BACKGROUND: Few useful interventions exist for patients with persistant low back pain. We suggest that a fibro-fatty nodule (“back mouse”) may be an identifiable and treatable cause of this and other types of pain.

METHODS: We describe 2 patients with painful nodules in the lower back and lateral iliac crest areas. In both cases, the signs and symptoms were unusual and presented at locations distant from the nodule. One patient complained of severe acute lower abdominal pain, and the other had been treated for chronic recurrent trochanteric bursitis for several years.

RESULTS: In both patients, symptoms appeared to be relieved by multiple injection of the nodule.

DISCUSSION: There is agreement that back mice exist. Referred pain from the nodules might explain the distant symptoms and signs in these cases. Multiple puncture may be an effective treatment because it lessens the tension of a fibro-fatty nodule.

CONCLUSIONS: Randomized trials on this subject are needed. In the meantime, physicians should keep back mice in mind when presented with atypical and unaccountable symptoms in the lower abdomen, inguinal region, or legs.

A part from symptoms caused by vertebral disk injury, the scientific evidence for the specific causation and effective treatment of low back problems is relatively weak. Clinicians who wish to adhere to evidence-based practice when treating persistent low back pain are faced with a limited number of useful interventions: short-term therapy with muscle relaxants and analgesics, and encouragement to return to daily routines as quickly as possible.^1-3

Though doubt has been cast on the real existence of subtypes or syndromes of low back pain, our clinical observation and experience suggest that a fibro-fatty nodule (“back mouse”) may be an identifiable and remediable cause of acute or chronic low back pain.^4,5 Given the extent of the costs and suffering caused by low back pain, effective therapy for even a few cases of recurrent or chronic pain would be helpful. We describe 2 patients for whom back mice were the likely cause of low back pain and were associated with unusual symptoms and signs, masquerading as other clinical problems. In both cases, the full medical records from the Family Practice Center and University of North Carolina hospitals were available for evaluation.

Methods and results

Bilateral Trochanteric Bursitis

In 1985 Ms C, a 53-year-old nursing aide, developed typical signs and symptoms of trochanteric bursitis following a vaginal hysterectomy. She also suffered from mild hypertension and chronic depression. The hip pain was intermittent and would affect one side and then the other, often radiating down the legs and limiting her ability to walk. Examination revealed marked tenderness to palpation over the greater trochanter in either hip area. There were no clear precipitating factors.

She was first treated with a variety of nonsteroidal anti-inflammatory agents, with little effect. Subsequently, she received physical therapy including ultrasound, exercise, and cushioned shoes, all of which produced only temporary relief. Because of the chronic pain antidepressants were also tried, which improved her depression but provided minimal improvement for the hip problem.

In 1991, orthopedic evaluation confirmed the findings of relapsing bilateral trochanteric bursitis with an otherwise normal physical examination. Lumbosacral spine and hip radiographs showed only mild degeneration of the L4/5 vertebral disk. The greatest relief for the patient came from injections of lidocaine hydrochloride and methylprednisolone acetate directly into each bursa, but relief lasted only a few weeks after each injection. They had to be repeated on a regular basis. She also needed acetaminophen and oxycodone twice daily to help control the pain, and in 1995 she applied for disability.

At that time, the orthopedic specialist discharged her back to her family physician with a diagnosis of chronic trochanteric bursitis for which no other treatment could be offered. In 1997 another orthopedic consultation led to the same opinion, based on typical symptoms and clinical findings. In 1998, during a discussion of the lack of treatment options for this chronic problem, it was suggested that the symptoms could be secondary to a lower back problem and a careful soft tissue examination might be of value. Detailed examination revealed 2 long rubbery and tender fibro-fatty nodules, each one lying on an iliac crest (right side=3 cm by l cm; left side=6 cm by l cm) These findings correlated with previous descriptions of fibro-fatty nodules in the back region.^4,7

Repeated testing using firm palpation of these nodules reproduced the pain over each greater trochanteric area where the patient had experienced pain in the past. Each nodule was treated with multiple puncture technique (6 to 8 punctures of the fibrous capsule of the nodule) and injected with 3 cc of lidocaine hydrochloride and 40 mg methylprednisolone acetate.^4,5 There was immediate and complete relief of the clinical symptoms and signs. Five months after the injections there has been no recurrence of the clinical characteristics of trochanteric bursitis, though there have been other symptoms of lumbosacral pain.

An Abdominal Emergency

In 1997 Ms W, a 35-year-old with type 2 diabetes (controlled by diet and exercise) was working as a nurse on the pediatric ward. She came to the urgent care clinic at the Family Practice Center with symptoms of mild low back pain and right-sided lower abdominal pain which were associated with dysuria and frequency of urination during the previous 2 months. She had suffered an episode of low back pain 2 years earlier. Her urine showed a mildly positive leukocyte esterase test and a trace of protein, and she was given a course of trimethoprim.

Ten days later she returned reporting that the abdominal pain had become much worse, particularly when sitting and lying down, and it kept her awake at night. She complained of a feeling of fullness in the right lower quadrant of the abdomen. There were no changes in her bowel habit, and she had no fever, nausea, or anorexia. She was taking maximum doses of ibuprofen and acetaminophen for the pain. A repeat urinalysis was normal. She had previously had a hysterectomy and appendectomy, as well as polycystic ovaries, but the results of a pelvic examination were normal. Examination of the abdomen revealed normal bowel sounds and some tenderness in the right lower quadrant, close to the superior iliac spine. Because of her history of persistent back pain and a lack of explanation for her abdominal symptoms, x-rays of the lumbosacral spine and pelvis were ordered.

When she was seen again 2 days later, the abdominal pain had become much more severe. It was sharp, intermittent, not colicky, and traveled down into the groin and right anterior thigh. There was considerably less pain when she was standing. She had normal bowel sounds but was again acutely sensitive to palpation in the lower abdomen just above the anterior superior iliac crest. Assessment for acute abdominal pain included an electrolyte panel and complete blood count, which were normal except for a white blood count of 11,200. The lumbar and pelvic radiologic studies showed mild degeneration of the hip joints and some spurring of the inferior aspect of both sacroiliac joints.

With a working diagnosis of lower abdominal abscess, she was referred to the emergency department to be seen by the family medicine inpatient team. A computed tomography scan of the abdomen and pelvis was normal, showing only a small ovarian cyst on the left ovary. However, a repeat white blood count (several hours later) was 13,100. A surgical consultation was requested, but the surgeon found no evidence of an acute abdominal process, giving his opinion that this was a musculoskeletal problem.

At the suggestion of one of the family medicine faculty a more detailed examination of the sacroiliac joints was performed, and the resident found significant point tenderness over the right sacroiliac joint. Deep palpation of this joint area also produced pain radiating to the right inguinal region. It was felt that the acute abdominal problem might be caused by referred pain from the sacroiliac joint. The patient received an injection of 60 mg ketorolac tromethamine, which produced considerable relief, and she was sent home. However, there was no clear explanation for the 2 occasions of elevated white blood cell count.

At follow-up 10 days later the abdominal pain had subsided, but she was complaining of much more pain in the right lower back with referral down the front of her thighs. There was still tenderness over the right sacroiliac joint, and she was sent to a physical therapist for evaluation and treatment. During her first visit to the physical therapist, in addition to the low back symptoms she again reported increasing right lower abdominal pain that was worse when sitting, better when standing. Careful examination revealed an extremely tender 3 cm long, partly mobile, fibro-fatty nodule along the mid-region of the right iliac crest, approximately 4 inches lateral to the spinous process of the lumbar vertebra. Repeated firm, direct pressure on this nodule made the patient cry and reproduced the right lower abdominal symptoms.

Following injection with multiple puncture technique, lidocaine hydrochloride 3 cc and methyprednisolone acetate 40 mg, the patient experienced immediate pain relief with no more abdominal symptoms and no difficulty in sitting or lying down. Since that time (2 years ago), there has been no recurrence of these symptoms. Although the cause of the abdominal pain might have been a polycystic ovary, the workup did not support this conclusion and revealed no other obvious cause for the severe symptoms. Injecting a painful nodular swelling in the lower back that fitted the characteristics of previously described fibro-fatty nodules provided immediate relief of the pain, suggesting a possible association with the anterior abdominal symptoms.

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.

Acknowledgements

We are grateful to Barry R. Howes, PT, and Robert E. Gwyther for guidance in the preparation of this report.

Fibro-Fatty Nodules and Low Back Pain