Twitch or no twitch? The cutaneous trunci reflex

Abstract

The localisation of spinal cord lesions in dogs has always been a controversial topic creating agreement, disagreement and discussion amongst veterinary clinicians. The different causes of injury to the spinal cord and the time elapsing from the onset of injury influence both the neurological signs and reflex changes detected during a neurological examination. For many years, clinical examination and neurological examination have been the only means of localisation, supported by the changes detected in radiographs of the vertebral column with or without myelographic contrast imaging. Veterinary students (including myself ) learned the value of the cutaneous trunci reflex (or panniculus reflex as it was incorrectly named) and how it was interpreted to help diagnose the site of a spinal cord lesion. I read about the cutaneous trunci reflex in Palmer’s textbook Introduction to Small Animal Neurology, a small but powerful book of the time. I learned the reflex’s subtleties: the different ways to elicit the reflex with pinch, pin prick or light irritation and, with understanding of the sensory dermatomes, to interpret the response and determine the possible localisation of the lesion. Dehydration affected the response and there were other unexplainable responses in some patients. The discrepancies between my localisation and that of radiological findings could be explained but not substantiated by ‘cord oedema’ or ‘more than one lesion’ or some other reason. Nevertheless, I was mostly happy with my subjective localisations. The advent of advanced imaging techniques such as magnetic resonance imaging (MRI) and computed tomography (CT) has now provided clinicians with a new dimension in diagnosis of spinal cord lesions. Young clinicians and students seem now to recruit these imaging modalities as part of their investigation of a spinal cord problem soon after examination, but frequently the basic neurological examination of the patient has been, rightly or wrongly, superficial at best. The paper by Gutierrez-Quintana and others (2012), in this issue of JSAP, attempts to evaluate the accuracy of the cutaneous trunci reflex in localising spinal cord lesions, to grade the severity of thoracolumbar spinal cord injuries and to correlate the neurological findings with the cord lesion identified by MRI in 41 dogs. The authors correctly state that the value of this reflex has neither been subject to audit with respect to its accuracy in determining the site of spinal cord lesions, nor in relation to its contribution to evaluating the severity of cord lesions. This paper identifies the cutaneous trunci reflex as having particular merit for the localisation of lesions cranial to L1, with the level for ‘cut-off’ of the reflex from zero to four vertebral bodies caudal to the maximal spinal cord lesion as determined by MRI in 80% of the dogs, and two to three vertebral bodies caudal in 16%. The authors also determined that the presence or absence of a cutaneous trunci reflex ‘cut-off’ correlated significantly with the severity grading according to the systems of Sharp and Wheeler (2005) and Olby and others (2001). The loss of the cutaneous trunci reflex occurred at a severity grade lower that that associated with an inability to walk (I cannot bring myself to say ‘non ambulatory’!). The reflex helps to allocate dogs with paresis into a less and more severely affected category without additional gait analysis. Veterinary clinicians and surgeons who investigate dogs with spinal cord lesions, especially those dogs with intervertebral disc disease, might have their subjective opinions reinforced by the results of this study. If that is not the case, this paper does provide some welcome objective data regarding the cutaneous trunci reflex that should assist decision making in dogs with spinal cord disease: in lesion localisation and determining the degree of severity of the cord lesion. Furthermore, the improved clinical localisation helps to plan appropriate diagnostic imaging with confidence of the location of the cord lesion and the determination of severity might help in deciding whether advanced imaging is required at all. However, the cutaneous trunci reflex must be elicited with skill and the results interpreted as part of a full neurological examination in every patient. It is a simple reflex to elicit and to interpret and the results from the paper by Gutierrez-Quintana and others (2012) have expanded its usefulness. It is the clinician’s responsi