Double Crush Syndrome (DCS) is described as compression of a peripheral nerve at more than one site. Scientists have theorized that compression at one site can be asymptomatic, but cause increase risk of impairment at another anatomic site, thus the double crush of the nerve. When the nerve is disrupted at both sites it can result in a change in nerve function nutrient flow at the axonal level and increase the chance that distal nerve axons also become compressed and often symptomatic.
Since being identified in 1973, there has been a lot of controversy surrounding the pathophysiology of double crush syndrome (DCS). There is no way to confirm objectively that the symptoms attributed to DCS are due to injury at two distinct sites and often the patients diagnosed with DCS have multiple comorbidities, other symptoms, and disability. Often the result of diagnosing an individual with DCS is encouragement to seek out surgical repair that may not be the most effective treatment, particularly when multiple comorbidities and disability coexist. Even with the surrounding controversy, it is important to identify that Double Crush Syndrome is a possibility and that a patient’s symptoms may not be related to only one site of nerve compression, multiple sites or even a systemic neuropathy can also be at play.
Many studies have attempted to identify the physiology, the risk factors and the frequency of DCS in the general population. The results have varied so significantly that it is difficult to define how common it is and which populations may be at greater risk. There is some consensus that there is an increased risk of nerve compression injury after systemic illness, such as diabetes, hereditary neuropathy, uremic neuropathy, hyperthyroidism, vitamin deficiency and chronic alcoholism. Researchers have found for possible mechanisms for DCS, the most common being a disruption in axonal nutrient flow due to the compression injury. Other reasons may be an immune response inflammation of the dorsal root ganglion portion of the nerve, an ion channel regulation issue, or a potential neuroma.
The most common diagnosis of DCS is with patients who are unsatisfied with a Carpal Tunnel release procedure. Researchers identified characteristics that differ between those with DCS compared to Carpal Tunnel Syndrome (CTS) alone and have found that in comparison to patients with only CTS, patients with DCS have greater incidence of radiating pain closer to the neck and shoulder, more parathesias and less numbness, decreased grip strength. Some of the classic tests for CTS, including Phalens and Tinnels, were also less frequently positive in those with DCS. It is important that the testing physician be able to identify these subtle differences that distinguish double crush syndrome from carpal tunnel syndrome, as it will allow for more appropriate treatment referral.
Double crush syndrome can also take place in the lower extremity, however there is less research available investigating the occurrence and risk factors for DCS in the legs. In general, the physiology of DCS can take place in any nerve. Examples of DCS in the lumbar nerve roots and associated peripheral nerve have been describes, as have injuries to the sciatic nerve and peripheral nerves. Trauma seems to be a large risk factor for DCS in the lower extremity, including acetabular Fractures of the Hip, posterior hip-dislocation-precautions/topic/45″ class=”alinks-link” title=”Hip Dislocation”>hip dislocation, and lumbar compression. DCS is also associated with ankle injuries resulting in tarsal tunnel syndrome where the posterior tibial nerve is compressed under the flexor retinaculum.
Treatment for double crush syndrome should be initiated with conservative measures that focus on distinct management of each individual lesion. This may include oral steroids, steroid injections, NSAIDs, relative rest to avoid irritating movements and positions, and physical therapy. It is important for the patient to understand that the treatment will involve both the area of pain and the secondary lesion contributing to symptoms, for example treating both the neck and the arm when dealing with carpal tunnel syndrome. If conservative measures fail, surgical consideration can be taken and may include cervical spine decompression and peripheral nerve decompression. This will include procedures such as cervical discectomy, fusion, total disc replacement, posterior laminoforaminotomy, 1st rib resection, or resection of a muscle. The decision as to where to focus surgical efforts first will depend on severity of symptoms and compression at each site. It is imperative that management, surgical or conservative, should focus first on accurate diagnosis and then treatment of all contributing elements.