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CLINICAL DEVELOPMENTS: A PHYSICAL THERAPY RESOURCE FOR PHYSICIANS

Volume 1, No.1:
FATTY INFILTRATION IN THE CERVICAL EXTENSOR MUSCLES IN PERSISTENT WHIPLASH-ASSOCIATED DISORDERS: A MAGNETIC RESONANCE IMAGING ANALYSIS

Elliott J, Jull G, Nobeboom JT, et al. Spine. 2006; 31(22):E847–E855
Abstracted by Peter M. Harris, PT, MS, COMP, Cert. MDT from Buffalo,NY
Reprinted by permission: IAOM-US Quarterly Review, Volume 61, 1st Quarter 2008
http://www.iaom-us.com

The purpose of this study was to evaluate several factors related to changes in the cervical extensor muscles in patients with whiplash associated disorders (WAD). The first was to assess for changes in MRI signal intensity in the cervical extensor muscles in patients with chronic WAD. The second was to quantify the level of fatty infiltration in the muscular tissue of these patients. The third purpose was to determine if there was a difference in fatty infiltration in the cervical extensor muscles of WAD patients and healthy control subjects. Lastly the study wanted to determine if the fatty infiltration had any association with age, Neck Disability Index (NDI) scores, compensation status, body mass index (BMI) and duration of symptoms.

The study sample consisted of 79 WAD subjects that met inclusion criteria and 34 healthy controls. Each subject underwent MRI of the cervical spine from the midpoint of the cerebellum to the T1 level to assure that all the extensor musculature was captured. These included the suboccipital muscles (rectus capitis posterior minor and major) and the cervical extensor muscles (multifidus, semispinalis cervicis and capitis, splenius capitis and upper trapezius).

Data analysis obtained from the MRI’s revealed specific patterns of muscle fatty infiltration throughout the entire cervical extensor muscle groups in both the healthy controls and the WAD subjects. The WAD group displayed a significantly larger amount of fatty infiltrate compared to the control subjects. The muscles in the WAD subjects that contained the largest amount of fatty infiltrate were the rectus capitis posterior minor, major and deep cervical multifidi muscles. Age of healthy and WAD subjects, duration of symptoms, compensation status, BMI, and NDI scores were not associated with the observed changes noted above.

Another observed pattern of interest noted by the authors was a decline in fat content in all of the cervical extensor muscles in both the experimental and control groups from proximal to distal. The WAD group had significantly higher levels of fat content in all of the muscles studied and at all levels. This pattern was seen in the deep suboccipital and multifidus muscles, and the intermediate and superficial muscles, the semispinalis cervicis and capitis, splenius capitis and upper trapezius. A higher level of fatty infiltration was found at the C3 level in the WAD group suggesting that greater damage occured here with subsequent greater changes in the muscles at this level.

The authors comment that the exact cause of the changes in muscle tissue cannot be determined from this study and they speculate, with support from the literature, that the fatty infiltration could be due to generalized disuse, chronic denervation, motor neuron lesions, metabolic disorders, aging or other muscle disorders. The authors draw the conclusion that because the fatty infiltrate was widespread the likely cause of the observed changes was due to generalized disuse. This conclusion is supported by the fact that muscle atrophy involves a greater amount of Type 1 fibers. The deep suboccipital muscles have a high density of Type 1 fibers vs. the superficial muscles that have a higher concentration of Type II fibers. Type II fibers are more resistant to fiber transformation in patients with cervical spine dysfunction. A second explanation for the fatty infiltration seen in the deep cervical muscles of WAD patients proposed by the authors may be due to nerve injury or demyelinated nerve tissue. They propose an inflammatory mechanism from either a minor nerve injury or damage to somatic structures may be the cause of the changes seen in the deep cervical muscle; however, they caution that further research is needed to support this notion. The muscle changes found in this study may offer insight into the sequelae often seen in patients with WAD.

IAOM-US Comment
The study authors found that patients with WAD displayed a significantly larger amount of fatty infiltrate compared to control subjects. Of interest was the finding of increased fatty infiltrate in the suboccipital muscles and deep cervical multifidi. It has been demonstrated that cervical muscular fatigue can cause increased postural sway in WAD patients and that physical therapy can effectively relieve susceptibility to impaired cervical muscle function by reducing the fatigability of the muscles.1 Falla studied the effects of chronic neck pain on the automatic feedforward control mechanism of the cervical spine.2 These authors found that the feedforward mechanism was significantly delayed when compared to healthy controls. This impairment may leave the cervical spine susceptible to injury.

There may be association between these findings and the degree of fatty infiltration seen in the cervical muscles.

It follows then that with physical therapy intervention we can have a positive effect in treating patients with WAD by teaching them cervical stabilization exercises to improve their strength and endurance. Research supports the use of therapeutic exercise and therapist facilitated postural correction in stimulating the cervical muscles.3.4 This in turn should reduce the onset of fatigue and perhaps improve the altered feedforward mechanism of control. The infiltration of fatty tissue may be due to muscle atrophy as well. This atrophy could result in fewer sensory afferents available and partially explain the loss of proprioception and joint position sense described in the abstract above.

References
1. Stapley PJ, Beretta MV, Dalla Toffola E, Schieppati M. Neck muscle fatigue and postural control in patients with whiplash injury.
2. Falla D, Jull G, Hodges PW. Feedforward activity of the cervical flexor muscles during voluntary arm movements is delayed in chronic neck pain. Exp Brain Res. 2004;157(1):43-48
3. Falla D, O’Leary S, Fagan A, Jull G. Recruitment of the deep cervical flexor muscles during a postural-correction exercise performed in sitting. Man Ther. 2007;12(2):139-143.
4. Ylinen J. Physical exercises and functional rehabilitation for the management of chronic neck pain. Europa Medicophysica. 2007;43(1):119-132.

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