Anomalous Course of the Extensor Pollicis Longus With Multiple Absences of Thumb Muscles
A 10-year-old girl visited the outpatient clinic to evaluate the flatness of the right hand, which was incidentally noticed by her parents. She is left-handed, and she had no history of specific discomfort or functional limitation in the right hand. There was no special problem during the prenatal and perinatal period, as well as any family history of any hand anomaly.\nSevere atrophy of the thenar eminence of her right hand was identified (). We also recognized that the IP joint of the thumb was not fully extended (). In addition, the EPB and APL tendons were not obvious enough to make skin fold during extension of the thumb. Passive range of motion of the thumb was normal. Active range of motion in adduction and flexion of her thumb was normal, whereas those of abduction and extension showed limitations. She could extend her thumb to neutral position, whereas the opposite thumb could hyperextend. This limitation did not seem to influence her functional movement of the thumb. Passive and active range of motions of her wrist and other fingers, except the thumb, were within normal range. There were no abnormal physical findings and function in the other extremities. In addition, we could not find any abnormalities in the head and neck, as well as chest and abdomen.\nMuscle power was checked by manual muscle testing and precision dynamometer G200 (Biometrics Ltd., Newport, UK). In manual muscle testing, the muscular strength of shoulder, elbow, wrist, and other fingers, except the right thumb was normal. In the right thumb, strength of flexion and adduction exhibited good grade, whereas extension and abduction were fair to good grade. Grip strength of the right hand, checked by the dynamometer, was 60% of that of the left hand. Sensory test of the right upper extremity, including the thumb, was normal. Tinel's sign and Phalen's test on the right side showed a negative result.\nIn the motor and sensory conduction studies on the right hand, compound muscle action potential of the median nerve was not evoked, but other nerve conduction studies, including the median sensory nerve, ulnar motor and sensory nerve conduction studies, were within a normal range. In the needle electromyography, there were neither insertional activities nor volitional motor unit action potentials in the APB muscle. These findings were in accordance with absence of this muscle.\nBones and joint of her right extremity showed no abnormality in the simple X-ray findings. On magnetic resonance imaging (MRI) study, the FPL tendon, AP and FPB muscles were observed in the palmar side, showing a normal course to an insertional point. However, the APB and OP muscles were not seen (). In the dorsal side, the APL and EPB tendons were not observed (). The EPL tendon was found, but it showed an abnormal insertion toward the radial side of the proximal phalanx, not to the distal phalanx ().\nShe could control her right hand as possible as the greater part of the functional movement, and there were no limitations of the activities of daily living, within the range of her age; therefore, we didn't consider surgical intervention. However, we recommend that she needs to get occupational therapy and education to prevent retardation of hand function development, because the affected hand was nondominant and has overt weakness.