Musculoskeletal complaints account for a significant number of outpatient visits to the pediatrician—up to 10% of non–well-child appointments in some studies. Only a minority of these visits result in hospitalization, but in many cases, even these admissions could have been avoided if a logical, stepwise approach to evaluation and management had been used. This chapter focuses on the entities to consider in a limping child and the appropriate approaches to take with regard to the history, physical examination, and diagnosis.

PATHOPHYSIOLOGY

The normal gait is the most efficient and stable means for humans to walk upright on two legs. For this gait to be altered, strong countervailing forces must be applied. These may be anatomic (e.g. broken bone, muscular weakness), neurologic (disrupted proprioception or balance), or nocioceptive in nature. Pain is the most common cause of an abnormal gait in children.

Recognizing specific gait aberrations can facilitate the localization and identification of musculoskeletal pathology. For example, a psoas abscess is often difficult to diagnose because of poor localization of discomfort within the pelvis; pain may be perceived as occurring anywhere from knee to the diaphragm. Trying to walk with a psoas abscess, however, results in a characteristically altered gait, as contraction of the psoas muscle is avoided to minimize discomfort. This causes leaning to the involved side and using the bones of the pelvis and upper leg to substitute for the support usually provided by the psoas muscle. Further, the contralateral hemipelvis dips to keep the psoas muscle relaxed and thus avoid pain. The result is the Trendelenburg gait, which can be caused only by a pathologic condition involving the proximal femur or muscles of the pelvis (Figure 30-1). Analogous effects of other sites of pathology on gait are listed in Table 30-1.

The key elements of the medical history that help identify the cause of a limp include the timing of the symptoms, the nature of the pain with regard to alleviating and exacerbating factors, particularly response to activity (Table 30-2), and the character of the pain, such as dull, sharp, radiating, or burning.

While pain is the most common cause of a limp in children, it is not the only cause. When children do complain of pain, it may originate in sensory nerves of the skin, soft tissues, muscles, bone, or nerves. Joint nocioceptors found in the joint capsule and adjacent connective tissue are activated only by extreme mechanical or chemical irritation. This high threshold for sensing joint pain likely accounts for the fact that up to 80% of children with arthritis do not report pain. For example, an Oklahoma study found that children with arthritis typically presented with stiffness, avoidance of activities, or regression of skills, but only 13 of 226 children referred to a rheumatology clinic with joint pain actually had arthritis.¹

INFLAMMATORY SYMPTOMS

The most characteristic feature of discomfort related to inflammatory processes is the classic morning stiffness of arthritis. Difficulty may also be reported after other periods of inactivity, such as long car rides or sitting in school. Inflamed synovial joint tissue produces less hyaluronic acid, which causes gelling of joint fluid and decreased lubrication. This can be reversed by warming of the joint, returning the synovial fluid to the liquid state and once again permitting efficient, low-friction movement. Thus children with arthritis typically feel better after a warm bath or after several minutes of activity. These children may suffer joint stiffness in the morning but be quite comfortable exercising strenuously later in the day. Cold, damp weather or swimming in cool water also tends to disturb children with arthritis. Nighttime awakening is unusual with arthritis. Any atypical symptoms—especially nighttime pain or discomfort with activity—should raise the suspicion of an alternative diagnosis, even in the setting of otherwise typical signs of joint inflammation.

MECHANICAL SYMPTOMS

The timing of symptoms caused by mechanical factors is the mirror image of manifestations of inflammation (Figure 30-2). Children typically feel well in the morning, but become more uncomfortable with increasing activity. Like inflammation, however, mechanical pain generally does not awaken children from sleep. Rest and ice tend to alleviate mechanical symptoms. The precise type of overuse syndrome or injury causing a child’s symptoms can generally be determined from a careful history (e.g. Osgood-Schlatter syndrome in an adolescent male athlete, iliotibial band syndrome in an adolescent female runner) and physical examination.

BONE PAIN

Pain originating in the osseous compartment tends to be constant and does not change significantly with activity. Bone pain raises concern for infection, trauma, and malignancy. This type of pain may awaken a child at night, particularly when it is related to leukemia or a tumor. Cytopenias are typically seen with leukemia, although a normal complete blood count does not exclude the possibility. Other tumors, such as sarcoma or metastatic neuroblastoma, are far less common but must be considered in children with significant nighttime pain. Osteomyelitis should be considered for patients with new onset of fever, localized pain, and decreased mobility; many patients will have a history of preceding trauma (see Chapter 70).

NEUROPATHIC PAIN

Nerve pain tends to be worst at bedtime, when the usual distractions of daily activities abate and children are left to focus on their discomfort. In patients old enough to describe the sensation, neuropathic pain typically has a sharp, burning, or shooting character. It is also commonly associated with allodynia (a sense of discomfort from stimuli that normally do not cause pain) and hyperesthesia (a decreased threshold for discomfort in overlying tissues). Although joints may be involved, neuropathic pain generally encompasses extra-articular areas as well and can follow a dermatomal distribution. Activity does not have a significant effect on neuropathic pain. When nerve pathology due to severe trauma, tumor, or vasculitis cannot be identified, pain syndromes such as fibromyalgia or complex regional pain syndrome should be considered.

The four cardinal signs of inflammation (dolor, tumor, rubor, and calor) are the hallmarks of arthritis. The musculoskeletal examination should begin with careful observation of the extremities in a child who has removed his clothing and is robed in a gown. Whether assessing pain, swelling, changes in color, or variations in temperature, the most sensitive aspect of the examination is comparing the two sides of the body. Decreased extension of the right knee compared to the left, or an identifiably warmer or more sensitive left ankle leads to rapid identification of likely pathology. Careful examination of all the joints is mandatory, even when the patient complains about a single joint, as the potential causes of a monoarticular process differ significantly from those of polyarticular conditions.

Warmth and swelling are most characteristic of inflammatory arthropathies, while overlying erythema is more indicative of septic arthritis. An inflamed knee may exhibit a ballotable effusion, meaning that applying pressure directly to the patella forces it downward, displacing synovial fluid and causing a bounce against the femur. The characteristic springiness is not noted when fluid does not intervene between the patella and femur, as in healthy children. The joint may also be swollen from synovial proliferation, which has a boggier consistency than the free fluid of a joint effusion. Osteomyelitis most typically causes point tenderness on palpation of the involved bone, though in younger children precise localization of the discomfort can be more difficult.

Careful examination of an inflamed joint may allow an estimation of the duration of the arthritis. Synovitis is characterized by increased blood flow, typically more pronounced in the portion of the joint compartment subjected to maximal force. In the knee, this is the medial aspect, where hyperemia leads to increased delivery of nutrients and accelerated growth. This may manifest initially as prominence of the medial femoral condyle, and later as genu valgus. Ultimately, the leg with the inflamed knee grows more rapidly, and a leg length discrepancy develops. The lower leg may bow to compensate for the greater length of the upper leg. At the same time, the knee loses extension and develops a flexion contracture, with resultant atrophy of the vastus medialis and wasting of the quadriceps muscle. In contrast, significant inflammation in the hip or the temporomandibular joint of the jaw often damages the growth plate and leads to shortening of the involved leg or hemimandible. Inflammation in other joints generally causes little demonstrable discrepancy in size.

When a child presents with a complaint referable to the lower extremities, the list of possible causes is long and varied (Table 30-3). In the absence of an obvious explanation such as known trauma, it is helpful to start the evaluation by categorizing the type of pain or discomfort according to the nature of its onset (acute vs. chronic), the number of joints involved, and whether extra-articular signs or symptoms such as fever or rash are present. Most normally active children have some history of trauma during the preceding 24 hours; however, unless the trauma is significant (e.g. football injury, automobile accident, bicycle fall), it is more likely to have unmasked a preexisting pathologic condition than to have caused damage to the child’s resilient tissues.