Again, anatomic breakdown of the leg into its various anatomic components is the basis of a sound differential diagnosis (Table 44). Before that, however, one should determine if the pain is actually originating from the hip or if it is the result of knee joint disease. If so, the differential diagnosis of these must be considered (see pages 231 and 282). Beginning with the skin, consider herpes zoster and various dermatologic conditions. In the subcutaneous tissue, one encounters cellulitis and occasionally filariasis, which may produce a similar picture. Beneath this layer the muscle and fascia suggest numerous causes of leg pain. There may be hematomas of the muscle, trichinosis or cysticercosis, nonarticular rheumatism, or fibromyositis. Muscle cramping from low sodium or other electrolyte disturbances must be considered. The superficial and deep veins are the site of thrombophlebitis, a prominent cause of leg pain. The arteries may be involved by emboli (from auricular fibrillation, acute myocardial infarction, and subacute bacterial endocarditis), thrombosis (especially in Buerger disease and blood dyscrasias), and vasculitis (from arteriosclerosis and collagen diseases). Acute trauma to the artery or veins may cause pain. As usual, when one moves centrally along the arterial pathways additional causes of pain come to mind. Leriche syndrome and dissecting aneurysm must be considered. When superficial or deep infections of the leg spread to the lymphatics, lymphangitis is important in the differential. The nerves may be involved locally, centrally, or systemically. Buerger disease, cellulitis, and osteomyelitis may involve the nerve locally. Neuromas may occasionally cause focal pain in the distribution of the nerve involved. More important are the central causes of nerve pain in the limbs. Probably herniated discs of the lumbar spine account for most of these cases, but Pott disease, lumbar spondylosis (osteoarthritis?), metastatic and primary tumors, multiple myeloma, fractures, spondylolisthesis, and osteomyelitis of the spine all may compress the cauda equina and cause pain in the lower limbs. Pelvic tumors, endometriosis, and sciatic neuritis are, in a sense, “central” causes of leg pain, and all patients deserve a rectal and pelvic examination when the diagnosis is obscure. Pelvic inflammatory disease and obturator hernias may rarely involve the obturator nerve. Meralgia paresthetica from diabetes mellitus and other causes must be considered in thigh pain and in causalgia. Finally, the thalamic syndrome and diseases of the cervical spine must be considered. Dissecting the limb layer by layer, we finally reach the bone, which suggests osteomyelitis, bone tumors, Osgood–Schlatter disease, tuberculous osteomyelitis, and Paget disease. Systemic diseases that may involve the nerves causing pain in the legs include tabes dorsalis, periarteritis nodosa, diabetes mellitus, metabolic and nutritional neuropathies, and blood dyscrasias.
The approach to the diagnosis of leg pain involves numerous ancillary examinations that one may not routinely do. Thus, arterial pulses must be checked all the way up. One should look for a positive Moses or Homans sign. Straight leg raising (SLR) and meticulous mapping of sensory changes are valuable. The SLR sign may be negative and the patient could still have a herniated disc higher up. Thus, a femoral stretch test is done,1 and when positive suggests a herniated disk at L2–3 or L3–4. Edema associated with phlebitis or atrophy associated with a herniated disc can be detected only with careful measurement of the calf and thigh. Deep vein thrombophlebitis can be diagnosed by ultrasonography or impedance plethysmography. Arterial circulation is best evaluated with an ultrasound flow study. Venography and arteriography may be necessary if plain x-ray films are unremarkable. One should almost always x-ray the spine, hips, knee joints, and, in difficult cases, the entire legs.
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Copyright notice for book excerpts: Copyright © 2008 Lippincott Williams & Wilkins. All rights reserved.
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More About This Book:
Title: Differential Diagnosis in Primary Care Authors: R. Douglas Collins MD, FACP Publisher: Lippincott Williams & Wilkins Copyright: 2007 ISBN: 0-7817-6812-8 
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