Imaging of the Eustachian tube and its function: a systematic review (2024)

Identification of studies

A comprehensive search strategy was developed to capture all published material relating to imaging of the Eustachian tube (available in supplementary material). MEDLINE, EMBASE and CENTRAL were searched on 11 February 2015. The reference lists of relevant studies were searched, and in addition, citation searches were performed.

Study selection

To be considered a viable clinical technique, authors must, as a minimum, have described test results in both ETD cases and healthy controls. Anatomical descriptions, techniques in development and non-controlled series were considered separately. Only publications from 1970 onwards were included as being relevant to contemporary practice. Isolated case reports, animal studies and descriptions relating to cancer were excluded.

Data extraction

Publications were assessed in the context of the questions stated earlier. Where possible, quantitative data were extracted for comparison.

Quality assessment

Sample size, risk of bias, generalisability and clinical relevance were assessed.

Data synthesis

The aims of this review dictated a qualitative data analysis through a narrative approach.

Which anatomical structures of the Eustachian tube can be defined radiologically?

The ET can be considered a continuous organ with the middle ear and mastoid cells. The tube is usually divided into three sections when described anatomically: cartilaginous, osseous and junctional, where the two main parts overlap. Particularly in adults, the ET can assume a gently curving, inverted ‘S’ shape, although there is a range from almost straight to acutely curved [10]. Combined with its angle, this shape means the ET is not easily assessed in axial or coronal planes and can be difficult to visualise in its entirety in any single plane, even using multiplanar reconstruction to obtain a plane perpendicular to the long axis of the ET [11, 12]. A number of identified papers address the imaging of the anatomical structures of the ET, almost all using CT and MRI. Whilst some present comparative findings from different dissected cadaver specimens, none comprehensively compare modalities or histological sections to imaging within the same specimen.

Length and angle

There is considerable growth of the ET in the first few years of life, and it continues to develop and enlarge into early adulthood. Takasaki et al. found a linear relationship between ET cartilage volume and age, with an increase in volume of 20mm³ per year after birth, up to the age of 20 [13]. Based on in vivo measurements from CT scans, the total length of the cartilaginous and bony ET is approximately 38mm in children (mean age of 5years), compared to 43mm in adults [14]. These values are slightly higher than those seen in cadaver studies, though the difference with age is consistent [15].

In adults, the ET forms an average angle of 34–36° with the horizontal plane, using the bony palate as a reference, although there is variation between individuals [16, 17]. The ET axis with reference to the sagittal plane is approximately 42° [17]. Takasaki et al. measured the ET angle against Reid’s standard plane (a plane passing through the inferior orbital margin and the upper margin of the external auditory meatus) using CT, and found that in young children, the ET angle is closer to horizontal than in adults, but that there was no significant difference between the angle in children with otitis media with effusion (OME) and controls [14]. The shallower angle of the ET in young children (as little as 10° in some reports) has been widely confirmed, and is hypothesised as being a predisposing factor for middle ear inflammation and infections in this group, due to an increased risk of reflux of nasopharyngeal secretions and pathogens [2]. Figure ​Figure22 demonstrates the difference in angle of the paediatric and adult ET.

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Fig. 2

Schematic diagram showing the relative angles of the ET from the horizontal plane in children and adults

Muscles

Four muscles are associated with the ET: the tensor veli palatini, the levator veli palatini, the tensor tympani and the salpingopharyngeus. The ET is passively closed at rest, and it is thought that the tensor veli palatini plays the main role in opening the lumen [1]. This muscle arises in two parts from the greater wing of the sphenoid bone and from the fibrocartilaginous ET, before passing downwards to hook around the pterygoid hamulus and insert into the soft palate aponeurosis. The levator veli palatini arises from the inferior aspect of the petrous temporal bone, running below and parallel to the ET floor. It is thought only to be related to the ET by loose connective tissue, opening the ET during contraction by rotating the medial part of the cartilage with its increased bulk [10]. The relationship between the ET and these muscles is shown in Fig.3.

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Fig. 3

A schematic of a transverse section through the ET in the mid-cartilaginous section, as seen in the closed state, with the collapsed lumen shown in black. The mucosal folds in the inferior part of the lumen can be seen. ML medial lamina of the cartilage, LL lateral lamina, OFP Ostmann fat pad, TVPM tensor veli palatini muscle, LVPM levator veli palatini muscle. Orientation: AL antero-lateral, PM postero-medial

The salpingopharyngeus arises from the medial and inferior part of the cartilaginous ET. This small muscle, which is often poorly formed, courses inferiorly embedded within connective tissue, to insert into the pharyngeal wall [22, 23]. The tensor tympani arises from the cartilaginous ET and sphenoid bone, and receives fibres from the tensor veli palatini, before ending in a tendon that inserts into the manubrium of the malleus. Despite their proximity, neither the salpingopharyngeus or tensor tympani are thought to influence ET opening [2, 23].

The tensor and the levator veli palatini muscles are usually well visualised using MRI [17, 24, 25], where they can be seen to be separated by a layer of fat [26] and their dimensions can be assessed [27] (Fig.4). Electromyographic and other studies have been inconclusive as to the role of the paratubal muscles in obstructive ETD [28, 29]. In humans, cranial nerve dysfunction has not been linked to ETD, though abnormal muscle function is thought to be the reason that ETD is prevalent in those with a cleft palate.

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Fig. 4

Coronal oblique proton density MRI of the right ET and paratubal structures in a 36-year-old healthy male volunteer. The tubal cartilage is indicated by the arrow, the levator palatini muscle by the arrowhead. LPM lateral pterygoid muscle. MPM medial pterygoid muscle. AL and PM denote antero-lateral and posteromedial orientation of the imaging plane

Ostmann fat pad

Ostmann fat pad is an area of fatty tissue running the length of the cartilaginous ET, infero-lateral to the lumen, that is thought to play a role in tube closure [2]. It is poorly visualised on CT [31], but consistently seen with T1-weighted MRI [24] (Fig.5). Amoodi et al. found Ostmann fat pad to be best visualised on axial T1-weighted post-gadolinium MR images, and calculated its surface area to demonstrate that unlike other soft tissue structures around the ET, it shrinks with adult ageing [27].

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Fig. 5

Axial T1 spin echo MRI of the nasopharynx in a 36-year-old healthy male volunteer. Ostmann fatty tissue is indicated on the left by the arrow

What are the radiological features of obstructive ETD?

The imaging findings associated with ETD have not been extensively described, with only six published papers identified as describing features detectable on imaging that are suggestive of ETD [12, 24, 32, 40–42]. All except one [24] are based on CT findings (Table ​(Table11).

Two groups found that the bony channel for the Eustachian tube within the temporal bone was of reduced cross sectional size in individuals with obstructive ETD [12, 40], and one group found that the tube ran at a shallower angle in diseased ears [42]. Kanzaki et al. highlight one of the weaknesses of using CT to assess soft tissues; as by scanning pre- and post-grommet insertion, they demonstrated that mucus and fluid cannot easily be distinguished from soft tissue obstructing the lumen [32]. After grommet insertion, they could localise what appeared to be mucosal thickening in some patients with ETD, a finding shared with Liang et al. who found that these mucosal changes correlated well with tympanometry results (tympanometry measures middle ear pressure, which is close to atmospheric pressure if the ET is functioning normally) [41].

In the only study to use MRI, Lukens et al. found good soft tissue definition, and were able to analyse ET opening with real-time turbo-gradient echo sequences during a Valsalva manoeuvre (Fig.6) [24]. Although all patients had ETD, in unilateral cases, unaffected ears all appeared to open normally, whereas those affected did not.

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Fig. 6

a, b TFE-SPIR (spectral presaturation with inversion recovery) sequence in neutral position (left) and during a Valsalva manoeuvre (right). Complete opening of the left ET (arrows) and distension of the pharyngeal recess of Rosenmüller (asterisk) are visible. Taken from Lukens et al. 2012 [24], with permission

What are the radiological features of patulous ETD?

The defining feature of a patulous Eustachian tube is patency of the lumen at rest, when it should be closed. This condition has been successfully imaged using CT. Seven published records were identified, most including only very small case series [43–47], and only one reporting the results from case and control cohorts [31] (Table ​(Table22).

The first published use of CT imaging was in 1990 by Tolley et al. who visualised tube patency in one patient with characteristic symptoms, and three patients with microsomia who were asymptomatic [43]. By reconstructing images in an oblique plane, several authors have been able to image the patent lumen over its entire length [44, 45, 47]. The most comprehensive study was published by Yoshida et al. in 2004, who examined whether patulous ETs were visible on CT scans. Of 31 patulous ETs examined, 13 appeared patent throughout their length, and the other 18 were seen to be mostly open. None of the 50 control (healthy) ETs examined were found to have visibly open lumens [31]. However, no mention is made of examiner blinding and the results have yet to be replicated. An image from the study is included (Fig.7). The group also reported good definition of soft tissues, with the exception of Ostmann fat pad.

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Fig. 7

Oblique CT image of the ET and its surrounding tissues in a patient with severe symptoms of patulous ETD (reconstructed 1-mm thick, parallel and perpendicular to the long axis of the ET). The ET can be seen to be patent along its entire length. Taken from Yoshida et al. 2004 [31], with permission

A drawback to the use of CT for imaging patulous Eustachian tubes is that patient symptoms often resolve when lying flat [4] suggesting that the patent tube closes. Because standard CT scanning is performed supine, there is a tendency to under-diagnose patulous ET using the technique. This problem has been overcome in two ways: either through the use of horizontal cone beam CT scanning with a seated patient [44, 45]; or with a tilting scanner and reclining chair, which facilitates positioning of the patient at 45° in a standard scanner [46]. Although the soft tissue contrast resolution of cone beam CT is inferior to standard CT, Yoshida et al. were still able to measure the Eustachian tube lumen long axis, short axis, cross-sectional area and total volume [44]. Interestingly, by using both cone beam and standard CT scanning techniques, this group was able to visualise closure of the abnormally patent lumen on supine positioning.

Rodrigues et al. took the CT scanning one step further in one patient, and used it to perform image-guided placement of a silicone elastomer suspension implant in the lateral wall of the ET, in order to narrow the ET lumen. Symptomatic improvement of patulous ET symptoms was reported [48].

How can imaging be used as a measure of Eustachian tube function?

b) Research and developing techniques

The inaccessible nature of the ET means that imaging techniques have often been employed in research into its function. Some novel techniques remain potential future clinical tests, whereas others have simply been of use in determining the ET opening mechanism.

Early experiments in patients using nasopharyngeal or middle ear contrast, with multiple radiographs or cine x-ray, demonstrated reflux up the ET during opening when assisted by gravity, and suggested a pump-like action of the cartilaginous ET with closure from the tympanic end, clearing any contents into the nasopharynx in partnership with mucociliary clearance [58, 59]. Two groups have also attempted insufflating the ET with a mixture of air and powdered radio-opaque material, to provide evidence of ET patency and size, under non-physiological conditions [60, 61]. These techniques have not been developed further due to their complexity and poor images with radiographs. More recently, transtympanic injection of silver nanoparticles into the middle ear in an animal model has been shown to demonstrate middle ear and ET anatomy, and mucociliary clearance on CT images [62].

CT imaging has been utilised in the analysis of ET opening, which is still not fully understood. In healthy volunteers, Tarabichi et al. found that by performing CT during a Valsalva manoeuvre, the nasopharyngeal third of the cartilaginous ET could be seen in 94%, and the whole length of the tube in 35% [63]. Interestingly, the group went on to use the technique in ears undergoing surgery for chronic disease, and visualised a similar rate of patency in the nasopharyngeal third of the ET, suggesting that the obstruction may lie in the protympanic part [64]. A 3D CT reconstruction from the group is shown in Fig.8. Another study comes from McDonald et al. who used cine CT to visualise an air bolus moving up the ET towards the middle ear [65]. While they were unable to follow the progress of a single opening over the entire ET length, the team hypothesised that their finding may explain why clinical tests of ET function suggest that the ET may not always open along its full length. This study examining gas exchange along the ET provides a clear contrast to other studies, which have looked at the clearance of secretions. Earlier work using radiographs suggested ET opening with progressive closure towards the nasopharynx as a method of pumping secretions out of the middle ear, and these findings were replicated by Niwa et al. who used intratympanic contrast with sequential CT repeats at 1 scan per second [66]. Interestingly, this group also tested the same ears with tubo-tympano-aerodynamic-graphy (TTAG), a manometric clinical test of ET opening during Valsalva. In a mixed patient group with chronic middle ear disease, they found that clearance and ventilation function did not always correlate with each other, suggesting that a single radiological or clinical test is unable to assess the function of the ET. As most middle ear pathologies arise from inadequate gas exchange across the ET, ventilation rather than clearance should be the focus of future imaging studies.

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Fig. 8

Three-dimensional reconstruction of the airspaces within the temporal bone from CT. Note the location of the isthmus of the ET and the relatively large distal aperture (circled). Taken from Tartabichi et al. 2015 [64], with permission

Karhuketo et al. successfully performed CT virtual endoscopy of the middle ear and osseous ET, though the closed passive state of the cartilaginous ET limits the usefulness of this technique outside the middle ear [67].

MRI has also been used to image the ET lumen opening. Krombach et al. asked healthy volunteers to perform Valsalva during image acquisition with a dynamic turbo-gradient echo sequence, and found that ET opening could be confirmed in 20 out of 22 ears [68]. The next step with this investigation would be to see if patients with ETD produce different results, and given the resolution of MRI, if areas or causes of lumen narrowing could be identified.

Patulous ETD may develop after weight loss or radiotherapy, but no anatomical cause for this has been identified. Two groups looked at Ostmann fat pad [27] and the ET cartilage shape [25] as potential candidates, but could find no relationship between MRI appearances and patulous disease. MRI has also been used to investigate the unknown mechanism of improved patulous ETD symptoms on lying supine. Oshima et al. measured the dimensions of the soft tissues around the ET on T1 contrast MRI before and during neck compression, and found that engorgement of the pterygoid venous plexus between the tensor veli palatini and the medial pterygoid muscles caused compression of the patent ET lumen from the antero-lateral side of the tube [69].

Dynamic anatomical imaging offers possible future applications. Yamawaki et al. used fast spin echo sequences to obtain T2 images to study dynamic changes induced by phonation [70]. They found that pharyngeal wall movements and ET cartilage movements seen with MRI correlated well with endoscopy findings, and hypothesised that rapid MRI may be a useful tool for the assessment of velopharyngeal function.

Given the ongoing uncertainty regarding the role of paratubal structures in ET opening and dysfunction, Leuwer et al. presented an interesting research technique using MRI voxel data to generate a 3D model of the ET and investigate muscle action [71]. Such modelling has been attempted before using histological sections [72–74], but the ability to perform this in vivo gives radiologists and otolaryngologists new opportunities for research into ETD.

Summary: How can imaging be used as a measure of Eustachian tube function?
(a) Clinical tests
• Plain film radiograph and scintigraphy tests have shown the greatest promise, but have not been developed and therefore have not entered clinical use
(b) Research and developing techniques
• Imaging has greatly added to our understanding of ET function and research is ongoing
• Dynamic imaging using new CT and MRI rapid acquisition techniques is beginning to provide new insight into ET function, and may yet provide a clinical test that allows diagnosis of ETD, and targeting of treatments

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Quality of existing studies

Even when case-control data were presented in identified studies, groups were small and often poorly defined. Although efforts at blinding were made by a few authors, most studies have a significant risk of bias and no controlled trials of any ET imaging techniques were identified. Where studies employed a case-control distinction, patient labels were applied prior to testing, and no published results have yet demonstrated the sensitivity or specificity of an imaging technique at assigning a diagnosis of ‘normal’ or ‘abnormal’ ET function to mixed population.

Current hurdles and future developments

A problem shared with any ETD research or outcome measure development is the poorly defined nature of the disorder, which is due to the spectrum of possible presentations, and lack of a ‘gold standard’ test [75]. There will still be issues with generalisation, as clinical tests have shown different results in different populations [5], and particularly, in children ETD appears to differ in aetiology [2]. Current imaging work has focused on adults, but ETD is far more common in children. Imaging in the paediatric population will present new challenges when considering permissible radiation exposure and patient cooperation.

Patient positioning is an important consideration, as many newer imaging techniques require supine positioning, and as identified by some authors, this will affect ET function. Using a manometric method, it has been noted that even in healthy individuals, the volume of air passing through the ET is reduced by one third when the body is positioned at 20° to the horizontal and by two thirds when the body is supine [76]. Although cone beam CT perhaps provides the current best combination of spatial resolution and position, it is an area for development.

Imaging and reconstruction protocols to optimise orientation, tissue definiton and capture ET movement need to be futher developed. Through collaboration between radiologist and otolaryngologists, it is possible that the use of intranasal or intratympanic contrast may again prove of benefit with the progression of cine CT and MRI techniques.

Most studies identified to date have made qualitative assessments of opening, although anatomical measures of size and volume have been possible. For maximum clinical utility, quantifying ET opening or function is the goal. Clinical tests are beginning to reach this point, though no single investigation has declared itself as superior; and currently, diagnoses are mostly made on expert opinion. For the foreseeable future, it is likely that the assessment of patients with obstructive or patulous ETD will reply on a combination of imaging, clinical tests, patient reported measures and expert opinion.

Conflict of interest

We declare that we have no conflict of interest.

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