Giant Repeater F-Wave in Patients With Anterior Horn Cell Disorders

Abstract

Conventional F-wave responses as well as single motor unit F-wave responses together with the volitionally recruited motor unit action potentials (MUAP) were studied in hand and feet muscles of 10 healthy subjects and 32 patients with anterior horn cell disorders. The amplitude of the largest F-wave (Fl) was significantly greater in the affected patients compared with healthy subjects. Giant repeater F-wave responses "up to 4 mV" were recorded in muscles having volitionally recruited giant MUAPs. Although, the group mean percentage of motor unit F-wave responses per stimulation in all tested orthodromic MUAPs was significantly decreased in amyotrophic lateral sclerosis patients, the group mean percentage of motor unit F-wave responses per stimulation in all tested orthodromic MUAPs that gave motor unit F-wave response was significantly increased compared with healthy subjects. The responding orthodromic MUAP gave identical motor unit F-wave response, even for complex polyphasic units. Enhanced monosynaptic (H-) reflex, proximal axon reflex (A-wave), and repetitive muscle response as possible explanations for the giant F-wave responses could be discounted. The electrophysiologic behavior of the giant late responses described here fits well with the criteria of F-waves modulated by newly formed distal (and or proximal) axonal branching.

Differences in Dysfunction of Thenar and Hypothenar Motoneurons in Amyotrophic Lateral Sclerosis

“…The differences in the F-wave characteristics between the APB and ADM may be attributed to the competing effects of degeneration and regeneration within the motor unit. With the degeneration of motor neurons, the surviving motor neurons compensate by reinnervating the denervated muscle fibers through axonal sprouting (Ibrahim and el-Abd, 1997 ). In ALS, the denervation process may begin earlier and progress more rapidly in the thenar region.…”

Differences in F-Wave Characteristics between Spinobulbar Muscular Atrophy and Amyotrophic Lateral Sclerosis

“…In anterior horn cell disorders, surviving motor neurons compensate for functional motor unit loss by increasing the size of their motor units through axonal sprouting and reinnervating denervated muscle fibers. The increased mean F-wave amplitudes are probably caused by a higher proportion of large motor units contributing to F-waves ( Ibrahim and el-Abd, 1997 ). The generation of giant F-waves in patients with anterior horn cell disorders may be attributed to axonal sprouting and subsequent reorganization of a muscle’s innervation ( Ibrahim and el-Abd, 1997 ).…”

“…The increased mean F-wave amplitudes are probably caused by a higher proportion of large motor units contributing to F-waves ( Ibrahim and el-Abd, 1997 ). The generation of giant F-waves in patients with anterior horn cell disorders may be attributed to axonal sprouting and subsequent reorganization of a muscle’s innervation ( Ibrahim and el-Abd, 1997 ). Other possible mechanisms have been discounted, such as an entire motoneuron pool responding to a stimulus recurrently, an enhanced H-reflex, a repetitive muscle response to an orthodromic motor impulse, and an axon reflex ( Ibrahim and el-Abd, 1997 ).…”

“…F-wave persistence was defined as the number of recordable F-waves per 100 stimuli and was expressed as a percentage. An F-wave with an amplitude exceeding the mean value plus 2 standard deviations (SD) of the maximum F-wave amplitudes recorded in normal subjects was considered a giant F-wave ( Ibrahim and el-Abd, 1997 ). There were no significant differences in absolute F-wave amplitudes between the same nerves on the left and right sides in normal controls.…”

“…Previous studies have demonstrated differences between SBMA and ALS in sensory action potential amplitudes, needle electromyography manifestations, cortical excitability testing, serum creatine kinase levels, and clinical presentations ( Ferrante and Wilbourn, 1997 ; Hirota et al, 2000 ; Vucic and Kiernan, 2008 ; Chahin and Sorenson, 2009 ; Hama et al, 2012 ; Jokela et al, 2015 ). It has been suggested that the frequencies of giant F-waves and repeater F-waves are significantly increased in patients with anterior horn cell disorders compared with the healthy subjects ( Petajan, 1985 ; Ibrahim and el-Abd, 1997 ). However, differences in the characteristics of F-waves between motor neuron diseases such as SBMA and ALS have not been explored up till now.…”