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When is a Vestibular Aqueduct Enlarged?

A ten-year-old boy with bilateral moderate sensorineural hearing loss underwent computerized tomographic (CT) imaging (GE Brightspeed, Wisconsin, USA) of the temporal bone as part of the work-up to determine the etiology of his condition. The formal radiologic interpretation of the scan stated that the vestibular aqueducts were not enlarged. However, independent review of the axial CT images appeared to indicate the presence of enlarged vestibular aqueducts. (Figure 1) This can be   contrasted with a scan from another patient with no evidence of sensorineural hearing loss. (Figure 2) 
What can explain the discrepancy between the two? 
  
If simple visual inspection of the vestibular aqueduct (VA) can lead to conflicting interpretations, then what radiographic parameters can be used to resolve the issue? Is there a more objective means of determining the presence of a clinically significant vestibular aqueduct enlargement? 
  
In 1978, Valvasorri and Clemis1 first described an association between congenital sensorineural hearing loss and an abnormality in vestibular aqueduct anatomy which they labelled as the “large vestibular aqueduct syndrome.” In this landmark study that utilized hypocycloidal polytomographic temporal bone studies, they proposed that a vestibular aqueduct is enlarged when its midpoint diameter is greater than 1.5 mm.  Although this parameter is generally considered to be the defining characteristic of the condition, one must realize that this measurement was based on less accurate imaging technology and measurement tools. Contemporary studies utilize high-resolution CT imaging with digital workstation measurement software to evaluate vestibular aqueduct anatomy. Currently, the two most commonly used radiographic parameters are the VA midpoint (MP) width and the VA opercular (OP) width. (Figure 3) 
More recently, Boston et al.2 in 2007 published normative values for these parameters based on a study population of 73 children without known sensorineural hearing loss. They considered a vestibular aqueduct enlarged when one or both of the measured widths were above the 95th percentile of the normal study group measurements. On this basis, a VA midpoint width of >0.9 mm and/or a VA opercular width of >1.9 mm was the criteria established to define an enlarged vestibular aqueduct. 
  
The patient’s measured vestibular aqueduct midpoint width on the right was 2.1 mm, while the vestibular aqueduct opercular width was 2.9 mm. (Figure 4) These measurements, when evaluated against either the original Valvassori criteria or the newer criteria of Boston et al., confirm what was visually apparent– the presence of a clinically significant enlargement of the vestibular aqueduct as the etiology of the patient’s sensorineural hearing loss. 
 