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Original Report |
1 Department of Radiology, Duke University Health System, Erwin Rd., Box 3808,
DUMC, Durham, NC 27710.
2 Current address: Spartanburg Radiological Associates, PA at 101 East Wood St.,
Spartanburg, SC 29303.
Received March 23, 2004;
accepted after revision June 30, 2004.
Address correspondence to R. L. Cothran, Jr.
Abstract
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CONCLUSION. Focal teres minor atrophy or abnormal signal suggesting quadrilateral space syndrome is an uncommon, although not rare, finding on MRI of the shoulder in our referral population and is rarely an isolated abnormality.
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The purpose of this report was to determine the incidence of MRI findings suggesting quadrilateral space syndrome in those patients referred for MRI of the shoulder and to correlate the imaging findings with the clinical setting. To our knowledge, the incidence on MRI has not been described previously in the radiology literature.
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All patients were imaged on a 1.5-T imaging platform (Signa, GE Healthcare). The imaging protocols for the shoulder varied over the 67-month period. However, the diagnosis of atrophy within the teres minor was made on the basis of disproportionate loss of muscle volume related to the other rotator cuff muscles or fatty infiltration of the teres minor muscle out of proportion to the other rotator cuff muscles or both. This determination was obtained on spin-echo T1-weighted images (TR range/TE range, 600–700/10–20) without fat suppression, in the sagittal plane.
In addition to atrophy of the teres minor muscle, diffuse increased T2 signal within the teres minor muscle suggesting neurogenic edema was included as a finding suggestive of quadrilateral space syndrome. This determination was made primarily on a sagittal fast spin-echo T2-weighted sequence (3,000–4,000/60–80) with fat suppression. These oblique sagittal T2-weighted images with fat suppression were a standard part of the imaging protocol at our institution throughout the time period evaluated in this review. Patients with diffuse symmetric atrophy of all visualized muscles or diffuse abnormal muscle signal specifically suggesting multiple nerve distribution involvement were not included. However, because of the rarity of teres minor muscle involvement in rotator cuff tears and the fact that no teres minor tendon tears were visualized in the included subjects, atrophy of a torn supraspinatus or infraspinatus muscle or both was not utilized as an exclusion criterion.
The patients' computerized clinical records were reviewed, when available, to determine the reason for referral and the clinical course. The computerized records were evaluated to determine what, if any, clinical follow-up was performed and whether any specific treatment for quadrilateral space syndrome was instituted.
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Six of the 19 patients had labral tears, with five of these being described as posterior labral tears. Of these five tears, three had associated posterior paralabral cysts. However, only one of these cysts was believed to be impinging potentially on the quadrilateral space at the time of the MRI interpretation (Figs. 4A, 4B, and 4C). No patients were found to have a soft-tissue mass in the region of the quadrilateral space on MRI.
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As with a large proportion of our shoulder referrals, 13 of the 19 patients were believed clinically to have a rotator cuff tear and were referred for imaging on that basis. Two patients were referred to evaluate for mass. One patient was referred for pain (not otherwise described), one for "popping," and one because of clinical concern for a labral tear or cyst. One of the 19 patients did not have any clinical data suggesting a specific reason for the shoulder MRI.
On review of the available computerized clinical records, specific follow-up was available in only 11 patients. No records indicated surgical exploration of the quadrilateral space in any of the patients with follow-up information. No patients underwent further imaging evaluation of the quadrilateral space to confirm the presence of compression of the neurovascular structures. Six of the 11 patients with clinical follow-up available underwent surgery on the imaged shoulder for rotator cuff repair, labral débridement, or subacromial decompression. One patient was believed not to have quadrilateral space syndrome clinically because of preserved deltoid strength on examination. This patient had clinical improvement in symptoms with subacromial injections and physical therapy. One patient underwent placement of a splint as treatment for a presumed brachial plexus injury. The patient with a humeral neck fracture underwent treatment for the fracture. One patient developed a rotator cuff tear in the shoulder in question within 1 year of the initial imaging and subsequently underwent surgical repair of the rotator cuff. One patient was diagnosed clinically with a "Parsonage-Turner variant" and underwent conservative therapy. This patient's electromyography results showed normal activity in the teres minor muscle but did show evidence of a suprascapular nerve injury. No abnormality of the supraspinatus or infraspinatus muscle was observed on the MRI.
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Cahill and Palmer [2] advocated arteriography as a useful technique, with the diagnostic finding of occlusion of the posterior circumflex humeral artery occurring in the abducted, externally rotated position in the symptomatic extremity. Subsequent authors have described MRI abnormalities in patients with the clinical syndrome including focal or isolated atrophy of the teres minor muscle [3]. MR angiography has been shown to reveal occlusion of the posterior circumflex humeral artery during abduction and external rotation, but this occurred in both symptomatic individuals and a significant portion of the asymptomatic individuals evaluated [4].
Tears of the supraspinatus and infraspinatus tendons will result in atrophy of these muscles if left unrepaired. One could then postulate that a tear of the teres minor muscle might cause focal atrophy of this tendon and mimic a quadrilateral space syndrome. However, tears of the teres minor tendon are extremely rare, and none of the patients included in this study had visible disruption of the teres minor tendon on the MR images.
Fibrous bands in the region of the quadrilateral space are believed to be the cause of nerve and artery compression in most patients with quadrilateral space syndrome [2, 5], although large paralabral cysts have been described as causing compression on the quadrilateral space and resulting in teres minor atrophy and clinical symptoms [6–8]. Surgical decompression of these paralabral cysts has provided pain relief in those reported cases in which surgery was performed [6–8]. Surgical débridement of the fibrous bands has also been reported to be successful in alleviating symptoms in those patients who do not respond to conservative measures [1, 2, 5, 9, 10].
Linker et al. [3] described the MRI appearance of atrophy of the teres minor or deltoid muscle or both as indicative of quadrilateral space syndrome in three patients with appropriate clinical findings. The largest reported series of patients with quadrilateral space syndrome is the 18 cases of quadrilateral space syndrome originally described by Cahill and Palmer [2]. However, this study did not include MRI findings. Other studies indicating the MR angiographic findings and case reports have been published, but we have been unable to find any reference to the incidence of this syndrome.
Our study suggests that MRI findings of teres minor atrophy or abnormal signal may be present in as many as 0.8% of patients referred for shoulder MRI. However, our study also suggests that most of these patients have other shoulder abnormalities on MRI apart from atrophy or abnormal signal in the teres minor or deltoid muscles or both. In addition, the diagnosis of quadrilateral space syndrome was not suspected clinically before imaging in most of these cases. This may be interpreted either as supporting evidence of the difficulty of diagnosing quadrilateral space syndrome clinically or as evidence that this atrophy or abnormal signal may exist as an asymptomatic or subclinical entity that becomes apparent only when patients are imaged for other symptomatic abnormalities.
Limitations of this study include its retrospective nature and the fact that there is no surgical proof of impingement on the axillary nerve or posterior circumflex humeral artery. However, surgical exploration of the quadrilateral space is rarely performed at our institution because the initial treatment is conservative via physical therapy.
A further limitation is that a database search of radiology interpretations was used to select the cases rather than directly reviewing the images from all 2,436 MRI examinations. However, all interpretations were performed by musculoskeletal radiologists with knowledge and understanding of the quadrilateral space syndrome, and the initial reports were therefore thought to be reliable and satisfactory. To address this limitation in a practical manner, however, we included the word "teres," in addition to "quadrilateral," to allow identification of cases in which the teres minor muscle was specifically noted for any reason, even if quadrilateral space syndrome was not mentioned. If teres minor signal abnormality or atrophy was mentioned, then the images were reviewed to determine whether the case should be included.
Using diffuse abnormal signal in the teres minor muscle in a pattern consistent with neurogenic edema as a diagnostic criterion is also a limitation. This finding was not included in the initial description of the imaging findings of this syndrome. However, edema without atrophy was present in only three of our patients, and because it was thought to be neurogenic in origin, inclusion of this finding as an indicator of axillary nerve dysfunction was believed to be appropriate in this setting. Finally, inclusion of patients with a known history of acute trauma and possible traumatic injury to the brachial plexus, nerve roots, or axillary nerve is a potential limitation, but we thought that the presence of acute trauma did not preclude a preexisting or coexisting quadrilateral space syndrome.
Atrophy or abnormal signal in the teres minor muscle on MRI suggesting quadrilateral space syndrome is uncommon in the patient population referred to our institution for imaging of the shoulder, with an incidence of 0.8%. The low incidence of clinical suspicion of quadrilateral space syndrome in these patients suggests that this may be an overestimate of the true incidence of the syndrome, but this conclusion is complicated by the difficulty in making the diagnosis of quadrilateral space syndrome clinically. Certainly the radiologist can suggest the diagnosis on the basis of the finding of teres minor atrophy or abnormal signal, but care must be taken to assess the remainder of the shoulder because rotator cuff tears, acromioclavicular joint degenerative disease, and biceps abnormalities were seen in most patients. In addition, traumatic injury to the axillary nerve, brachial plexus, or nerve roots must also be considered in any patient with muscle atrophy or neurogenic edema without a visible cause.
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