Opponensplasty With Abductor Pollicis Brevis Rerouting for Types II and IIIA Hypoplastic Thumbs.

PURPOSE: To evaluate the functional results after opponensplasty using an abductor pollicis brevis rerouting technique in type II and IIIA hypoplastic thumbs. METHODS: Eleven hypoplastic thumbs in nine children with type II and IIIA hypoplastic thumbs were treated with abductor pollicis brevis rerouting. The mean follow-up period was 70 months (range, 12-172 months). We assessed preoperative to postoperative changes in the angles of the first and second metacarpal axes and the longitudinal axis of the first metacarpal and proximal thumb phalanx as well as grip and pinch strengths. RESULTS: The mean angle of the first and second metacarpal axes showed a significant improvement to 64° (range, 47° to 89°), and the mean angle of the first metacarpal and proximal phalanx of the thumb showed a significant reduction to 8° (range, 1° to 21°) after surgery. The mean postoperative grip and pinch strengths were 77% (range, 63% to 106%) and 72.0% (range, 33% to 97%), respectively, relative to the unaffected side. CONCLUSIONS: Abductor pollicis brevis rerouting for type II and IIIA hypoplastic thumbs can produce joint stability and a strong pronation effect in addition to the opponens function. TYPE OF STUDY/LEVEL OF EVIDENCE: Therapeutic V.

The senile hand: Age effects on intrinsic hand muscle CMAP amplitudes influence split-hand index calculations.

INTRODUCTION/AIMS: Age can affect hand muscles non-uniformly. We investigated the influence of age on the compound muscle action potential (CMAP) amplitude of the hand muscles and the derived split-hand index (SHI). METHODS: We studied 244 subjects investigated for suspected myasthenia gravis but without neuromuscular disorders. Abductor pollicis brevis (APB), first dorsal interosseous (FDI), and abductor digiti minimi (ADM) CMAPs were obtained by supramaximal stimulation at the wrist, recording with surface electrodes while checking the best recording site. We applied Tukey's HSD and Kruskal-Wallis one-way analysis of variance for comparing age groups defined by median and interquantile ranges (IQRs). Spearman's rank correlation coefficient and linear regression were used for testing age-dependence of measurements. RESULTS: Median age was 61.5 y (first IQR, 44.5; third IQR, 72.0; range 18-89). Age and neurophysiological measurements were similar between genders. APBCMAP , FDICMAP , ADMCMAP , and SHI were correlated with age (P < .001). Median and cutoff values were significantly different between age groups. APBCMAP , FDICMAP , and ADMCMAP decreased by 0.8/0.7/0.3 mV/y, respectively, and SHI decreased 0.15/y. DISCUSSION: The CMAP amplitudes of hand muscles and derived SHI were strongly age-dependent, although this effect was less in ADM. This represents a physiological phenomenon. Future studies using the SHI should consider age effects.

Fiber type composition of contiguous palmaris longus and abductor pollicis brevis muscles: Morphological evidence of a functional synergy.

The palmaris longus (PL) tendon is used in surgical opponensplasty to restore functional hand movements in thenar paralysis. Although successful PL autologous tendon transfer has been attributed to an established synergistic relationship between the PL and abductor pollicis brevis (APB) muscles in vivo, this functional relationship may be dependent on the quality of their spatial relationship and properties of their constituent muscle fibers. The purpose was to compare the proportion of type I and type II muscle fibers in the APB based on its contiguous morphological relationship with the PL tendon for indirect insight into their functional synergy, contractile capacity, and digastric arrangement. Twenty-four contiguous PL and APB specimens were harvested from the upper limbs (12 right and 12 left) of twelve formalin-embalmed cadavers (mean age: 74 ± 10 years). The fiber type composition of these muscles was determined by labeling serial cross sections with myosin heavy chain (MyHC) type I and type II monoclonal antibodies. The PL consisted of a relatively heterogeneous fiber type composition irrespective of the presence of a discrete (type I: 41 ± 11%; type II: 55 ± 12%; hybrid: 4 ± 3%) or rudimentary (type I: 49 ± 10%; type II: 45 ± 9%; hybrid: 6 ± 4%) tendinous connection with the APB. The APB fascicles arranged contiguously with the PL through a discrete tendon had significantly greater proportions of type II fibers (41 ± 19%) compared to those with rudimentary PL connections (type II: 15 ± 8%). Therefore, the APB fascicles arranged in a digastric relationship with the PL may have the capacity to produce more powerful contractions than those with rudimentary PL tendons based on the known contractile properties of type II muscle fibers. Knowledge of the spatial relationship between the PL and thenar musculature prior to PL autologous tendon transfer may be a useful indicator of the quality of established synergy in vivo.

A rare anatomic variation of the superficial palmar branch of the radial artery causing pain.

BACKGROUND: The superficial palmar branch of the radial artery (SPBRA) normally pierces through the thenar muscles and unites with the ulnar artery to form the superficial palmar arch. Rarely, a subcutaneous course of the SPBRA is described in which the artery lies superficial to the thenar muscles. CASE REPORT: We report about a 17-year-old female patient with pain at the thenar eminence due to a unique course of the SPBRA. Duplex sonography and magnetic resonance angiography revealed a subcutaneous course of the artery over the thenar muscles. Arterial transposition by splitting of the abductor pollicis brevis muscle was performed. At 12-month follow-up, the patient is still free of symptoms. Duplex sonography confirmed patency of the SPBRA. CONCLUSION: While a subcutaneous course of the SPBRA has been described before, we present an adolescent patient with this anatomical variation causing pain. Our specifically tailored treatment strategy consisting of arterial transposition by splitting of the abductor pollicis brevis muscle was efficient and feasible in our patient and hand surgeons should be aware of this anatomical variation.

Severe and extreme idiopathic median nerve lesions at the wrist: new insights into electrodiagnostic patterns and review of the literature.

INTRODUCTION: We evaluated the electrodiagnostic (EDX) pattern of severe and extreme median nerve lesions at the wrist (MNLW) in patients with idiopathic carpal tunnel syndrome. METHODS: EDX data were recorded in 229 patients with 314 severe and extreme MNLW and 447 patients with 777 non-severe MNLW. We recorded distal motor latency (DML) to abductor pollicis brevis (APB) and second lumbricalis (2L), sensory conduction (SC) from digits 2 and 3, and needle examination of APB. RESULTS: Preservation rate of DML to APB and 2L and of SC from digits 2 and 3 were 100%, 100%, 85%, and 76%, respectively, for severe MNLW, and 37%, 90%, 36%, and 26% for extreme MNLW. Active denervation, as demonstrated by fibrillation potentials, was found in 1% of non-severe NMLW, 7% of severe MNLW, and 56% of extreme MNLW cases. CONCLUSIONS: In idiopathic severe and extreme MNLW, the DML to the 2L and averaged SC from digit 2 can be present when other responses are absent.

Camitz tendon transfer using flexor retinaculum as a pulley in advanced carpal tunnel syndrome.

PURPOSE: To report the outcomes of modified Camitz abductor plasty using the released flexor retinaculum as a pulley in patients with advanced carpal tunnel syndrome. METHODS: A retrospective review of 46 hands in 43 patients who underwent modified Camitz abductor plasty was performed. Active palmar abduction of thumb and pulp pinch strength were assessed. Patient-reported outcome measures were assessed using the Disabilities of the Arm, Shoulder, and Hand and Carpal Tunnel Syndrome instrument. As an electrophysiological assessment, compound muscle action potential (CMAP) from abductor pollicis brevis (APB) was investigated. RESULTS: At 3 months, active palmar abduction of thumb and pulp pinch strength significantly improved. Although pulp pinch strength further improved, active abduction of thumb did not improve at the final follow-up. Both the patient-reported outcome measures improved at 3 months and further improved at final follow-up. Approximately 75% of improved scores were obtained at the first 3 months after surgery and the balance of improved scores (25%) was obtained by the time of final follow-up. Useful recovery of postoperative APB-CMAP (amplitude > 1.8 mV) was obtained in 3 hands (7%) at 3 months after surgery and in 23 hands (50%) at final follow-up. There was no statistical significance of the postoperative results including active palmar abduction of thumb and improvement of patient-reported outcome measures at final follow-up between the hands with useful recovery of postoperative APB-CMAP and the hands without it. CONCLUSIONS: Modified Camitz abductor plasty benefitted the early improvement of activity of daily living in patients with advanced carpal tunnel syndrome. It acted not only as an internal orthosis in patients who eventually recovered thenar muscle function but also as the sole palmar abductor of the thumb in patients who failed to recover useful thenar muscle function. TYPE OF STUDY/LEVEL OF EVIDENCE: Therapeutic IV.

Schwannoma in the Thenar Eminence: a Case Report.

This report describes a rare presentation of schwannoma in the thenar aspect of a surgeon and reviews the literature. A 35-year-old surgeon had a slow-growing swelling in his left thenar eminence. Clinical and radiological findings suggested it was a well-encapsulated mass within the abductor pollicis brevis muscle. A complete surgical excision was done, and the histopathological findings confirmed schwannoma, a benign peripheral nerve tumor. The surgeon remained symptom-free and had no recurrence at 1 year of follow-up. Though a benign peripheral nerve tumor is rare in the hand, it remains one of the differential diagnoses for a thenar eminence swelling. Surgical enucleation preserving the nerve fascicles achieves an excellent functional outcome.

[A case of T1 radiculopathy caused by intervertebral disc herniation with cervical angina].

A 65-year-old man presented to the emergency department with the complaints of left anterior chest and back pain, numbness in the left medial arm, and weakness in the left hand grip. Myocardial infarction was suspected, but later ruled out, and the patient was subsequently referred to our department. Among the intrinsic muscles, the left abductor pollicis brevis was the most severely weakened, and there was a sensory disturbance in the left T1 region and left Horner's sign. An MRI T2-weighted image of the cervical spine showed a herniated disc on the left lateral side at the T1/2 level, suggesting compression of the T1 nerve root. Cervical angina is a rare, angina-like anterior chest pain due to cervical spine diseases. It is often reported in patients with C6 and C7 radiculopathy. The most severe weakness in the abductor pollicis brevis muscle in T1 radiculopathy is important to distinguish it from C8 radiculopathy.

Relationship between excitability, plasticity and thickness of the motor cortex in older adults.

The relationship between brain structure, cortical physiology, and learning ability in older adults is of particular interest in understanding mechanisms of age-related cognitive decline. Only a few studies addressed this issue so far, yielding mixed results. Here, we used comprehensive multiple regression analyses to investigate associations between brain structure on the one hand, i.e., cortical thickness (CT), fractional anisotropy (FA) of the pyramidal tract and individual coil-to-cortex distance, and cortical physiology on the other hand, i.e. motor cortex excitability and long-term potentiation (LTP)-like cortical plasticity, in healthy older adults (mean age 64 years, 14 women). Additional exploratory analyses assessed correlations between cortical physiology and learning ability in the verbal domain. In the regression models, we found that cortical excitability could be best predicted by CT of the hand knob of the primary motor cortex (CT-M1HAND) and individual coil-to-cortex distance, while LTP-like cortical plasticity was predicted by CT-M1HAND and FA of the pyramidal tract. Exploratory analyses revealed a significant inverse correlation between cortical excitability and learning ability. In conclusion, higher cortical excitability was associated with lower CT and lower learning ability in a cohort of healthy older adults, in line with previous reports of increased cortical excitability in patients with cortical atrophy and cognitive deficits due to Alzheimer's Disease. Cortical excitability may thus be a parameter to identify individuals at risk for cognitive decline and gray matter atrophy, a hypothesis to be explored in future longitudinal studies.

Corticomotor facilitation in vegetative state: results of a pilot study.

OBJECTIVE: To investigate whether corticomotor facilitation induced by transcranial magnetic stimulation (TMS-CF) could evoke a simple purposeful motor behavior in patients with a diagnosis of vegetative state. DESIGN: Cross-sectional survey. SETTING: Post-coma and rehabilitation care unit. PARTICIPANTS: Patients (N=6) with a diagnosis of vegetative state. INTERVENTIONS: A cascade of consecutive motor-evoked potentials (MEPs) was elicited under 3 different conditions: in the first condition, patients were at rest (Rest); in the second, they were asked to open and close the right hand (Execution); in the third, the examiner modeled a movement of abduction of the thumb in front of the patient who was encouraged in advance to imitate the action (Observation to Imitate). MAIN OUTCOME MEASURES: Changes in MEP values from the abductor pollicis brevis muscle and improvement in scores on the Coma Recovery Scale-Revised. RESULTS: TMS-CF alone or combined with verbal instructions did not yield any change; only the combination with imitation caused changes in MEPs (shorter latency and increased amplitude) associated with behavioral improvement in 4 patients. CONCLUSIONS: Encouraging observation to imitate may favor the transformation of some perceived actions into motor images and performances, probably depending on the activation of mirror motor neurons. In our opinion, combining visual input with TMS-CF might have reinforced the coupling between movement planning and execution, promoting the recovery of elementary motor activities in some patients. The proposed protocol may contribute to unmasking signs of preserved consciousness in patients with latent capacities for recovery.

Reconstruction of chronic thumb metacarpophalangeal joint radial collateral ligament injuries with a half-slip of the abductor pollicis brevis tendon.

PURPOSE: To evaluate a reconstructive method for chronic radial collateral ligament (RCL) injuries of the thumb metacarpophalangeal (MCP) joint using a combination of RCL advancement and the transfer of a half-slip of the abductor pollicis brevis tendon. METHODS: Eight patients (4 male and 4 female; mean age, 25 y) with chronic RCL injury of the thumb MCP joint were enrolled. All patients were referred to our institution because of continuing pain and instability on the radial side of the MCP joint when grasping or pinching objects. The mechanism of the injury was adduction stress to the thumb during sporting activities in 5 patients, a heavy object falling on the thumb in 1, and a fall in 2. The mean duration from RCL injury to surgery was 20 weeks. The average postoperative follow-up was 51 months. We evaluated postoperative outcomes including pain, range of motion of the thumb MCP joint, grip strength, key pinch strength, Disabilities of the Arm, Shoulder, and Hand score, and ability to return to preinjury work or sporting activities. RESULTS: No patients demonstrated continuing symptoms, and the MCP joint was stable after surgery. Postoperative grip and pinch strength (37 and 6.3 kg, respectively) were increased compared with preoperative values (34 and 3.9 kg, respectively). All patients returned fully to their preinjury work or sporting activities within 6 months after surgery. Although postoperative flexion was decreased by an average of 6°, no patients noted functional deficiency. CONCLUSIONS: We recommend the reconstructive method of RCL advancement and transfer of a half-slip of the abductor pollicis brevis tendon to alleviate pain and improve grip and pinch strength in chronic RCL injuries of the thumb MCP joint.

Sonographic evaluation of the abductor pollicis brevis muscle reflects muscle strength recovery after carpal tunnel release.

PURPOSE: This study aimed to examine the associations between sonographic measurements of the abductor pollicis brevis (APB), grip and pinch strength, and distal motor latency (DML) in patients with carpal tunnel syndrome (CTS) before and after surgery. METHODS: We prospectively studied patients (46 hands) who underwent 1 year of postoperative follow-up after endoscopic carpal tunnel release. The patients underwent ultrasound (US) scans, grip and pinch strength assessment, a nerve conduction study, and patient-reported outcome measures (Carpal Tunnel Syndrome Instrument and Michigan Hand Outcomes Questionnaire) before and 1 year after surgery. The standardized response mean was calculated to compare the sensitivity of clinical changes in these measurements. RESULTS: US measurements (thickness of the APB and the cross-sectional area of the APB) and muscle strength (grip strength, key pinch, and tip pinch) were greater, and DML was reduced after surgery compared with those before surgery (all P < 0.05). Patient-reported outcome measures also showed clinical improvement 1 year after surgery (P < 0.05). US measurements of the APB were significantly correlated with grip and pinch strength (all P < 0.05), but not with DML, before surgery and 1 year after surgery. The standardized response mean showed a large responsiveness for US measurements of the APB and patient-reported outcome measures. CONCLUSION: US evaluation of the APB after CTS can complement the evaluation of grip and pinch strength in the clinical setting. Postoperative recovery of the APB leads to improved motor dysfunction in CTS. Therefore, US measurement of the APB could be a useful tool for evaluating motor function.

Abductor pollicis brevis rerouting and first web deepening for clasped thumb deformity in arthrogryposis multiplex congenita.

LEVEL OF EVIDENCE: III.

MScanFit motor unit number estimation of abductor pollicis brevis: Findings from different experimental parameters.

MScanFit motor unit number estimation (MUNE) based on the recording of the compound muscle action potential (CMAP) scan has wide applications. This study evaluated the effect of different CMAP scan settings on MScanFit MUNE. CMAP scan of the abductor pollicis brevis (APB) muscle was performed in 10 healthy subjects at a United States (US) research center using different stimulus pulse widths (0.1, 0.2 ms) and total number of stimuli or steps (500, 1,000), and in 12 healthy subjects at a China research center using a 0.1 ms pulse width and 500 steps. MScanFit MUNE was derived using the default model parameters. A significantly higher MUNE was obtained using the shorter than longer pulse width; 84.70 ± 21.56 (500 steps) and 77.90 ± 27.62 (1,000 steps) at a pulse width of 0.1 ms vs. 67.60 ± 18.72 (500 steps) and 62.20 ± 15.82 (1,000 steps) at a pulse width of 0.2 ms (p < 0.05). However, MUNE was unrelated to the number of steps (500 vs. 1,000, p > 0.1). MUNE was significantly higher in persons studied in the China center (136.42 ± 32.46) than the US center (84.70 ± 21.56) despite each center using the same pulse widths and steps (p < 0.001). After excluding the ethnicity, age and experimenter factors, this significant difference is speculated to be partly related to different electrode size used in the two centers. The findings suggest that CMAP scan experimental parameters should remain consistent, so the MScanFit MUNE will not be compromised by non-physiological factors.

Neurophysiological differentiation of upper motor neuron damage in neurodegenerative disorders.

Objective: Using transcranial magnetic stimulation (TMS) to delineate upper motor neuron (UMN) signs of two neurodegenerative disorders: amyotrophic lateral sclerosis (ALS) and multiple system atrophy (MSA). Methods: Medical records including clinical signs for UMN damage and TMS results were reviewed retrospectively. The UMN signs were classified into none, mild, and severe based on neurological examination of various reflexes. Then TMS-elicited motor evoked potentials (MEPs) were recorded from a hand and a leg muscle to calculate the central motor conduction time (CMCT), which represents fast, mono-synaptic conduction along the corticospinal tract. Relations between the UMN signs and CMCT were analysed for the two diseases. Results: Prevalence and severity of the UMN signs for ALS and MSA were comparable for both upper and lower limbs. However, abnormality in CMCT was found more frequently in ALS: CMCT abnormalities were found in upper limbs for 44% in ALS patients but only for 7% in MSA patients; CMCT abnormalities in lower limbs were 55% in ALS and 20% in MSA. Some ALS patients showed abnormal CMCT in limbs without UMN signs, which was not true for most MSA patients. Conclusions: The abnormalities of CMCT were different in ALS and MSA, even for those who clinically had similar UMN signs. Sometimes, CMCT can reveal UMN damage in the absence of clinical UMN signs. Differences presumably derive from selective degeneration of different fibres in the motor descending pathways. Longitudinal studies must be conducted to accumulate neuroimaging and pathological findings. Significance: CMCT can be useful to differentiate ALS and MSA.

Fast acquisition of resting motor threshold with a stimulus-response curve - Possibility or hazard for transcranial magnetic stimulation applications?

OBJECTIVE: Previous research has suggested that transcranial magnetic stimulation (TMS) related cortical excitability measures could be estimated quickly using stimulus-response curves with short interstimulus intervals (ISIs). Here we evaluated the resting motor threshold (rMT) estimated with these curves. METHODS: Stimulus-response curves were measured with three ISIs: 1.2-2 s, 2-3 s, and 3-4 s. Each curve was formed with 108 stimuli using stimulation intensities ranging from 0.75 to 1.25 times the rMTguess, which was estimated based on motor evoked potential (MEP) amplitudes of three scout responses. RESULTS: The ISI did not affect the rMT estimated from the curves (F = 0.235, p = 0.683) or single-trial MEP amplitudes at the group level (F = 0.90, p = 0.405), but a significant subject by ISI interaction (F = 3.64; p < 0.001) was detected in MEP amplitudes. No trend was observed which ISI was most excitable, as it varied between subjects. CONCLUSIONS: At the group level, the stimulus-response curves are unaffected by the short ISI. At the individual level, these curves are highly affected by the ISI. SIGNIFICANCE: Estimating rMT using stimulus-response curves with short ISIs impacts the rMT estimate and should be avoided in clinical and research TMS applications.

Reduction in median nerve cross-sectional area at the forearm correlates with axon loss in carpal tunnel syndrome.

OBJECTIVE: To explore the relationship between axon loss and measured cross-sectional areas of the median nerve (MN) in severe carpal tunnel syndrome (CTS). METHODS: In this retrospective study of 158 examined wrists, we compared axon loss to the ultrasound parameters MN cross-sectional area at the wrist (wCSA), MN cross-sectional area at the forearm (fCSA) and wrist-to-forearm ratio (WFR), in patients with moderate to extreme CTS. Axon loss was evaluated by needle electromyography (EMG) of the abductor pollicis brevis muscle (spontaneous activity and reduction of interference pattern). RESULTS: Both the spontaneous activity and interference pattern reduction correlated negatively to fCSA (r = -0.189, p = 0.035; r = -0.210, p = 0.019; respectively). In moderate CTS, both the spontaneous activity and interference pattern reduction correlated positively to WFR (r = 0.231, p = 0.048; r = 0.232, p = 0.047; respectively). The WFR was highest when slight spontaneous activity was detected. Neither wCSA nor WFR correlated with axon loss in severe and extreme CTS. CONCLUSIONS: The fCSA is smaller when axon loss in CTS is more prominent. The WFR is highest when CTS is associated with slight axon loss of the MN. SIGNIFICANCE: CTS might cause retrograde axonal atrophy detected as small fCSA. Prominent axon loss in CTS may reduce the diagnostic value of WFR.

Iatrogenic neurological injury after radiofrequency ablation and epidural steroid injections: illustrative cases.

BACKGROUND: Neck pain is often chronic and disabling. Cervical facet joint injections and epidural steroid injections are frequently used to manage chronic neck pain and cervicogenic headaches. While minimal side effects are commonly associated with these treatments, severe complications are exceedingly rare. OBSERVATIONS: The authors report 4 cases of iatrogenic neurological injury after radiofrequency ablation (RFA) and epidural steroid injections. One patient experienced left shoulder, scapular, and arm pain with left arm and hand weakness that developed immediately after RFA for chronic neck pain. Electromyography/nerve conduction velocity (EMG/NCV) studies confirmed denervation changes in the left C8-T1 distribution. Three patients complained of numbness and weakness of the hands immediately after an interlaminar cervical epidural block. One of these patients underwent EMG/NCV that confirmed denervation changes occurring in the left C8-T1 distribution. LESSONS: Spine surgeons and pain management specialists should be aware of neurological injuries that may occur after cervical RFA and epidural steroid injections, especially after a multilevel cervical procedure and with severe cervical spinal stenosis. EMG/NCV studies plays an important role in detecting and localizing neurological injury and in differentiating from conditions that mimic cervical root injuries, including brachial plexus trauma due to positioning and Parsonage-Turner syndrome.

Sonographic morphometry of abductor pollicis brevis: can direct contact yield images comparable with those obtained by the water bath technique?

PURPOSE: This study sought to compare ultrasound-guided measurements of the abductor pollicis brevis (APB) using the water bath technique (WBT) and the direct contact method (DM) and investigate whether the DM can reproduce the measurements that would be obtained with a non-contact method, such as the WBT. METHODS: The APB muscles of 80 hands (40 healthy adults) were measured. The WBT was performed in a plastic container filled with water. The probe was placed adjacent to the skin surface without contact. In the DM, sonographic images were obtained with the probe and skin separated by sufficient transmission gel. The muscle thickness and cross-sectional area (CSA) were calculated with both methods. All subjects were examined three times by two examiners to estimate the inter- and intra-observer reliability. Bland-Altman analysis was performed to examine the agreement between the methods. RESULTS: No significant differences in the thickness or CSA of the APB were found. The interclass correlation coefficients for the WBT and DM showed almost perfect intra- and inter-observer reliability (range 0.87-0.94). There was no systematic bias between the techniques in the Bland-Altman analysis. CONCLUSION: Similar to the WBT, the DM provides measurements of the APB thickness and CSA without causing morphometric changes.

Site-dependent thenar compound muscle action potential: Comparison between surface and needle recordings.

BACKGROUND: Within the thenar eminence, the median nerve innervates three muscles: abductor pollicis brevis (APB), flexor pollicis brevis (FPB), and opponens pollicis (OP). Of these muscles, APB was often considered as the sole contributor to the thenar compound muscle action potential (CMAP). OBJECTIVE: To evaluate subcomponents of the thenar CMAP from the median nerve innervated muscles. METHODS: Surface and needle CMAPs were recorded in normal human subjects from three recording sites: proximal (site-I), middle (site-II), and distal (site-III) aspects of the thenar eminence when the median nerve was activated at the wrist. RESULTS: In the site-I and -II, both the surface and needle CMAPs shared many similar characteristics although the needle CMAPs were larger (∼ 5 folds) and briefer (∼ 60%, needle/surface duration). In addition, on the surface recording, the CMAP was larger (by ∼ 1.9 mV) when recorded from the site-I comparing to that of the site-II. In the site-III, the surface recordings registered a delayed (by ∼ 3.8 ms) CMAP. The muscle fiber action potential (MFAP) study suggested a predominant FPB contribution in the site-III. CONCLUSION: The optimal recording site for APB derived thenar CMAP is the site-I and for FPB is the site-III. The CMAPs registered by the needle recordings are more robust than the surface ones.