WS3.8. Cranial Nerve Assessment (with Demonstration)

Abstract

The cranial nerve condition commonly tested in an EMG laboratory comprise mono or polyneuropathies affecting the accessory, trigeminal or facial nerve. A posterior triangle tumor or surgical procedures can damage the spinal accessory nerve. Other causes include stretch induced injury, carotid endarterectomy, and ligature injury during surgical exploration. The paralysis of the sternocleidomastoid causes weakness when the face is rotated toward the opposite shoulder. Stimulation of this nerve behind the sternocleidomastoid elicits a compound action potential (CMAP) recordable from the upper trapezius muscle. The differential diagnosis of facial palsy, which constitutes the most common disorder of the cranial nerve seen in an EMG laboratory, deserves a special mention. Weakness of the orbicularis oculi and frontalis suggests a peripheral type of facial palsy, which may herald other symptoms of multiple sclerosis, Lyme borreliosis and AIDS. Patients with Guillain Barre Syndrome and Charcot Marie tooth disease type 1 may develop bilateral, or rarely unilateral, involvement. An acoustic neuroma may compress the facial nerve. Bell's palsy develops sporadically, 80% showing a good prognosis with demyelination, and 20%, a less favorable outcome with axonal degeneration as in herpes zoster infection. The conduction study of the facial nerve and blink reflex reveal various degrees of abnormality in most of these patients. Stimulation of the facial nerve evokes CMAP usually recorded from the nasalis. Blink reflex elicited by supraspinal nerve consists of ipsilateral R1 and bilateral R2 components. The latency of R1 represents the conduction time along the trigeminal and facial nerves and poutine relay. Inherent latency variability makes R2 less reliable for diagnostic purpose. During the session, participants will observe various electrodiagnostic techniques currently in use for the evaluation of the cranial nerves, and learn pit falls which may lead to an erroneous interpretation of the acquired results. All these should lead to a better understanding of the use of electrodiagnostic studies in attending patients with a disorder of cranial nerves in the clinical practice. In patients with a diffuse process such as polyneuropathy, examination of the cranial nerves complements the routine studies of the upper and lower limb nerves.