Changes in electromyography and F wave responses in two cats with presumed local tetanus: implications for diagnosis and prognosis

Abstract

Two cases of focal tetanus in the cat are described. Clinical findings included severe muscular spasms of the pelvic limbs in one cat, and involvement of the thoracic limbs and muscles of the neck and face in the other. Electromyography in both cats showed spontaneous activity characterised by the presence of motor unit potentials. F waves, never previously reported in focal tetanus in animals, showed significantly increased F/M amplitude ratio in both cats and increased F wave duration in one cat. The electrodiagnostic findings provided relevant diagnostic and, possibly, prognostic information.

Case Report

Cats are considered very resistant to tetanus infection because of the reduced ability of tetanospasmin to enter the nervous system and bind to specific target sites. ¹ Two forms of tetanus have been recognised in domestic animals based on the clinical presentation: one characterised by generalised hyperactivity and one by localised hyperactivity of the motor system. ²

Clinically, affected animals present a sustained muscle contraction characterised by the prevalence of the extensor muscles, along with episodic muscle spasm affecting both agonist and antagonist muscle groups.^1–3 As a consequence of the natural resistance, in cats tetanus has a low prevalence and the localised form is apparently predominant over the generalised one.^4,5 Electromyography (EMG) may be particularly helpful in the diagnosis of localised tetanus, ⁵ and is reported as part of the diagnostic work-up of these patients.^4,6–9

Diagnosis of focal tetanus may be challenging, especially in cases in which anamnesis is not clear and post-traumatic muscle contractions cannot be excluded. In human tetanus the most striking electrodiagnostic abnormalities are observed during EMG and F waves studies.^10,11 In particular, an exaggerated F-response and the simultaneous EMG activity of both agonist and antagonist muscles are important electrophysiological features of patients with the focal form. ¹⁰ Normal values of F/M amplitude ratio and F wave duration — two parameters reflecting motor neurons excitability, usually increased in tetanus — are not available in veterinary medicine.

The present report describes the electrodiagnostic findings in two cats with suspected localised tetanus. The diagnostic and prognostic implications of the results of EMG and F waves responses are discussed. With the aim of emphasising the role of electrodiagnostic tests in affected patients, normal values of F/M amplitude ratio and F wave duration recorded in 15 healthy cats are also provided.

Two young adult, female, stray domestic shorthair cats were referred for the presence of severe muscle rigidity. Cat 1 presented pelvic limbs hyperextension, a mummified left hind limb distally to the stifle joint and abrasions to the left forelimb. Neurological examination showed hypertonic paraplegia with increased tail muscle tone (Supplementary video 1). Hyperactive reflexes were present in the pelvic limbs. Cat 2 showed severe muscular rigidity of the neck and forelimbs, and had infected necrotic wounds on both thoracic limbs. On neurological examination, the cat showed thoracic limb diplegia. The muscle rigidity prevented any movement of the head and the neck, and passive flexion of the thoracic limbs. Persistent contraction of the facial muscles with narrowing of the palpebral fissures was also recorded (Supplementary video 2).

In both cats electrophysiological investigations under general anaesthesia were performed using conventional electrodiagnostic equipment (Myoquick, Micromed) to support the presumptive diagnosis of focal tetanus. The electrodiagnostic evaluation consisted of EMG, motor nerve conduction studies (MNCS) and F wave evaluation. EMG recordings were obtained from the main appendicular, epaxial and head muscles, followed by the simultaneous recording from agonist and antagonist muscles in the affected limbs. The tibial nerves were evaluated with MNCS and F waves in both cats. The presence of severe wounds prevented the specific evaluation of the ulnar nerves in cat 2. The F waves were recorded from the plantar interossei muscle using needle electrodes, stimulating the nerve proximally to the tarsal joint. Two trains of 10 electric stimulations of supramaximal intensity, with a duration of 0.2 ms and a frequency of 1 Hz, were applied. Reference intervals for F/M amplitude ratio and F wave duration were obtained from 15 normal cats of similar age tested with the same methodology. These cats were anaesthetised for neutering procedures and the tests were performed after owner’s gave informed consent.

In cat 1 the electrodiagnostic study was performed 2 weeks after the first evaluation, following the amputation of the mummified limb, while the cat was receiving treatment with metronidazole. EMG showed the presence of spontaneous pathological activity represented by fibrillation potentials, positive sharp waves and occasional motor unit action potentials (MUAPs) in the majority of the muscles of the hind limbs and tail (Figure 1a, b). In the other muscles evaluated no MUAPs or spontaneous pathological activity were present. MNCS of the right sciatic-tibial nerve were normal. F waves of the right tibial nerve showed normal minimum latency and mean amplitude of the F waves. The cat had a significantly increased F/M amplitude ratio and F wave duration (Figure 2; Table 1). The cat continued to improve after the electrodiagnostic study and was clinically normal 1 month later.

Figure 1

(a) Electromyography (EMG) tracing of the left quadriceps femoris muscle of cat 1 showing two motor unit action potentials (MUAPs) and positive sharp waves. Forty ms/Div; 150 mV/Div. (b) EMG of the tibialis cranialis muscle of cat 1 showing fibrillations. Forty ms/Div; 50 mV/Div. (c) EMG of cat 2 showing multiple MUAPs (multiplets) discharging simultaneously in biceps brachii muscle (upper tracing) and triceps brachii muscle (lower tracing) of the right fore limb. Twenty ms/Div; 50 mV/Div. (d) EMG of cat 2 showing doublets in the extensor carpi radialis. Twenty ms/Div; 20 mV/Div

Figure 2

F wave test of the tibial nerve of the two cats with focal tetanus and of a normal subject. Dotted arrow = M wave amplitude; thin arrow = F waves amplitude; wide arrow = F waves duration. Five ms/Div; M Zone 2 mV/Div, F Zone 0.2 mV/Div

Table 1

Normal values of M wave maximum amplitude, F wave mean amplitude, mean F amplitude/maximum M amplitude expressed as a percentage (F/M amplitude ratio) and F wave duration measured from the onset to the final return to the baseline, and values recorded in two cats with focal tetanus. Two SD above mean normal values are given in bold

	M Max amplitude mV	F Mean amplitude mV	F/M amplitude %	F Duration ms
Normal values ± SD	21.17 ± 3.53	1.06 ± 0.34	4.95 ± 1.12	7.26 ± 2.82
Cat 1	18.1	1.3	7.2	13.3
Cat 2	16.7	1.7	10.2	7.6

In cat 2 the electrodiagnostic study, performed the day after the first neurological examination showed, on EMG, MUAPs as single potentials, doublets, triplets or multiplets (Figure 1c, d), recorded simultaneously in agonist and antagonist muscles of the front limb, in epiaxial muscles of the neck and in the tibialis cranialis muscles of the hind limbs. MNCS of the right sciatic-tibial nerve were normal. F waves of the same nerve were normal apart from a significantly increased F/M amplitude ratio (Figure 2; Table 1).

The cat was treated with metronidazole (12.5 mg/kg q12h) and was clinically normal 2 months later.

The diagnosis of tetanus is usually based on history, clinical signs and evidence of infection. ⁵ Diagnosis may be less obvious when the source of infection is not evident and in animals with the localised form. Owing to the lack of a reliable serological test and the frequent failure to isolate tetanus bacilli, the identification of both localised and generalised tetanus depends largely on their clinical and electrophysiological characteristics. In our cases EMG and F waves confirmed the suspected diagnosis and possibly provided useful prognostic information.

The muscles affected by contractures are usually electrically silent on EMG. ¹² MUAPs, reflecting the activity of a single anatomic motor unit, are readily recordable in awake subjects during voluntary muscle contraction and are occasionally present under general anaesthesia, depending on the anaesthetic plan. ¹³ The presence in the anaesthetised cats of our study of MUAPs is suggestive of a disinhibition of the alpha motoneurons as the main cause for the muscle rigidity. ³ In human medicine, single, double, triple or multiple MUAP, known as singlets, doublets, triplets and multiplets, can be recorded in tetanus and many other conditions associated with hyper-excitability of the motor neuron pool. ¹³ In cat 2, MUAPs could be recorded simultaneously in agonist and antagonist muscles (Figure 1c). This finding, that in human medicine was proposed as typical for localised tetanus,^10,14 was reported previously in veterinary medicine only in an awake dog with focal tetanus. ¹⁵ The EMG of cat 1 also showed the significant presence of fibrillation potentials and positive sharp waves, commonly recorded in the later stages of tetanus as a possible result of the degenerative changes induced in the striated muscles by the prolonged neuromuscular hyperactivity.^1,3,16

The severity and the extension of the abnormalities depend on the toxin load, which is one of the most important factors influencing the clinical evolution. ² EMG evaluation can be very useful in assessing the stage and the extension of the abnormalities. In cat 1, EMG results are in agreement with the clinical findings of an exclusive involvement of both hind limbs and tail. In cat 2, the involvement of the forelimbs, neck and face and the mild electromyographical changes observed in the hind limbs (tibialis cranialis muscle) were supportive of a more widespread form. The earlier clinical normalisation of cat 1, characterised by a more localised form of tetanus and the presence of fibrillations and positive sharp waves and a low number of MUAPs in respect to cat 2, confirmed the prognostic information provided by EMG.

MNCS were normal in both cats. Like MNCS, F waves are pure motor events as they are the result of the antidromic activation of a pool of motoneurons. Differently from MNCS, F responses also reflect proximal nerve conduction and central motor neuron excitability. F wave mean amplitude, F/M amplitude ratio (mean F amplitude/maximum M amplitude) and F wave duration are parameters often increased when spinal motoneuron excitability is higher. ¹¹ F wave mean amplitude for the tibial nerve has been published in cats. ¹⁷ For clinical purposes, F waves’ mean amplitude is routinely expressed as a percentage of the M response (F/M amplitude ratio). F wave duration and F/M amplitude ratio are positively correlated and reflect the proportion of a motor neuron pool activated by antidromic stimulation. When increased, these parameters are suggestive of a disinhibition of the spinal motor neurons. ¹¹ F/M amplitude ratio and F wave duration were measured in our cats and compared with a population of 15 normal cats of similar ages (Figure 2; Table 1). Both affected cats showed increased F/M amplitude ratios, and cat 1 had also an increased duration of F waves. Similar findings have been reported in human tetanus. ¹¹ The markedly increased F/M amplitude ratio of the tibial nerve in the diplegic cat (cat 2) confirmed the subclinical involvement of lumbosacral spinal cord segments.

Conclusions

To our knowledge this is the first report describing the results of a thorough electrodiagnostic study in cats affected by focal tetanus. The neurophysiological tests, especially EMG and F waves, may provide useful diagnostic and, possibly, prognostic information in these cases. The results encourage the inclusion of a complete electrodiagnostic evaluation in the diagnostic work-up of tetanus.