Back mice are small, firm nodules of fatty tissue found just beneath the skin of the lower back, typically near the top of the pelvis. Despite the quirky name, they are a real and recognized (if underdiagnosed) cause of low back pain. The medical term for them is episacral lipomas or episacroiliac lipomas, and they were first described over a century ago. In a study of 1,000 randomly selected backs, these nodules were found in roughly 30% of people examined, though not all of them caused symptoms.
What Back Mice Actually Are
Back mice are small collections of fat that sit in or just beneath the layer of tough connective tissue (fascia) covering the muscles of the lower back. They tend to form near the sacroiliac region, the bony area where your spine meets your pelvis. The nodules can feel like small, rubbery lumps that may slide slightly under your fingers when pressed. Some are as small as a pea, while others grow to a few centimeters across.
What makes them painful isn’t always the fat itself. The leading theory is that these fatty nodules herniate through small openings in the fascia and become trapped, putting pressure on the small cutaneous nerves that run through the area. When that happens, the nodule can become tender to the touch and generate pain that radiates into the buttock or even down the leg, mimicking sciatica or other more commonly diagnosed conditions.
How They Feel
The hallmark of a symptomatic back mouse is a localized, tender spot in the lower back that you can actually feel with your fingers. Pressing on it typically reproduces or worsens the pain. Many people describe a dull, persistent ache in the lower back that flares with certain movements, prolonged sitting, or bending. Because the trapped fat can irritate nearby nerves, the pain sometimes refers outward into the hip, buttock, or upper thigh, which is one reason the condition gets mistaken for disc problems or sacroiliac joint dysfunction.
Not every back mouse causes pain. Plenty of people carry these nodules without knowing it. The ones that become symptomatic tend to be firmer, less mobile, and more sensitive to pressure, possibly because they’ve become pinched within the fascia or have developed minor inflammation around the nerve fibers they’re pressing against.
Why They’re So Often Missed
Back mice are one of the more overlooked causes of low back pain. Part of the problem is that they don’t show up reliably on standard imaging. Ultrasound can sometimes reveal a well-defined, oblong, echogenic mass beneath the skin, and MRI can identify tissue that matches the signal of subcutaneous fat. But studies have shown ultrasound has low accuracy for diagnosing lipomas in general, and most typical lipomas are diagnosed clinically (by touch) rather than through imaging. If a doctor isn’t specifically looking for them during a physical exam, they’re easy to miss entirely.
Another issue is confusion with trigger points. Both back mice and myofascial trigger points cause localized tenderness in the lower back. The key difference is that a back mouse is a discrete, palpable lump of fatty tissue, while a trigger point is a tight band within the muscle itself. Trigger points often produce a local twitch response when pressed, while back mice do not. Knowing the difference matters because the treatments diverge.
Treatment Options
The most widely reported treatment for painful back mice is injection of a local anesthetic directly into the nodule, sometimes combined with a corticosteroid. The anesthetic provides immediate pain relief by numbing the irritated nerve, while the steroid reduces local inflammation. Many case reports describe good results with this approach, especially when combined with repeated direct needling of the nodule. However, the one clinical trial that compared anesthetic injection against a saline placebo (without the needling component) found only mild, temporary benefit. This suggests the mechanical disruption of the nodule through needling may matter as much as the medication itself.
For nodules that don’t respond to injections, surgical excision is an option. The procedure is straightforward: a small incision, removal of the fatty nodule, and closure. Surgical removal is generally considered when the nodule causes persistent discomfort, compresses nearby structures, or significantly limits daily function. It remains the most definitive treatment, with a high likelihood of resolving symptoms for good. Because these are small, superficial lumps, the surgery is minor and recovery is typically quick.
Back Mice vs. Other Lipomas
Back mice are technically a type of lipoma, which is just the medical term for a benign fatty growth. Lipomas are extremely common throughout the body and are almost always harmless. What sets back mice apart is their specific location along the lower back and pelvis, where the fascia is thick and nerves run close to the surface. A lipoma on your forearm is unlikely to cause much trouble beyond a cosmetic bump. A lipoma wedged into the thoracolumbar fascia sits in exactly the wrong spot to irritate the nerves responsible for low back sensation.
In rare cases, deeper lipomas in the pelvic region can compress larger nerves. One documented case involved an intermuscular lipoma pressing directly on the sciatic nerve, causing neuropathic pain in the posterior thigh and lower leg. This is unusual for typical back mice, which are superficial, but it illustrates how fatty tissue in the wrong location can produce significant nerve-related symptoms.
What to Do if You Think You Have Them
If you have chronic lower back pain and can feel one or more small, tender lumps near the top of your pelvis, back mice are worth considering. The simplest test is to press firmly along the area where your lower back meets the pelvis, slightly off to either side of the spine. A back mouse will feel like a small, somewhat firm, rubbery nodule that’s tender when you push on it. If pressing it reproduces the pain you’ve been experiencing, that’s a strong clue.
Bringing this up with a healthcare provider is worthwhile, especially since the condition is underrecognized and a physical exam is the most reliable way to identify it. Many people with back mice have already been through rounds of imaging and treatment for other suspected causes of low back pain without relief. Simply knowing this diagnosis exists can be the first step toward getting the right treatment.