral sensory and motor nerve conduction velocities in CP. The excitability properties of nerves in the CP subjects were reported as abnormal in studies elsewhere.^[5] The motoneurons are hyperexcitable in CP. Abnormal internodal properties of axons were reported in CP adults that may be attributed to the altered architecture of axons. Simultaneously, a reduced brain inhibitory inputs would increase motoneuron excitability and conductivity. The above explains that higher conduction in the H-reflex pathway leads to shorter HRL in CP children.

Despite increased motoneuron excitability and shorter HRL, the HRCV and MNCV were within the normal range in our observations. This suggested adequate myelination of nerve in CP children. The prenatal ischemia in animal models shows hypomyelination, and axonal degeneration persisting in many parts of the brain and is not detected in white matter zones below the primary motor cortex or the corticospinal tracts.^[21,22] HRCV was higher than MNCV in all CP children; however, in hypotonic CP, the HRCV was significantly more than MNCV. The previous studies in normal children reported that the HRCV was higher than MNCV.^[12,23,24] As the myelination progresses from the spinal root toward the periphery, the value of proximal conduction velocity would be higher than the distal segment in the developing nerve trunk.^[25,26] The difference between proximal and motor distal conduction velocities diminishes with age.^[25] An increased HRCV than MNCV in hypotonic CP indicates an increased growth spurt in the myelination process in the proximal segment of a reflex arc than distal motor nerves and some myelination process defect/white matter injury in motor neurons. It leads to higher HRCV than MNCV in hypotonic CP. Spinal motor neuron pool excitability could not alter the conduction velocity in peripheral nerves of spastic and hypotonic CP children. Changes in the peripheral sensory and MNCV are unclear because CP is caused by neurological difficulty in the higher brain, not by problems in the peripheral nerves.

This is a preliminary study of a small sample size. We did not observe the electrophysiological parameter in the left lower and upper limbs of CP children as study was fully dependent on the OPD follow-up and cooperation of parents. We could not co-relate magnetic resonance imaging and computed tomography scan with electrophysiological findings. H-reflex parameters have their limitations, that is, the value of H-reflex in this capacity decreases as spasticity increases due to the development of minor contracture and has some limitations for direct measurement of central spasticity and hypotonia in children.

CONCLUSION

In a present study, the electrophysiological parameters, that is, H-reflex and nerve conduction, were altered in CP children. The presence of H-reflex in all CP children suggested that the reflex arc was anatomically and functionally intact. Increased Hmax, Hmax/Mmax ratio and decreased HRL in spastic CP patients suggested the increased excitability of motor neurons; however, HRCV and MNCV were not altered. H-reflex parameters were not altered in hypotonic CP children suggesting that motor neuronal pool was not suppressed. However, HRCV was significantly more than MNCV in hypotonic CP suggesting some myelination process defect/white matter injury in motor neurons. The excitability status of the spinal motor neuronal pool has not altered the conduction velocity in a peripheral nerve in CP. The electrophysiological parameters of nerve conduction study have the potential as a quantitative measure of reflex excitability and conductivity of neurons in CP children. It can be used as a test to assess the treatment response of CP children. It also helps the physician to start the treatment modality in the early childhood so that disabilities arising in CP children may be minimised or prevented.