Masseter reflex latencies and amplitudes are not influenced by supratentorial and cerebellar lesions

Electrophysiology of Cranial Nerve Testing: Trigeminal and Facial Nerves

The clinical examination of the trigeminal and facial nerves provides significant diagnostic value, especially in the localization of lesions in disorders affecting the central and/or peripheral nervous system. The electrodiagnostic evaluation of these nerves and their pathways adds further accuracy and reliability to the diagnostic investigation and the localization process, especially when different testing methods are combined based on the clinical presentation and the electrophysiological findings. The diagnostic uniqueness of the trigeminal and facial nerves is their connectivity and their coparticipation in reflexes commonly used in clinical practice, namely the blink and corneal reflexes. The other reflexes used in the diagnostic process and lesion localization are very nerve specific and add more diagnostic yield to the workup of certain disorders of the nervous system. This article provides a review of commonly used electrodiagnostic studies and techniques in the evaluation and lesion localization of cranial nerves V and VII.

[Chiari II malformation. Supportive and predictive value of brainstem reflex and EAEP recordings]

BACKGROUND

In about 20% of patients with Chiari II malformation brainstem signs and symptoms occur. Ventilatory dysfunction is the main cause of death in these patients. The indication for craniocervical decompression is based on clinical examination because supporting electrophysiological or radiological methods were lacking.

METHODS

In a prospective study the clinical courses of 106 patients were documented during a 3-year period. In addition brainstem diagnostic procedures using the masseter reflex (MR), blink reflex (BR) and early auditory evoked potentials (EAEP) were done. Based on the model of binary logistic regression the odds ratio (OR) of progression over time was calculated.

RESULTS

The combination of MR and late BR components showed the highest correlation with clinical findings (OR: 23). The highest predictive value regarding clinical progression over a 3-year period was shown by the combined evaluation of MR, late BR components and EAEP interpeak latency I-V (OR: 17.6). Signs and symptoms had no predictive value.

CONCLUSIONS

Combined brainstem reflex recordings (MR and late BR components) support the clinical examination. To evaluate the long-term prognosis brainstem reflexes and EAEP recordings should be used.

Brainstem reflex circuits revisited

Our current understanding of brainstem reflex physiology comes chiefly from the classic anatomical-functional correlation studies that traced the central circuits underlying brainstem reflexes and establishing reflex abnormalities as markers for specific areas of lesion. These studies nevertheless had the disadvantage of deriving from post-mortem findings in only a few patients. We developed a voxel-based model of the human brainstem designed to import and normalize MRIs, select groups of patients with or without a given dysfunction, compare their MRIs statistically, and construct three-plane maps showing the statistical probability of lesion. Using this method, we studied 180 patients with focal brainstem infarction. All subjects underwent a dedicated MRI study of the brainstem and the whole series of brainstem tests currently used in clinical neurophysiology: early (R1) and late (R2) blink reflex, early (SP1) and late (SP2) masseter inhibitory reflex, and the jaw jerk to chin tapping. Significance levels were highest for R1, SP1 and R2 afferent abnormalities. Patients with abnormalities in all three reflexes had lesions involving the primary sensory neurons in the ventral pons, before the afferents directed to the respective reflex circuits diverge. Patients with an isolated abnormality of R1 and SP1 responses had lesions that involved the ipsilateral dorsal pons, near the fourth ventricle floor, and lay close to each other. The area with the highest probabilities of lesion for the R2-afferent abnormality was in the ipsilateral dorsal-lateral medulla at the inferior olive level. SP2 abnormalities reached a low level of significance, in the same region as R2. Only few patients had a crossed-type abnormality of SP1, SP2 or R2; that of SP1 reached significance in the median pontine tegmentum rostral to the main trigeminal nucleus. Although abnormal in 38 patients, the jaw jerk appeared to have no cluster location. Because our voxel-based model quantitatively compares lesions in patients with or without a given reflex abnormality, it minimizes the risk that the significant areas depict vascular territories rather than common spots within the territory housing the reflex circuit. By analysing statistical data for a large cohort of patients, it also identifies the most frequent lesion location for each response. The finding of multireflex abnormalities reflects damage of the primary afferent neurons; hence it provides no evidence of an intra-axial lesion. The jaw jerk, perhaps the brainstem reflex most widely used in clinical neurophysiology, had no apparent topodiagnostic value, probably because it depends strongly on peripheral variables, including dental occlusion.

Masseter reflex in childhood and adolescence

We report normative data of masseter reflex from a group of 54 children 2-16 years of age. For statistical analysis, the patients were divided into five age groups: 2-4, 5-7, 8-10, 11-13, and 14-16 years of age. A tap to the chin, using a hammer with a trigger device, elicited the masseter reflex. The response was recorded by surface electrodes. The onset latency and peak-to-peak amplitude of the averaged curve of eight reflex responses were measured. The reflex response could be recorded in all children and adolescents of all groups. The mean latency shortened from age 2 to 7 and was stable at the age of 8 years. As a sign of maturation, the increase of amplitude corresponded to the shortening of latency and was also stable at the age of 8 years. Abnormal side differences in latency of 0.9 ms (age group 2-4 years), 1.1 ms (age group 5-7 years), and 0.8 ms (age group 8-16 years) were evaluated. An amplitude ratio (lower amplitude divided by higher one) above 0.33 was calculated as normal.