A major factor in the successful use of BTX injections in muscular spasm is the correct assessment the muscles involved. This determination is made predominately by history and physical examination with potential contributions from electrodiagnostic testing and imaging in select cases. Often, the decision on the muscular targets chosen for injection will change over time based on the patient's response to past injections and the progression of their underlying pathology.
In neuropathic pain, BTX is injected subdermally into the skin affected. Multiple small (usually 0.1 cc) injections are used at regular intervals until all affected skin is covered. Clinical assessment should emphasize the patient's historical account of pain location and physical examination should identify dysesthetic and allodynic areas of skin. Causes of focal pain that are nociceptive and not neuropathic in etiology, such as infection, bone infarct, and oral caries, should be excluded because the treatment of these conditions may be more successful with modalities other than BTX injection.
Aside from bleeding and infection, the major potential complication of BTX injection is focal muscular weakness, which can be as benign as a droopy eyelid or as serious as major dysphagia requiring the placement of a feeding tube. Neck drop is particularly common in the head and neck cancer population because the target muscles are often weak. In general, BTX should only be used in relatively strong muscles. Overzealous use of BTX injections in weakened muscles, such as the trapezius or cervical paraspinal muscles for pain or spasm, can make the symptoms worse by forcing compensation from muscle fibers not affected by the injection. Great care should be used when clinical weakness is found and starting doses of BTX should be small.
Patients should be continually re-assessed for worsening of both benign and malignant pathology because these pathologies may significantly alter if, where, and how BTX injections are used. A patient with worsening upper cervical metastases, for instance, may develop instability pain that requires surgical cervical stabilization and may be worsened by BTX injections. Progression of radiation fibrosis with worsening paraspinal muscle strength can facilitate the development of neck drop from BTX injections in a patient who had previously benefited from them.
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