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Original Research |
1 Institute of Diagnostic Radiology, Department of Medical Radiology, University
Hospital Zurich, Raemistrasse 100, Zurich CH-8091, Switzerland.
2 Division for Plastic, Hand, and Reconstructive Surgery, Department of Surgery,
University Hospital Zurich (Academic Medical Center), Zurich CH-8091,
Switzerland.
Received October 22, 2004;
accepted after revision November 15, 2004.
Address correspondence to D. Weishaupt.
Abstract
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MATERIALS AND METHODS. MRI of 21 hands was performed in 19 patients with clinically evident or suspected intrinsic hand muscle abnormalities. All MRI was performed on a 1.5-T scanner using transaxial T1-weighted, T2-weighted, or STIR as well as contrast-enhanced T1-weighted sequences. Two observers reviewed all MR images retrospectively in a blinded fashion with regard to the exact anatomic location of the muscle abnormality, signal abnormalities, muscle atrophy, and the cause. Kappa statistics were used to calculate interobserver variability. MRI findings were compared with clinical findings using Spearman's rank test. A panel of experts assessed the impact of MRI on the diagnostic workup.
RESULTS. On the basis of MRI findings, abnormalities (either MR signal abnormality or atrophy) of both the lumbrical and interosseus muscles were noted in 10 (48%) of 21 hands, of the thenar muscles in eight (38%) of 21 hands, and of the hypothenar muscles in 12 (57%) of 21 hands. The correlation between clinical and MRI findings was moderate to strong for the interosseus, thenar, and hypothenar muscles (0.43-0.84). MRI was judged to be useful for establishing the final diagnosis in 17 (81%) of 21 hands.
CONCLUSION. MRI of the hands is useful and correlates well with clinical findings in patients with intrinsic hand muscle abnormalities.
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Based on the experience from imaging other body regions, there is general agreement that MRI is also valuable in assessing skeletal muscles and their abnormalities, including the sequelae of traumatic muscle injuries and muscle overuse syndromes [8]. Moreover, MRI is considered to be useful for the diagnosis and evaluation of inflammatory and infectious muscle disorders, muscle atrophy, muscle infarction, and neoplasms involving the skeletal muscles [3, 9, 10].
To our knowledge, studies pertaining to MRI characterization of intrinsic hand muscle pathology and the potential usefulness of MRI for the assessment of clinically suspected abnormalities of these muscles are limited in the literature. Beyond reports on anatomic variants of the hand muscles [11, 12] only a few reports exist on MRI of patients with abnormalities of the hand muscles caused by peripheral nerve injuries [6, 13, 14] or by a primary affliction of the muscles themselves [15, 16].
From a clinical point of view, MRI does not yet play an important role in the normal diagnostic workup of abnormalities of the intrinsic hand muscles. So far, diagnosis of such muscle disorders is based on clinical evaluation, including patient history, physical examination, and electrodiagnostic testing. However, despite thorough clinical evaluation, in some cases the final diagnosis may remain unclear. In particular, the distinction between a primary myogenic abnormality (i.e., an abnormality caused by a primary affliction of the muscle) and a secondary abnormality caused by nerve injury or by nerve compression syndrome may be difficult based on clinical evaluation. Particularly in these patients, MRI may be potentially helpful.
The purpose of this study was to describe the spectrum of intrinsic muscle pathology in patients with clinically evident or suspected abnormalities of the intrinsic hand muscles and to correlate MRI findings with clinical findings. In addition, the usefulness of MRI in the management of the intrinsic hand muscles abnormalities was assessed.
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Between 2001 and 2004, 19 patients (13 female, six male; mean age, 40.8 years; age range, 14-68 years) with a clinically evident or suspected abnormality of the intrinsic hand muscles were referred to our institution for MRI of the hand. In all these patients, a more proximal cause for their hand muscle problems had been excluded. All patients were referred from the department of surgery (division for plastic, hand, and reconstructive surgery), where all patients underwent a detailed clinical and neurologic examination by a hand surgeon specialized in peripheral neuropathies before undergoing MRI. In addition to clinical evaluation, conventional radiographs and electrodiagnostic studies, including motor and sensory conduction velocities and electromyography, were performed (see following text). All these patients with clinically evident or suspected abnormalities of the intrinsic hand muscles were referred for MRI because the diagnoses based on the clinical findings were unclear or ambiguous (i.e., discrepancies between the history, symptoms, and results from physical examination and the electrodiagnostic studies). Beyond the hand muscle problem, none of the patients showed signs or symptoms of a generalized polyneuropathy or of a more proximal neuropathy, such as cervical spondylosis, lower cervical radiculopathy, or lower trunk brachial plexopathy. In these 19 patients, a total of 21 MRI examinations of the hands were performed (13 right hands, eight left hands). In 17 patients only one hand was examined, and in two patients MRI of both hands was performed.
The intrinsic muscles of the hand evaluated in our study included the lumbrical muscles I-IV; the dorsal and palmar interosseus muscles; the abductor, flexor, and opponens muscles of the small finger; and the flexor pollicis brevis, abductor, adductor, and opponens muscles of the thumb [17]. Because the MR appearance of the intrinsic hand muscles is not well described in the radiology literature, we performed MRI of the hand in 21 hands of 21 volunteers (women, 11; men, 10; mean age, 30.5 years; age range, 25-60 years; 18 right hands and three left hands) to gain a more detailed understanding of the normal MRI appearance of the intrinsic hand musculature. All healthy volunteers were recruited from the hospital staff. None of these volunteers had a history of hand muscle abnormality or a history of significant trauma to the hand or wrist requiring medical consultation or treatment. None of these volunteers suffered from an underlying condition that might have affected the hand musculature. In addition, the hands of all these volunteers were examined clinically by an experienced hand surgeon and were subsequently judged to be normal. The local institutional review board approved the imaging of normal volunteers for this purpose, for use as an internal control, and for comparison with the findings of patients with clinically evident or suspected abnormalities of the intrinsic muscles of the hand. Informed consent was obtained from all volunteers before MRI.
MRI Techniques
All MRI was performed on a 1.5-T scanner (Signa Horizon, GE Healthcare)
using a high-resolution 4-channel wrist coil. All patients and asymptomatic
volunteers were placed in a prone position with the elbow extended overhead
and with the pronated hand positioned in the center of the wrist coil at the
scanner's isocenter ("Superman position").
The imaging protocol for both the patients and the volunteers included the following sequences in the transaxial plane: T1-weighted spin-echo (TR range/TE range, 300-640/9-35), T2-weighted fat-suppressed fast spin-echo (3,500-5,500/95-140; echo-train length, 8), or STIR (TR range/TE, 3500-5,500/33; inversion time, 150 msec). In addition, a transaxial T1-weighted fat-suppressed spin-echo sequence after the IV administration of 0.1 mmol/kg of body weight of gadopentetate dimeglumine (Magnevist, Schering) was acquired in all patients. No contrast agent was used in imaging the volunteers. MRI parameters for the transaxial sequences were as follows: field of view, 8-12 cm; section thickness, 3 mm with an intersection gap of 0.5 mm; image matrix, 256 x 224; 3 acquisitions. In addition to these sequences, our protocol included the following sequences: T1-weighted spin echo (300-500/9-35), intermediate-weighted fast spin-echo (4,000/45; echo-train length, 4), and STIR (3,500-5,500/33; inversion time, 150 msec) in the coronal plane, and a T1-weighted spin-echo sequence (320-600/9-20) in the sagittal plane.
Clinical Examination
All 21 hands of the 19 patients were examined by one of two hand surgeons
specialized in peripheral neuropathies. In addition, all patients underwent a
detailed neurologic examination that included electrodiagnostic examinations
of the median and ulnar nerves by an experienced neurologist. In affected
hands, the compound muscle and the sensory action potential of both the median
and ulnar nerves were recorded. Conduction velocity, sensory amplitude, and
distal latency in both the motor and sensory branches were measured. Standard
techniques of supramaximal percutaneous stimulation and surface electrode
recording and electromyography with concentric needle electrodes of individual
muscles were used. Muscle and nerve function were assessed on the basis of
CMAP and CSAP amplitudes of nerve conduction and electromyographic findings
[18,
19]. All electrodiagnostic
studies were performed according to the consensus criteria of the American
Association of Electrodiagnostic Medicine
[20].
MRI Analysis
Two radiologists independently analyzed all MR images of all patients and
volunteers in a retrospective, blinded, and randomized fashion. Criteria for
MR image analysis were available in a written form for both observers.
Consensus was reached in case of disagreement. Image analysis was performed on
a separate workstation (Advantage Windowing Workstation 4.1, GE Healthcare).
All MR images were evaluated with regard to abnormalities of the individual
intrinsic hand muscles, which included the following muscles: lumbrical
muscles I-II and III-IV; the dorsal and palmar interosseus muscles; the
abductor, flexor, and opponens muscle of the small finger; and the flexor
pollicis brevis, abductor, adductor, and opponens muscle of the thumb.
The observers were asked to classify each individual intrinsic hand muscle using a 3-point classification system: grade 0, normal muscle, no atrophy; grade 1, moderate atrophy; and grade 2, severe muscle atrophy. Grade 1 atrophy was defined as significant muscle volume loss not exceeding more than 50% of normal muscle volume, with concomitant fatty streaks. Grade 2 corresponded to severe fatty degeneration with greater than 50% reduction of the expected muscle volume. The muscle volume was assessed visually at three levels on transaxial planes (at the level of pisiform bone, at the level of the hook of hamate, and at the level of mid diaphysis of metacarpal bone).
The grading system for the assessment of fatty muscle degeneration was developed on the basis of a similar classification system for semiquantitative estimation of rotator cuff muscle atrophy of the shoulder as described by Goutallier et al. [21] and Fuchs et al. [22]. Furthermore, signal abnormalities or abnormal contrast enhancement in the individual intrinsic hand muscles was noted. MR signal for hand muscles was considered normal if it was generally much lower than that of fat and slightly higher than that of water on T1-weighted images, and much lower than that of both fat and water on T2-weighted images. On STIR images, normal signal intensity should be higher than that of fat but much lower than that of water. Muscle edema was presumed in the event of high MR signal on T2-weighted or STIR images [23].
In addition, if a muscle abnormality was present, each observer determined whether it was caused by either a myogenic (i.e., primary affliction of the muscle) or a neurogenic cause (i.e., primary affliction or lesion of the nerve). A neurogenic cause was considered if the pattern of muscle signal changes or the pattern of muscle atrophy matched the typical innervation pattern of the intrinsic muscles. A myogenic cause (e.g., myositis, necrosis, hematoma, hemangioma) was suspected in case of typical involvement of surrounding soft tissues, bones, or vessels [23]. Finally, both observers noted the presence or absence of abnormalities of the median and ulnar nerves, which were evaluated along their entire course with special attention to the segments in the carpal tunnel and Guyon's canal, respectively. Nerve abnormalities included hyperintense signal on T2-weighted images, atypical courses, thickening, external compression, and tumors of the nerve or nerve sheath [6].
Correlation of MRI and Clinical Findings
On a case-to-case basis, a panel consisting of a hand surgeon and a
neurologist reviewed the clinical findings that were gathered until the time
MRI was performed. For this purpose, the study coordinator made all clinical
data available (including clinical history, physical examination, and
electrodiagnostic studies) as far as these examinations were performed
immediately before MRI. The panel was blinded to the MRI findings. On the
basis of the clinical data gathered before the MR examination, the panel
reached a clinical diagnosis for each individual intrinsic muscle abnormality
of the hand using the same 3-point classification system used in evaluating
the MR images: grade 0, normal muscle; grade 1, moderate atrophy; grade 2,
severe atrophy. In addition, the panel attempted to determine whether the
muscle abnormality was caused by a primary affliction of the muscle or was
related to a neuropathy.
In a second phase, a clinical panel of experts consisting of a hand surgeon, a neurologist, and a radiologist, reviewed all patients' charts again. For this review, all clinical data, including clinical history, physical examination, electrodiagnostic findings, follow-up studies, surgical reports (if written), and MRI findings, were available. The purpose of this final review was to reach a final, definitive diagnosis and to assess the impact of MRI on the diagnostic workup. For the latter, the panel of experts recorded whether the MR examination provided additional information that helped in establishing the final diagnosis and changed therapeutic considerations, or whether the MR examination influenced the decision to perform surgery or helped in planning the surgery. In addition, in those patients in whom surgery was performed, the surgical findings were correlated with the MRI findings.
Statistical Analysis
The interobserver agreement for all analyzed MRI characteristics was
calculated using the kappa coefficient (moderate agreement, 0.41-0.60;
substantial agreement, 0.61-0.80; almost perfect agreement, > 0.80)
[24]. MRI findings were
compared with the clinical findings (as assessed immediately before the MR
examination) using the Spearman's (p) rank correlation test. Positive
p values between 0 and 0.2 indicate no correlation. Values between
0.2 and 0.5 represent a weak correlation; between 0.5 and 0.8, a moderate
correlation; and between 0.8 and 1.0, a strong to perfect correlation
[25]. The correlation was
tested for statistical significance. For all statistical analyses, we used
SPSS software (Statistical Package for the Social Sciences).
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MRI Findings
All MRI findings of the healthy volunteers were considered to be normal. In
particular, no signal abnormality or atrophy of the intrinsic hand muscles nor
any other abnormality was seen on the MRI examinations.
The MRI findings of the 21 hands of the 19 patients are shown in Table 2. Abnormalities (either MR signal abnormality or atrophy) of at least one muscle group were present in 19 (90%) of 21 hands. Abnormalities of the lumbrical and interosseus muscles were noted in 13 (62%) of 21 hands, of the thenar muscles in nine (43%) of 21 hands, and of the hypothenar muscles in 12 (57%) of 21 hands. MR signal abnormalities on T2-weighted or STIR MR images were present in nine (43%) of 21 hands (Figs. 1A, and 1B). Muscle atrophy (either grade 1 or 2) was seen in a total of 16 (76%) of 21 hands In eight hands, atrophy of the thenar muscle was seen (grade 1, seven hands; grade 2, one hand). In a total of 12 hands, atrophy of the hypothenar muscles was detected (grade 1, two hands; grade 2, 10 hands). Ten hands showed atrophy of the interosseus and lumbrical muscles. The lumbrical muscles of the index and middle finger were affected in one hand, whereas the lumbrical muscles of the ring and small finger were affected in three hands. The palmar interosseus muscles were atrophied in eight hands (grade 1, four hands; grade 2, four hands). In eight hands the dorsal interosseus muscles were affected by atrophy grade 1 or 2 (grade 1, four hands; grade 2, four hands). Abnormalities of the median and ulnar nerves were detected in 13 (62%) of 21 hands. Both reviewers considered a neurogenic cause for the muscle abnormality in a total of 16 (84%) of these 19 hands in which a muscle abnormality was present. A myogenic cause was suspected in three (16%) of 19 hands.
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The kappa statistics between reviewer 1 and reviewer 2 for classifying the individual hand muscles ranged from moderate to perfect interobserver agreement: lumbrical muscles I-II, 1.0; lumbrical muscles III-IV, 0.57; dorsal interosseus muscles, 1.0; palmar interosseus muscles, 0.76; abductor digiti minimi muscle, 1.0; opponens digiti minimi muscle, 0.76; flexor digiti minimi muscle, 0.74; abductor pollicis brevis muscle, 1.0; adductor pollicis muscle, 0.86; and opponens pollicis muscle, 1.0. The mean interobserver agreement for MRI analysis between reviewers 1 and 2 was 0.87.
Correlation of MRI and Clinical Findings
The statistical correlation of the clinical findings obtained immediately
before or after MRI and the MRI findings is shown in
Table 3. Data analysis revealed
that this correlation was statistically significant with regard to all
individual intrinsic muscles of the hand except the lumbrical muscles I-II.
Thus, the correlation between the MRI findings and clinical findings was
moderate to strong except in the lumbrical muscles.
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The data considered by the clinical panel of experts in establishing the final diagnosis are shown on Table 4. In 16 (76%) of 21 hands, the muscle abnormality was a sequela of a peripheral neuropathy caused by either a mass lesion (n = 13) or atypically thickened fibrous tissue (n = 1) (Figs. 2A, 2B, 2C, and 2D). In two instances, the diagnosis of posttraumatic muscle necrosis was established on the basis of clinical history, signal abnormalities on T1- and T2-weighted spin-echo or STIR sequences combined with grade 2 atrophy of the flexor digiti minimi muscle (Figs. 3A, 3B, and 3C). In two hands, neither signal abnormalities nor atrophy of the muscles was present on MRI. However, in both of these examinations, MRI revealed relevant findings. In one of these two hands, a ganglion at the level of the Guyon's canal was discovered; in the other hand, MRI revealed findings consistent with a posttraumatic carpal tunnel syndrome that correlated well with electrodiagnostic findings. In one hand with increased T2 signal in all thenar muscles, interpretation of the clinical and MRI findings in conjunction led to the diagnosis of myositis. One patient suffered from a congenital hemangioma of the forearm that extended distally to the fingers and affected the interosseus, the opponens pollicis, and the flexor pollicis muscles (Figs. 4A, 4B, and 4C).
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The intrinsic muscles of the hand are of paramount importance in efficient hand function. So far, the role of MRI for the routine assessment of intrinsic hand muscle abnormalities has been limited. In clinical practice, the evaluation of abnormalities of the intrinsic hand muscles relies primarily on an accurate clinical history, a thorough physical examination, and electrodiagnostic testing, including electroneurography with nerve conduction studies, recording somatosensory evoked potentials, and electromyography [6, 18, 19]. In integrating all clinical and diagnostic information, the location and severity of the underlying disorder may be determined in most cases. However, all of this clinical and diagnostic information is sometimes not sufficient to identify the underlying cause of nerve or muscle dysfunction. In addition, determination of the degree of muscle atrophy may be difficult because not all muscle groups can be tested easily with electrodiagnostic studies; in particular, the lumbrical muscles are difficult to evaluate [19]. This fact may be the reason that correlation between MRI findings and clinical findings in our study was insufficient for the lumbrical muscles.
A few reports have considered MRI useful for the assessment of muscle abnormalities of the hand, especially regarding neurologic causes and anatomic variants [6, 11, 13-16]. Accessory abductor digiti minimi and extensor digitorum brevis hand muscles are known, as well as variants of the lumbrical, the palmaris longus, and the flexor digitorum superficialis muscles [11]. Although representing incidental normal variants, anomalous intrinsic and extrinsic hand muscles are pitfalls on MRI. It is helpful for radiologists to be familiar with these anatomic variants because they can cause compressive neuropathies, particularly when the muscles are hypertrophied [12, 26].
The anatomy of the intrinsic muscles of the hand is complex and description of the normal MR appearance is limited in the literature [17, 27, 28]. Because the volumes of the individual muscle are relatively small and visible fat planes do not separate the muscles, it may be difficult to localize the margins of the individual intrinsic hand muscles on MRI. Contrary to the findings of Jacobson et al. [27], who showed differences between the intrinsic hand muscles with regard to the anatomy and muscle volume in cadaveric specimens, Homma and Sakai [28] considered the division of the thenar muscle mass into different muscles to be artificial. However, in our experience using dedicated wrist coils, the signal-to-noise ratio is sufficiently high to identify the individual muscle groups.
A methodologic problem of our study lies in the assessment of the degree of muscle atrophy using MRI because no clear anatomic references are defined in the literature for classifying atrophy of intrinsic hand muscles. To overcome this limitation, we included a series of healthy volunteers in this study who underwent MRI of their hands in the same fashion as the patient group. However, detecting a small loss of muscle volume may remain difficult in some patients or volunteers because of normal muscle volume variants. MRI of the opposite hand may be useful to determine the presence of muscle atrophy in these instances.
Increased signal intensity in the individual muscles or muscle groups on fat-suppressed T2-weighted or STIR sequences and the presence of muscle atrophy are considered sequelae of denervation [29]. Other causes of signal abnormalities in the absence of muscle atrophy include exertional muscle injuries [30], muscle necrosis [9], intramuscular hemorrhage [31], tumor edema [32], polymyositis [33], effects of irradiation [34], and even short-term changes after exercise [35]. In nerve damage, high signal intensity on STIR and T2-weighted fat-suppressed sequences in the absence of atrophy are indicative of the acute and subacute stages of denervation, whereas muscle atrophy with or without fatty changes represents a more chronic stage of denervation [29, 36]. With the recognition of these abnormal signal patterns and correlative anatomy, MRI may be useful to localize and characterize the lesion as well to distinguish acute from chronic nerve damage.
As shown in this study, MRI is particularly useful in the assessment of intrinsic hand muscle abnormalities. On the basis of physical examination and electrodiagnostic studies, it may be difficult to distinguish between myogenic and neurogenic abnormalities, particularly if the clinical history is ambiguous. Our series had one patient with a clinically evident abnormality of the abductor and flexor digiti minimi muscles. On the basis of clinical evaluation, it was not possible to differentiate between a primary myogenic abnormality and sequelae of posttraumatic neurogenic damage. Because MRI showed signal abnormalities in the abductor digiti minimi muscle, the diagnosis of posttraumatic necrosis with secondary scarring and fibrosis was established. In another patient, MR signal abnormalities on STIR sequences without atrophy were present in all thenar muscles. Because the distribution of the signal abnormalities of the affected muscles did not correspond to any typical distribution pattern expected in peripheral nerve injury, the final diagnosis of myositis was established.
The presence and degree of muscular atrophy may aid in predicting the likelihood of functional recovery after surgery. Previous studies of carpal tunnel syndrome and rotator cuff disorders [7, 21, 22] have shown that the presence of muscle atrophy with or without increased muscle signal intensity may indicate irreversible muscle damage. As shown in this study, MRI provides additional information useful in establishing a definitive diagnosis in patients with intrinsic hand muscle abnormalities. Information regarding irreversible atrophy of the intrinsic muscles of the hand may aid in surgical planning and in predicting clinical outcome. In our experience, hand surgeons use this information in deciding whether a surgical intervention such as nerve decompression is worth undertaking.
We acknowledge several limitations of our study. First, this was a retrospective study with selected cases in which the clinical findings were unclear. This may have resulted in a selection bias. However, because the same three referring physicians referred all patients during the study period, the rationale for ordering an MRI examination was uniformly applied. Another limitation is that the patients had a broad spectrum of abnormalities affecting the intrinsic hand muscles. Furthermore, in most patients we evaluated chronic stages of muscle abnormality. However, in clinical practice, acute onset of a muscle abnormality (i.e., functional loss due to a traumatic nerve injury) is usually not an indication for MRI because the clinical findings are usually considered to be sufficient.
In conclusion, this study has shown that MRI is useful in patients with a clinically evident or suspected hand muscle abnormality because MRI often reveals the precise distribution and the cause of the hand muscle abnormalities. MRI correlates well with the clinical findings obtained by clinical examination and electrodiagnostic studies and therefore is a valuable adjunct to these techniques in selected cases.
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