Discussion
Do Back Mice Really Exist?
There is general agreement in a number of published case series from various rheumatologic and hospital populations that these nodules do exist, with an estimated prevalence of 15% in the general population.5,7 These studies all report characteristic locations and clinical findings of a rubbery, well-defined, often mobile, round or oval swelling in the deep subcutaneous tissues. Though these can be painful, they never show evidence of infection and are frequently asymptomatic. The only other frequently found nodules with a similar anatomical location are lipomas (more superficial and soft to palpation) and sebaceous cysts (more superficial and usually circular).
Why Would Back Mice Masquerade as Other Conditions?
It is difficult to postulate a direct mechanical effect in which a low back nodule would produce pain at a distant location, so it is more likely that this is some form of referred pain. The authors of previous studies have reported that these nodules may mimic sciatic nerve root compression and lead to a diagnosis of vertebral disk prolapse.6-8 The 2 cases we report appear to show a pattern different from that of the sciatic nerve. The first case is similar to data reported by Collee and colleagues9 in a study of hospital and primary care patients suffering from low back pain. They found that a substantial proportion of patients (45% and 25%, respectively) with low back pain also complained of greater trochanteric pain syndrome. There may be a specific referral pathway linking the 2 areas. It is more difficult to explain an association between a back mouse and the symptoms of abdominal pain in the second case. One possible mechanism could be a referral pattern, well reported in the literature, that comes from musculoskeletal structures (fascia, muscle, fibrous tissue, tendons) and follows sclerotomal distribution rather than the segmental dermatomes shown in most anatomical texts.10-12 There is ample evidence that anterior abdominal pain can be referred from musculoskeletal elements in the low back region and can be quite severe.13 Referred pain from the fibrous capsule surrounding fibro-fatty nodules might explain the distant symptoms and signs in our 2 cases.
Where Do Back Mice Come From?
Pathologic studies show that these nodules consist of a fibrous capsule containing fat divided by fibrous septa and some fine blood vessels and nerve fibers, and they usually have a stalk connecting them to the tissues below the deep fascial layers.8 It has been suggested that these nodules and their stalks are formed by extrusion through the neurovascular foramina in the deep fascia as a result of mechanical stresses.
Why Would Back Mice Cause Pain?
The answer to this is not known. Nodules that have been examined after surgery have not shown evidence of inflammation as a cause of pain, though nerve fibers are found in the fibrous capsule. In our experience, the immediate relief of pain frequently produced by multiple needle puncture, compared with poor relief with a single puncture, suggests that the pain might be caused by raised intranodular pressure.6
Why Would Injections Relieve Pain Caused by Back Mice?
A recent long-term follow-up study of 35 patients showed that 89% had lasting relief. Generally, placebo effects from injections do not exceed 50% improvement.6 If lidocaine hydrochloride is used, its possible beneficial effects may come from washing out irritant chemicals (substance P) from the area, from local vasodilation that facilitates the removal of metabolites, or from the interruption of the neural feedback mechanism.13 However, the local anesthetic effect is too short to explain lasting relief, and a randomized controlled study of injection therapy of the iliac crest syndrome showed no difference in pain outcome between lidocaine hydrochloride and saline.14 This would tend to support the possibility that multiple puncture, rather than the injected material, is effective because it lessens the tension of the innervated fibrous capsule. Does Injection Treatment of the Back Mouse Really Work? There have been no randomized controlled trials and, until recently, no published long-term follow-up studies of treatment. It is not clear if multiple puncture of the nodule followed by deep massage alone is effective or whether the addition of local anesthetic and steroid improves outcome.
Conclusions
Until a rigorous study demonstrates the reliability and reproducibility of the clinical diagnosis of the back mouse and the effectiveness of treatment in a randomized controlled trial, our best advice for physicians, based purely on clinical and biased observation, is to consider the back mouse when atypical and unaccountable symptoms and signs are found in the lower abdomen, inguinal region, or legs.