KB Tam, KY Chan, S Fong, EKC Yau

Abstract

Carpal tunnel syndrome (CTS) is rare in children. Lysosomal storage diseases including mucopolysaccharidosis and mucolipidosis, account for most of the underlying causes of childhood onset CTS. Rare causes such as trauma, aberrant ligaments, and muscle slings in the carpal tunnel and sports-related CTS have also been reported in children. Classical CTS occurs due to compression of the median nerve in the wrist when it runs through the carpal tunnel and usually presents with early sensory and late motor symptoms related to the territory of median nerve. Children with CTS however often have very few complaints despite long standing difficulty in manual tasks. Most of them are diagnosed late in the course of nerve compression with severe thenar weakness and atrophy. We describe the clinical features, hand function assessment, MRI findings and surgical outcome in a 12 years old girl with thenar muscles atrophy due to bilateral carpal tunnel syndrome with entrapment of the recurrent motor branches of median nerves.

Introduction

Carpal tunnel syndrome (CTS) is the most common entrapment neuropathy with a prevalence of about 3% in the adult population.¹ It occurs two to three times more often in women than in men, most frequently in middle-aged women. Conditions including pregnancy, diabetes mellitus, rheumatoid arthritis, chronic renal failure and amyloidosis are important risk factors for CTS. The diagnosis of CTS is based on the typical clinical history of numbness, paresthesia, pain and weakness and supported by provocative tests including percussion or compression of the transverse carpal ligament.² Properly performed electrophysiological studies could provide information on severity and progression of CTS and at the same time rule out concomitant peripheral nervous system abnormalities.³ Carpal tunnel syndrome is very rare in children especially in healthy kids without history of trauma or underlying predisposing conditions such as genetic lysosomal storage diseases, local etiology like macrodactyly, melorheostosis, and aberrant muscle slips and tendons.^4,5 We report the clinical features, hand function assessment and surgical outcome in a young healthy girl with bilateral carpal tunnel syndrome due to compression of median nerves and the thenar motor branches as a result of congenital narrow carpal canal and aberrant origin of the motor branch of median nerves.

Case Report

This 12 year old girl was referred to child neurology clinic because of 3 months history of progressive muscle weakness of both hands. There was no complaint of any sensory symptoms. Her past health was good and there was no injury to upper limbs. Physical examination showed severe wasting of thenar muscles of both hands with weakness in thumb abduction and opposition. No sensory deficit was elicited. Both Tinel sign and Phalen's test were negative. X-ray hands showed no bony abnormalities. MRI cervical and upper thoracic spine showed normal findings. Motor nerve conduction velocity studies (NCV) over bilateral median nerve showed a very severe decrease in CMAP (compound motor action potential) of 0.5 mV and severe delay in distal latency of up to 9-10.2 ms. There was also a moderate slowing in bilateral median nerves motor conduction velocities. Sensory NCV studies showed normal distal latencies and SNAP (sensory nerve action potential) but a mildly decrease in sensory conduction velocities. Electromyogram of left abductor pollicis brevis showed pattern of neuropathic changes. The abnormal electrophysiological findings over bilateral median nerves are suggestive of bilateral median neuropathy at the wrist, likely to be CTS. But the absence of sensory symptoms and the absence of typical sensory conduction abnormal findings for classical CTS raise the suspicion of also block or abnormality involving the recurrent motor branches of median nerves. MRI of both wrists showed increased T2 signal and swelling over proximal right and left median nerves, compression/flattening of bilateral median nerves and atrophic thenar muscles (Figure 1). There is no space occupying lesion inside the carpal tunnel. However the recurrent motor branches of the median nerves could not be demonstrated by MRI. Preoperative hand function assessment showed normal protective sense, but impairment in power grip and writing speed (Table 1). Below standard performance were noted in skills in handwriting, picking up objects and simulated feeding in the dominant right hand by Jebsen Taylor Hand Function Test.

Exploration of the median nerves was performed on both sides and tight transverse carpal ligaments and flattened median nerves were noted during operation. The left and right recurrent motor nerves branched unusually from the anterior aspect of the median nerves and went through an acute turn radially towards the thenar eminence (Figures 2a & 2b). The right recurrent motor branch arising at the level of the distal margin of the transverse carpal ligament was found impinging by the distal margin of the ligament when it went up before turning towards the thenar muscle. The left recurrent motor branch arose 0.5 cm distal to the distal margin of the transverse carpal ligament and was not impinged by the ligament. Both recurrent motor branches were traced 0.5 cm further after they entered the substance of the thenar muscles. No impingement was found at the entry point of the muscle. The transverse carpal ligaments were left open after release and the skin wound was closed. The causes of the compression of the recurrent motor branches were due to twisting of the nerve when it turned acutely towards the thenar muscle for the left side and twisting of the nerve plus impingement by the distal margin of the transverse carpal ligament for the right side. The postoperative course was uneventful and the patient was discharged the next day.

Hand function assessment eight months after operation showed a significant improvement in power grip and writing speed. Skills in handwriting, turning cards, simulated feeding and picking up large objects were also significantly improved (Table 1).

Figure 1 MRI of right wrist. Transverse view of the wrist showing flattened and atrophic median nerve (circled) underneath the transverse carpal ligament (left) compared with the shape and size of the median nerve (circled) just proximal to the transverse carpal ligament. No space occupying lesion is seen inside the carpal tunnel. Similar view is seen in left wrist.

Discussion

Carpal tunnel syndrome is very rare in children and adolescents and is usually seen following trauma such as fractures, burns and crush injuries. Healthy children can acquire CTS through excessive overuse of hands in athletic endeavors including weight lifting, skiing, bicycling, basketball and golfing.⁴ It is very rare to have CTS in a young healthy child without trauma or underlying genetic, systemic or local disorders.⁶ In Idiopathic Familial CTS, abnormal thickening of flexor retinaculum or congenitally narrow carpal canal were postulated to be the possible predisposing factors for development of symptoms in CTS.⁶ Bilateral CTS. with such a severe degree of thenar muscle atrophy and weakness in thumb abduction and opposition without sensory symptoms as reported in our patient is even more unusual. Underlying causes should be extensively looked into before contemplating invasive treatment.

There are three types of presentation in CTS. The most common one encountered in daily practice is classical CTS with sensory and motor symptoms without genuine secondary compression of recurrent motor branch and carpal tunnel release per se could relieve the compression. The other two including CTS with superimposed motor branch compression and isolated recurrent motor branch compression without associated CTS. The variation in the pattern of presentation should be seriously considered before surgical decompression particularly when there is significant thenar muscle atrophy.⁷ The thenar motor branch often originates from the radial side of the median nerve or the beginning of the first division.⁸ Hurwitz⁹ in the prospective study of 61 patients with CTS found 21% hands with an anomalous origin of the motor branch of median nerve. It was noted that 12.5% from the anterior side, 6.25% from the second division, 1.25% from the ulnar side and 1.25% from the dorsal side of the first division of the median nerve. Bennett and Crouch⁷ reported two types of independent recurrent motor branch compression as a result of either direct fascial penetration and entrapment or acute angulation of the nerve with apparent impingement by the transverse carpal retinaculum in his cohort of patients.

The diagnosis of CTS is usually accomplished by specific clinical features and neurophysiological studies. The classical clinical features of CTS include predominantly sensory symptoms (pain and paraesthesia) in the territory of the median nerve and less frequently thenar muscle weakness. Neurophysiologic studies show slowing of sensory and motor nerve conductions in the median nerve at the wrist, as a result of demyelination in the early phase of compressive neuropathy; and decrease in amplitude of sensory and motor action potential in late stage when axonal damage has occurred.¹ Children with CTS usually have modest complaints despite long-standing hand clumsiness or poorly localised pain. Some children will be presented with profound thenar muscle atrophy per se and the degree of atrophy is severe enough to suggest the possibility of congenital thenar hypoplasia.⁴ Similarly, the clinical presentation of our patient is not typical of classical CTS and the major abnormality is bilateral thenar atrophy. The neurophysiological studies of our patient are also atypical with prevalent electrophysiological involvement of median motor fibres and a borderline decrease in median sensory conduction velocities. In CTS, selective neurophyiological involvement of median motor fibres is rare and varies from 0.6 to 1.2%. Prevalent or exclusive motor fibres involvement in CTS may be related to the intraneural topography of motor fibres or to the aberrant course of the recurrent motor thenar branch.¹⁰ Imaging is useful to define the exact cause or when clinical findings and nerve conductions studies are not specific. Carpal tunnel imaging could be done by routine radiography, ultrasonography, CT and MRI. Using MRI evaluation, the size, shape, signal intensity of median nerve and abnormalities within the carpal tunnel are easily visualised. Swelling of the proximal part of median nerve, distal flattening, increased signal intensity and pronounced palmer bowing of the transverse carpal ligament at the level of the hook of hamate are the four major MRI features in carpal tunnel syndrome.^11,12 MRI wrist of our patient showed compression and flattening of median nerves within the carpal tunnel, swelling of the proximal part of the nerves and atrophy of bilateral thenar muscles. As our patient did not have classical presentation of CTS, preoperative MRI could be very useful to further delineate the nerve or tunnel abnormalities and at the same time exclude space occupying lesions such as neurofibroma or ganglion underneath the transverse carpal ligament. However possible abnormality of the recurrent motor branches of the median nerves could not be demonstrated in MRI though it was suspected preoperatively from the clinical signs and electrophysiological study. During the operation, compression of bilateral median nerves due to congenital narrow carpal canals were noted and aberrant origin of the recurrent motor branches causing twisting and impingement of the nerves were also demonstrated. The first finding was known preoperatively in MRI and the second finding was only suspected before surgery and was confirmed after surgical exploration.

Congenital narrowing of the carpal tunnel is well documented in adults with CTS. In children, it was not well depicted in literature as idiopathic CTS is very rare. The normal dimensions of the carpal tunnel in various age groups have not been reported. Although quantitative measurement could not be made in the MRI, the diagnosis in this case was derived by the longitudinal profile of the nerve in MRI and on surgical exploration. The nerve was also noted to be flattened under the transverse carpal ligament but swollen immediately proximal to it.

Although this patient did not have the typical finger numbness or pain in adult CTS, the preoperative electrophysiological study did show a mild decrease in sensory conduction velocities. So it is postulated that the sensory involvement of this patient was small and she could adapt to it with time.

Muscle power assessment, sensibility test, Jebsen Hand Function test and copying speed are useful tools for assessment of hand function before and serially after operation.¹³ Significant improvement in power grip, two-point discrimination, speed of writing/picking up objects/copying were shown after the release surgery in our patient. Therefore, it is important to have early diagnosis and operation in severe case of CTS because irreversible neurological damage will occur in long standing compression and neuromuscular function will not improve even after release surgery.

Conclusion

Carpal tunnel syndrome is extremely rare in children without underlying genetic, systemic and local disorders. In young children, sensory symptoms are often not well aware and usually passed unnoticed without any intervention until there is severe thenar muscle atrophy. It is always a diagnostic challenge in cases devoid of typical clinical features and neurophysiological findings. A more meticulous approach in the search for underlying aetiologies has to be carried out before embarking on explorative surgery. MRI wrist is definitely indicated to delineate local abnormalities and improve the diagnostic accuracy of CTS especially for atypical cases. However one should also remember the limitation of MRI which cannot demonstrate subtle anatomical anomaly such as kinking of a nerve. So all the preoperative symptoms, signs and investigations results should be combined to give the most likely diagnosis before proceeding to surgery and sometimes the final diagnosis can only be known after surgical exploration. For our patient, the clinical phenotype, electrophysiological results and operative findings favor the diagnosis of bilateral CTS with superimposed recurrent motor branches entrapment. One has to look for anatomical variation in the recurrent motor branches of median nerves when the clinical presentation for CTS is so atypical with predominantly motor weakness.

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