Nerve decompression

Diagnostic nerve blocks can confirm the clinical diagnosis for chronic pain as well as identify the entrapment site.[5] A diagnostic block is like an inverted palpation in the sense that palpation will cause a sensory nerve to send a signal (action potential) and a block will prevent a sensory nerve from sending a signal. By blocking nerve signals, the pain-contributing nerves can be identified or ruled out. Nerves are predisposed to entrapment in certain anatomical regions such as in an osteofibrous tunnels, through a muscle, adjacent to fibrous tissue.[6] Consequently, knowledge of these anatomical regions as well as peripheral nerve anatomy is an essential component to planning successful diagnostic blocks.[5] Ultrasound is a common form of image-guidance to place the needle properly, but it faces limitations visualizing small and deep nerves.[7] CT- or MRI- guidance are better positioned to access deep nerves as well as identify the anatomic level of the needle.[7]

MRI may be used to identify certain causes of entrapment such as a structural lesions pressing on a nearby nerve, but is prone to false negatives/positives and has poor correlation with the clinical examination.[8] A major limitation with MRI is that nerve tissue is resistant to imaging. An advancement of MRI that takes advantage of the tissue properties of nerves, called MR neurography (MRN), provides more detail. MR tractography (MRT) can also be of use in surgical planning as it can identify peripheral nerve abnormalities with a high correlation to intraoperative findings and has higher accuracy than MR neurography alone.[9] MRT uses diffusion tensor imaging to visualize the directional movement of water molecules along nerve tracts. Often an abnormality can be identified along tracts of nerve where water is not diffusing normally along the axis. MRT has been used to identify sacral nerve entrapment by the piriformis muscle, which would otherwise only be diagnosable with exploratory surgery.[10]

Carpal tunnel surgery has a clinical success rate of 75-90%.[17] Success is most frequently measured with the Boston Carpal Tunnel Questionnaire, physical examination (sensory function, motor function, pain, electrodiagnostic, trophic function), and patient self-assessments. One study found that while 86% of patients improved, only 26% had complete recovery of clinical and electrodiagnostic findings. Of the functional assessments, pain showed the greatest improvements following surgery. [18] Another study compared carpal tunnel syndrome patients who elected surgery with those who chose not to. 77% of the surgery group said they were cured compared to 16% who did not elect surgery. [19] While some of the success of surgery may just be due to the natural history of the disease, the surgery groups still have an improvement in outcomes over conservative measures. A systematic review found that surgical treatment outweighed the benefits over conservative treatment overall all outcome measures, however conservative treatment caused fewer complications.[20]

A systematic review has found that 90% of surgery patients see improved pain scores with scores improving on average from 6.7 preoperatively to 2.1 postoperatively.[21] In the literature, the most common outcome measurement for sciatic nerve decompressions is the visual analog scale, where patients rate their pain on a 100mm horizontal line that gets converted into a numeric score from 0-10 or 0-100. The main disability questionnaires used are the modified Harris Hip score (mHHS) and the Oswestry low back disability questionnaire. One study found that all deep gluteal syndrome surgery patients who were taking narcotics for pre-operative pain (n = 21) no longer needed narcotics for the initial complaint after decompression surgery. [22]

A systematic review has found that the improvement is seen in 68-100% of surgery patients and complete migraine elimination is seen in 8-86% of surgery patients.[23] The outcomes are usually measured in migraine intensity, frequency, and duration (an early measurement, the migraine headache index, was just the product of these numerical values). The most common migraine disability questionnaires are the migraine disability assessment (MIDAS), headache impact test (HIT), and migraine specific quality of life questionnaire (MSQ). [24]

One randomized study compared the efficacy of migraine surgery to pharmacologic treatment and found that surgical treatment had a significantly higher success rate than medical treatment. Notably, 36% of patients in the surgical treatment group experienced complete elimination of migraine headaches, compared to and 4% in the medical treatment group.[25] Another randomized study compared surgery to sham surgery. 57% of the surgery group experienced complete elimination of migraine headaches, compared on only 4% of the sham surgery group.[26] A separate study examining outcomes found that there was a bimodal distribution (two main outcomes), where approximately >80% of patients saw either at least an 80% reduction in symptoms or less than 5% reduction. Of the patients seeing significant improvement, the mean improvement was 96%. Of the patients seeing minimal improvement, the average improvement was 0%.[27]

Paying special attention to complete elimination of migraines or measuring outcomes after long follow ups (e.g. years) may be important for assessing the efficacy of migraine surgery because headache research has found a strong placebo effect. [28] A large meta-analysis found that the placebo effect in acute migraine treatments was greatly reduced when the treatment outcome was "pain-free" (9% of patients) compared to "improved" (30% of patients).[29] Studies that have compared migraine surgery to a control group have found similarly low placebo cure rates, both at 4%.[25][26]