Limitations of Current Methodology for Assessment of Compression of the Optic Chiasm by Macroadenoma: The Neuroradiologic Perspective

Invited Commentary Limitations of Current Methodology for Assessment of Compression of the Optic Chiasm by Macroadenoma: The Neuroradiologic Perspective Ari M. Blitz, MD, Sachin K. Gujar, MBBS T he extent of visual compromise due to chiasmatic compression by a pituitary macroadenoma is variable as reported by Ryu et al (1). What accounts for this variability? Although the clinical variability may puzzle the neuro-ophthalmologist, evaluating the extent and significance of compression also presents a challenge to the neuroradiologist. The factors assessed in the accompanying article (including tumor height, tumor volume, and severity of chiasmatic compression) relate principally to mechanical compression of the optic chiasm. Technical factors relating to assessment of compression detected on magnetic resonance imaging (MRI) may prevent an accurate assessment of the degree of compression and deserve discussion. Regardless, the degree of chiasmatic compression observed is likely, as suggested in the work by Ryu et al, a necessary but not sufficient condition for loss of vision and, therefore, inadequate to predict visual recovery. The method of measurement of chiasm elevation described by the authors involves the "standard line" which, despite the name, is not a commonly used measurement in neuroradiology. As described in a previous publication (2), these authors define the standard line as connecting the inferolateral aspects of the chiasm, and it involves measuring the perpendicular distance between the standard line and the inferior aspect of the chiasm at maximum compression. This appears to be a measure of chiasmatic deformity rather than chiasmatic elevation per se. Also, it is not clear how this measurement has been validated (as the authors acknowledge this as a limitation). As in cases with large macroadenomas with suprasellar extension, the entirety of the chiasm may be elevated and the implied assumption that the "standard line" represents the native position of the chiasm is by its nature flawed. Unfortunately, for most patients, no imaging of the premorbid anatomy exists, rendering a measurement of the true elevation of the optic chiasm challenging, particularly without normative population-based data. Division of Neuroradiology, Department of Radiology and Radiological Sciences, Johns Hopkins Hospital, Baltimore, Maryland. Address correspondence to Ari M. Blitz, MD, Johns Hopkins Hospital, Neuroradiology, 600 North Wolfe St., Phipps B-126A, Baltimore, MD 21287; E-mail: [e-mail] Blitz and Gujar: J Neuro-Ophthalmol 2017; 37: 239-241 There also are potential inherent limitations to 2dimensional measurement of the optic chiasm and optic nerves. Namely, as the axis of the chiasm or nerve varies with respect to the imaging plane, varying measurements will be obtained. This problem is accentuated by compression and deformation of these structures. However distorted the morphology of an elongated anatomical structure, the greatest possibility of consistent and accurate measurement of compression may be obtained through measurement of the shortest cross-sectional dimension of the structure and not necessarily of the height (Fig. 1). Uniform measurement of deformation of the optic chiasm would also be limited by the lack of a consistent MRI protocol. MRI sequences with similar names (e.g., T1, T2, etc.) are not standardized and may vary from institution to institution in voxel size and slice thickness, not to mention the signal-to-noise ratio. Differences between MRI units in magnetic field strength will further compound difficulties in comparison between varying protocols. The acquisition of 3-dimensional (3D) volumetric images may, to some extent, offset such measurement limitations in the future. Duration of compression also may be of importance. In the setting of acute hemorrhage within a macroadenoma with resultant abrupt chiasmatic compression, visual symptoms are expected (3). Yet the volume of the tumor and the extent to which it deforms the optic chiasm may be similar to a slowly growing macroadenoma. This points to a general weakness of using measurements obtained on imaging in the evaluation of chiasmatic compression; namely, that in the absence of hemorrhage, the degree of compression may be more obvious than the rate at which the compression has developed. One observational study of the natural history of non-operated adenomas found significant enlargement in half of the macroadenomas over a mean observation period of 42 months (4). Unfortunately, MRI at a single point in time may simply not provide sufficient information to identify prospectively those tumors which are liable to grow more rapidly. The same study reported that 57% of the enlarging tumors demonstrated new or worsening visual field abnormalities, suggesting that early resection may be the best means of avoiding visual field loss. Likewise, it has been noted that a shorter duration of compression may 239 Copyright © North American Neuro-Ophthalmology Society. Unauthorized reproduction of this article is prohibited. Invited Commentary FIG. 1. Two-dimensional measurements vary with the plane of measurement with respect to the structure measured. A. Demonstration of a tubular structure to simulate the optic nerve seen in the lateral perspective passing directly through the anterior-posterior plane of measurement in a direct coronal 2D image. An accurate measurement of diameter is obtained. If the optic nerve passes through the plane of section at an oblique angle (B), the measurement obtained will change and may not accurately reflect the true diameter of the structure in question. result in a better visual prognosis, and chronic compression is associated with worse visual recovery (3,5). Likewise measurements of tumor size are of limited utility as macroadenomas of equivalent size may demonstrate differing vectors of enlargement, with some tumors growing caudally to invade the sphenoid sinus or the clivus or laterally to invade the cavernous sinus rather than extending superiorly into the suprasellar cistern to impinge on the visual pathway. Among macroadenomas which impinge on the visual pathway structures, asymmetry of compression is a common finding, and simple linear measurements may not adequately capture this complexity. Again, future evaluations with 3D volumetric MRI may aid in better defining the relative deformity of the various components. Apart from the size of the tumor and the degree to which it compresses the optic chiasm, are there other factors which influence the prognosis with respect to vision? It is reasonable to assume that a denser, stiffer tumor might be more prone to causing visual field defects than softer tumors, assuming the same degree of compression (although how this might translate into prognosis for visual recovery is uncertain). Variation in consistency of macroadenomas has been reported to be on the basis of differences in collagen content (6). Can MRI reveal the consistency of a pituitary tumor preoperatively? Various MRI techniques have been reported to correlate with intraoperative tumor consistency including tumoral perfusion (7), elastography (8), enhancement on postcontrast steady-state free-precession imaging (9), diffusion-weighted imaging, and T2 imaging (6). There are conflicting reports regarding the relationship of signal on diffusion-weighted imaging with adenoma consistency (6,10). The mechanism by which macroadenomas are thought to produce impairment of visual acuity other than field defects involves not simple mechanical compression but rather vascular compromise (11,12). The optic chiasm has a relatively complex vascular supply, arising from branches of the anterior communicating, anterior cerebral, posterior 240 communicating, posterior cerebral, and basilar arteries (13,14), with branches from the superior hypophyseal artery contributing to the blood supply of the prechiasmatic optic nerve, chiasm, and optic tracts (15). These vessels fall largely below the resolution of even our high-resolution MRI techniques. Furthermore, current clinical magnetic resonance techniques are not adequate to evaluate chiasmatic perfusion, particularly in a thinned, stretched, and compressed optic chiasm. Tokumaru et al (11) also attribute chiasmal signal abnormalities to damage by compression and stagnant anoxia at the chiasm and Wallerian degeneration along its ventral aspect. One study of fiber tract integrity using diffusion tensor imaging preoperatively found that a lower fractional anisotropy and higher mean diffusivity, indicative of axonal damage and/or demyelination, was predictive of a lack of improvement in vision after endoscopic macroadenoma resection (5). Predicting surgical outcomes on the basis of imaging studies alone is a perilous task and presupposes uniformity of surgical technique. Amongst different surgeons, decisions on whether to use fat packing and with what volume vary significantly, with the attendant possibility of overpacking leading to chiasmal compression (16,17) or underpacking leading to potential late chiasmatic prolapse (18), which may alter visual prognosis. Furthermore, surgical complications, chief among them postoperative bleeding (19) but also direct surgical trauma and ischemia (18), may lead to worsening of vision in a manner not directly related to the findings on preoperative imaging. Ryu et al (1) focus on predictors of visual recovery after surgery and appropriately conclude that no current commonly used imaging method is predictive. Notwithstanding these observations, is it possible that more precise measurements of anatomical deformation of the optic chiasm might provide additional prognostic value? High-resolution postcontrast 3D constructive interference in steady-state (similar to fast imaging employing steady-state acquisition cycled Blitz and Gujar: J Neuro-Ophthalmol 2017; 37: 239-241 Copyright © North American Neuro-Ophthalmology Society. Unauthorized reproduction of this article is prohibited. Invited Commentary 6. 7. 8. 9. 10. FIG. 2. A. Coronal postcontrast TI-VIBE demonstrates an enhancing mass (asterisk) extending into the suprasellar cistern. B. The extent of chiasmatic elevation, compression, and degree of deformation (curved arrow) are better visualized on postcontrast coronal reformatted constructive interference into steady-state imaging. VIBE, volume interpolated breath-hold examination. phases) imaging (20,21) is particularly promising in this regard, as the relationship of tumor mass to the optic chiasm and the extent of chiasmatic deformity may be significantly better appreciated than on postcontrast T1 images (Fig. 2). The more commonly obtained precontrast T1 thin-section images may also occasionally have advantages over the postcontrast T1 sequences illustrated by Ryu et al in the accompanying article (1). Future research might profitably be directed to comparison of these various techniques. REFERENCES 1. Ryu WHA, Starreveld Y, Burtar JM, Liu J, Costello F; PITNET Study Group. 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