The arrow-tipped loop is a marker of radiculomedullary vein thrombosis linked to the anti-reflux mechanism--angiographic anatomy and clinical implications

INTRODUCTION

This article proposes that the "arrow-tipped" loop or anastomosis classically described as an anatomic variant represents in fact a pathological phenomenon resulting from the partial thrombosis of a radiculomedullary vein (RMV) with a duplicated origin (double-rooted RMVs).

METHODS

The arrow-tip loop concept proposed in this report is illustrated with angiographic observations of patients with underlying pathologies of the spinal venous system, three cases of spinal arteriovenous fistulas, and one case of spinal venous insufficiency.

RESULTS

In each clinical case, the presence of arrow-tip loops was associated with diffuse alteration of the perimedullary venous system, including the lack of detectable RMVs. The angiographic appearance of the arrow-tip loops suggested partially thrombosed double-rooted RMVs, with rootlets originating either from the anterior or posterior spinal veins, or from both.

CONCLUSION

While a thrombosed single-rooted RMV typically becomes anatomically and angiographically undetectable, double-rooted MRVs keep a flowing proximal segment made of their two rootlets of origin. This residual proximal segment takes the appearance of an arrow-tip loop, which therefore be seen as an indicator of spinal venous thrombosis.

Angiographic suppression of the artery of Adamkiewicz by venous hypertension resolving after embolization in a case of spinal epidural arteriovenous fistula

A case of complete angiographic suppression of the artery of Adamkiewicz and anterior spinal artery in a patient with a spinal epidural arteriovenous fistula (AVF) is reported. Slow flow AVFs typically present with progressive myelopathy secondary to spinal venous hypertension (SVH). The lack of a normal venous phase during angiography and its restoration after treatment is commonly observed with these lesions, yet a similar phenomenon seems exceptional at the arterial level. Right T11 intercostal artery angiograms obtained before and after treatment of a left L4 epidural AVF documented the initial suppression of the artery of Adamkiewicz and anterior spinal artery, and their normal appearance immediately after correction of the SVH by embolization. This report confirms that SVH can angiographically suppress prominent and functionally important spinal arteries, re-emphasizing the potential role played by secondary arterial changes in SVH induced myelopathy. This hemodynamic phenomenon also represents a potential pitfall during diagnostic and therapeutic endovascular procedures.

Planning evaluation of C-arm cone beam CT angiography for target delineation in stereotactic radiation surgery of brain arteriovenous malformations

PURPOSE

Stereotactic radiation surgery (SRS) is one of the therapeutic modalities currently available to treat cerebral arteriovenous malformations (AVM). Conventionally, magnetic resonance imaging (MRI) and MR angiography (MRA) and digital subtraction angiography (DSA) are used in combination to identify the target volume for SRS treatment. The purpose of this study was to evaluate the use of C-arm cone beam computed tomography (CBCT) in the treatment planning of SRS for cerebral AVMs.

METHODS AND MATERIALS

Sixteen consecutive patients treated for brain AVMs at our institution were included in this retrospective study. Prior to treatment, all patients underwent MRA, DSA, and C-arm CBCT. All images were coregistered using the GammaPlan planning system. AVM regions were delineated independently by 2 physicians using either C-arm CBCT or MRA, resulting in 2 volumes: a CBCT volume (VCBCT) and an MRA volume (VMRA). SRS plans were generated based on the delineated regions.

RESULTS

The average volume of treatment targets delineated using C-arm CBCT and MRA were similar, 6.40 cm(3) and 6.98 cm(3), respectively (P=.82). However, significant regions of nonoverlap existed. On average, the overlap of the MRA with the C-arm CBCT was only 52.8% of the total volume. In most cases, radiation plans based on VMRA did not provide adequate dose to the region identified on C-arm CBCT; the mean minimum dose to VCBCT was 29.5%, whereas the intended goal was 45% (P<.001). The mean volume of normal brain receiving 12 Gy or more in C-arm CBCT-based plans was not greater than in the MRA-based plans.

CONCLUSIONS

Use of C-arm CBCT images significantly alters the delineated regions of AVMs for SRS planning, compared to that of MRA/MRI images. CT-based planning can be accomplished without increasing the dose to normal brain and may represent a more accurate definition of the nidus, increasing the chances for successful obliteration.

Focal compression of the upper left thoracic intersegmental arteries as a potential cause of spinal cord ischemia

BACKGROUND AND PURPOSE

This study was prompted by recurrent angiographic observations of focal stenoses involving the proximal segment of the left upper thoracic intersegmental arteries, a few centimeters distal to their origin. The purpose was to investigate the nature and prevalence of this anomaly.

MATERIALS AND METHODS

The existence of non-ostial thoracic intersegmental artery stenoses was evaluated in 50 angiograms; the contribution of stenosed branches to the anterior spinal artery was recorded. Angiograms performed in 3 patients with right-sided aortas were also reviewed. The topographic relationships of the upper thoracic intersegmental artery with surrounding structures were investigated in 3 cadavers.

RESULTS

Thirty-seven non-ostial stenoses were found in 26 patients (52%), predominantly on the left side (92%), between T3 and T8, most frequently at T4 and T5 (54%). Severe lesions were observed in 10% of cases. Patients with stenoses had fewer detectable anterior radiculomedullary arteries between T3 and T5 (35% versus 54%). Upper intersegmental artery stenoses, documented on the left side of all 3 specimens, appeared to be caused by the recurrent path of these intersegmental arteries related to the leftward position of the thoracic aorta, and by their course around reinforced paramedian longitudinal strands of the endothoracic fascia.

CONCLUSIONS

Upper thoracic intersegmental artery stenoses are frequent. They result from the leftward deviation of the descending aorta and the existence of a fixed point along the course of the intersegmental arteries related to the endothoracic fascia. Because contributors to the spinal vascularization often originate at similar levels, these stenoses may play a role in the susceptibility of the upper and midthoracic spinal cord to ischemia.

Diagnostic quality and accuracy of low dose 3D-DSA protocols in the evaluation of intracranial aneurysms

BACKGROUND

3D-DSA is the 'gold standard' imaging technique for the diagnosis and characterization of intracranial aneurysms.

OBJECTIVE

To compare the image quality and accuracy of low dose 3D-DSA protocols in patients with unruptured intracranial aneurysms.

MATERIALS AND METHODS

The standard manufacturer 5 s 0.36 μGy/f protocol and one of three low dose 3D-DSA protocols (5 s 0.10 μGy/f, 5 s 0.17 μGy/f, 5 s 0.24 μGy/f) were performed in 12 patients with unruptured intracranial aneurysms. Three interventional neuroradiologists, two neurosurgeons, and two neurologists rated the image quality of all 3D reconstructions as good, acceptable, or poor. Three interventional neuroradiologists measured two dimensions of each aneurysm for all protocols. The radiation dose metric Ka,r (reference point air kerma, in mGy) was recorded for each 3D-DSA protocol.

RESULTS

The standard 5 s 0.36 μGy/f protocol earned the highest average subjective rating of 2.76, followed by the 5 s 0.24 μGy/f (2.72), and 5 s 0.17 μGy/f (2.59) protocols. The ranges of differences in aneurysm measurements between the 5 s 0.24 μGy/f protocol and the standard were <0.5 mm. The median Ka,r metrics for each protocol were as follows: 5 s 0.36 μGy/f (89.0 mGy), 5 s 0.24 μGy/f (57.7 mGy), 5 s 0.17 μGy/f (45.9 mGy), and 5 s 0.10 μGy/f (27.6 mGy).

CONCLUSIONS

Low dose 3D-DSA protocols with preserved image quality are achievable, and can help reduce exposure of patients and operators to unnecessary radiation. The 5 s 0.24 μGy/f protocol generates one-third smaller radiation dose than the standard 5 s 0.36 μGy/f protocol without compromising diagnostic image quality or accuracy.

The supreme intercostal artery includes the last cervical intersegmental artery (C7) - angiographic validation of the intersegmental nomenclature proposed by Dorcas Padget in 1954

In 1954, Dorcas Padget wrote a note that settled the debate over the labeling of the intersegmental arteries (ISAs) and the level of origin of the subclavian artery (SubA), in agreement with earlier observations made by Hochstetter in the rabbit (1890) and Schmeidel in man (1932). In her proposed nomenclature, Padget introduced the name of proatlantal artery and confirmed that the SubA was associated with the sixth ISA rather than the seventh. However, a generally unnoticed consequence of this now widely accepted terminology lays in its incompatibility with the traditional view of the costocervical trunk (CT), regarding in particular the anatomy of the supreme intercostal artery (SIA). If the SubA derives from the sixth cervical ISA (ISA 6), and the CT provides the first and second posterior IAs (ISA 8 and 9), the seventh cervical ISA (ISA 7) then remains unaccounted for. The purpose of this study was to examine Padget's nomenclature in the light of modern angiographic material to identify the missing seventh ISA. Our findings validate the terminology used by Padget, including the addition of a pro-atlantal artery and the identification of the ISA most commonly associated with the SubA as the sixth ISA, but they also emphasize the need to redefine the anatomy of the SIA to incorporate the seventh cervical ISA.

A single burr hole approach for direct transverse sinus cannulation for the treatment of a dural arteriovenous fistula

A 55-year-old woman with a symptomatic Borden II/Cognard IIa+b transverse sinus dural arteriovenous fistula underwent an attempted percutaneous transvenous embolization which was ultimately not possible given the fistula anatomy. She then underwent a partial percutaneous transarterial embolization but the fistula recurred. Given the failed percutaneous interventions, the patient underwent a combined open surgical/transvenous embolization using neuronavigation and a single burr hole craniectomy. She has remained symptom free for 3 months. This case report illustrates the feasibility of combining minimally invasive open surgical access to allow for direct venous cannulation for endovascular embolization of a dural arteriovenous fistula when traditional percutaneous methods are not an option.

A single burr hole approach for direct transverse sinus cannulation for the treatment of a dural arteriovenous fistula

A 55-year-old woman with a symptomatic Borden II/Cognard IIa+b transverse sinus dural arteriovenous fistula underwent an attempted percutaneous transvenous embolization which was ultimately not possible given the fistula anatomy. She then underwent a partial percutaneous transarterial embolization but the fistula recurred. Given the failed percutaneous interventions, the patient underwent a combined open surgical/transvenous embolization using neuronavigation and a single burr hole craniectomy. She has remained symptom free for 3 months. This case report illustrates the feasibility of combining minimally invasive open surgical access to allow for direct venous cannulation for endovascular embolization of a dural arteriovenous fistula when traditional percutaneous methods are not an option.

A critical inventory of preoperative skull replicas

INTRODUCTION

Physical replicas of organs are used increasingly for preoperative planning. The quality of these models is generally accepted by surgeons. In view of the strong trend towards minimally invasive and personalised surgery, however, the aim of this investigation was to assess qualitatively the accuracy of such replicas, using skull models as an example.

METHODS

Skull imaging was acquired for three cadavers by computed tomography using clinical routine parameters. After digital three-dimensional (3D) reconstruction, physical replicas were produced by 3D printing. The facsimilia were analysed systematically and compared with the best gold standard possible: the macerated skull itself.

RESULTS

The skull models were far from anatomically accurate. Non-conforming rendering was observed in particular for foramina, sutures, notches, fissures, grooves, channels, tuberosities, thin-walled structures, sharp peaks and crests, and teeth.

CONCLUSIONS

Surgeons should be aware that preoperative models may not yet render the exact anatomy of the patient under consideration and are advised to continue relying, in specific conditions, on their own analysis of the native computed tomography or magnetic resonance imaging.

Anterior spinal artery syndrome presenting as cervical myelopathy in a patient with subclavian steal syndrome

The anterior spinal artery syndrome (ASAS), described by Preobraschenski in 1908,¹ is characterized by bilateral spastic paraparesis and dissociated sensory deficits involving bilateral loss of temperature and pain sensation but preserved proprioception and touch. The ASAS typically develops acutely, over minutes to hours. While initially linked to infections, in particular syphilis, many other etiologies are now recognized, including aortic atheromatous disease and thromboembolic phenomenon. We report a case of ASAS in a patient with a steal phenomenon secondary to a right-sided aortic arch with interruption of the left subclavian artery (SCA).

Reducing radiation dose while maintaining diagnostic image quality of cerebral three-dimensional digital subtraction angiography: an in vivo study in swine

BACKGROUND

Three-dimensional digital subtraction angiography (3D-DSA) is a modern technique that allows for better appreciation of complex vascular lesions. This study evaluates the impact of various dose reduction strategies on 3D-DSA image quality.

METHODS

The standard manufacturer 5 s 0.36 μGy/frame setting was modified to create lower dose 3D-DSA protocols by varying the acquisition time (5 or 3 s) and/or dose per frame (0.36, 0.24, 0.17, and 0.10 μGy/f). All protocols were evaluated in three swine. Four raters measured a segment of the external carotid artery on two-dimensional multiplanar reconstruction images. The raters were also presented with three-dimensional volume rendered images from all protocols in a blinded manner and asked to choose the superior image. A full model analysis of variance with repeated measure factors was performed to compare mean differences in measurements between protocols.

RESULTS

Measurement differences between the standard and low dose protocols were not clinically significant (<0.5 mm). All raters demonstrated high inter-rater reliability. The 5 s protocols were considered as qualitatively superior to the 3 s protocols. Delivered system doses ranged from 43.8 to 6.5 mGy. The 5 s 0.10 μGy/frame protocols generated 65-68% less delivered dose compared with the 5 s 0.36 μGy/frame setting.

CONCLUSIONS

Low dose 3D-DSA protocols with preserved image quality are achievable, and can help reduce unnecessary radiation exposure to both patients and operators. The 5 s low dose protocols generated clinically acceptable and superior images compared with the 3 s protocols, suggesting a more important role for acquisition time than dose per frame to maintain image quality.

The Artery of von Haller

BACKGROUND:

Although recognized since the 18th century, radiculomedullary arteries (RMAs) originating from upper thoracic intersegmental arteries are generally considered to be inconstant branches of little clinical importance. Yet, such vessels are commonly observed during spinal angiography.

OBJECTIVE:

To evaluate the angiographic prevalence of upper thoracic RMAs, in particular, branches supplying the anterior spinal artery (ASA).

METHODS:

Fifty spinal angiograms were reviewed. Anterior and posterior RMAs originating in the upper thoracic region (T3 to T7) were recorded. The level of origin of the artery of Adamkiewicz (T8 to L3) was also noted.

RESULTS:

Forty-three patients (86%) had at least 1 ASA contributor between T3 and T7. Of the other 7 patients, 4 had one at an immediately adjacent level (T2 or T8). The most frequent origin of upper thoracic anterior RMAs was left T5 (n = 10). Only left T9 (n = 12) and left T8 (n = 11) were more common. When combining the left and right sides, an ASA contributor was more frequent at T5 (n = 16) than at any other level (n = 15 for T9, n = 14 for T8). The sum of ASA contributors at T4 and T5 (n = 27) represented 54% of all upper thoracic anterior RMAs, and 23% of all anterior RMAs between T3 and L3.

CONCLUSION:

A significant upper thoracic anterior RMA distinct from the artery of Adamkiewicz appears to be a constant anatomic feature, which undermines the classic concept of an arterial watershed zone in the thoracic region. We propose to name this artery after Albrecht von Haller, who documented its existence in 1754.

The endovascular occlusion system for safe and immediate peripheral vessel occlusion during vascular interventions

Endovascular occlusion of blood vessels is an important part of interventional therapy concepts. Here, we evaluate the feasibility, procedural safety and efficacy of the novel endovascular occlusion system (EOS) in the arterial system in a porcine model. Thirteen devices were deployed in the iliac and femoral arteries (diameter: 4-5 mm) of five adult swine. Post-deployment angiography was performed at 1, 5 and 10 min and 6 h. All devices (n = 13) could be successfully delivered without any complications, such as dissection, perforation or rupture. The devices could be easily advanced to the target vessel segment, deployed at the intended target location and produced immediate and complete vessel occlusion which was confirmed to be maintained after 6 h. No leaks, recanalization or device migration was observed. In this pilot study, we demonstrate the feasibility, safety and efficacy of immediate vessel occlusion with the EOS device in the peripheral arterial system in a porcine animal model. Our data indicate that this novel device allows precise delivery without the occurrence of cardiovascular complications. Owing to its long-term safety and efficacy the EOS may represent a promising and effective alternative to currently available devices for vessel occlusion during vascular interventions.

Perinatally HIV-infected youth presenting with acute stroke: progression/evolution of ischemic disease on neuroimaging

BACKGROUND AND PURPOSE

Although HIV infection is decreasing in infants and children, there is a steady cohort of perinatally HIV-infected (PHIV) children that are growing older. Increased risk of acute stroke has been reported in PHIV children. Our goal was to evaluate evolution/progression of neuroimaging findings in PHIV youth initially presenting with acute stroke.

MATERIALS AND METHODS

The medical records of PHIV pediatric patients (n = 179) from 1996 to 2010 were reviewed and patients with clinical documentation of acute stroke referred to the neuroradiology service were eligible for the study. Neuroimaging (brain CT, MRI, and MRA) and charts were evaluated; clinical and neuroimaging findings at the initial acute stroke and at the last presentation to the neuroradiology service were documented and analyzed.

RESULTS

Eight PHIV patients with clinical findings of acute stroke referred to the neuroimaging were identified. CT and MRI findings of infarction were found in all (8/8) patients in their first and/or last neuroimaging study; including basal ganglia-thalami (BGT) infarction (7/8), focal cortical infarction (4/8), and internal capsule infarction (4/8). Imaging depicted cortical atrophy (5/8), BGT calcification (3/8), and posterior reversible encephalopathy syndrome, wallerian degeneration, and periventricular white matter hyperintense T2 signal each in one patient. No tumors or infectious masses, cysts or abscesses were identified. Subsequent available neuroimaging revealed progression of the cerebrovascular disease in seven patients, 5/7 in the absence of new clinical signs or symptoms. Segmental occlusion, narrowing or narrowing/dilatation in the circle of Willis was found in 6/6 patients who underwent MR angiography and fusiform aneurysms were detected in three of them, a saccular aneurysm in one patient.

CONCLUSION

Asymptomatic progression of cerebrovascular disease was found in PHIV adolescents with prior stroke. These findings may have implications for long-term risk and outcomes for this patient population. There should be a low threshold to evaluate for CNS pathology even with minor symptoms in this population. More studies are necessary to determine if there is a benefit from screening of asymptomatic patients.

Angiographic detection of cerebral cavernous malformations with C-arm cone beam CT imaging in three patients

Cerebral cavernous malformations (CMs) are typically not seen during conventional digital subtraction angiography (DSA) and are therefore classically referred to as angiographically occult malformations. We present three cases in which DSA with selective intra-arterial contrast injection in the common carotid artery and C-arm cone beam CT imaging was able to demonstrate a CM. In addition, an associated developmental venous anomaly (DVA) was present in all three cases, although detected by MRI in only one of them. In light of this finding, we suspect that the incidence of DVA associated with CM is probably higher than previously reported.

Flat panel catheter angiotomography of the spinal venous system: an enhanced venous phase for spinal digital subtraction angiography

BACKGROUND AND PURPOSE

While spinal DSA remains the reference standard technique for spinovascular imaging, visualizing spinal veins remains challenging due to their small size and motion artifacts. This study evaluates the ability of FPCA to overcome these obstacles.

MATERIALS AND METHODS

Sixty-three FPCAs, performed by intersegmental artery injections in 57 patients, were prospectively evaluated. Entrance skin doses were compared with standard spinal DSA runs, including a venous phase and cerebral FPCAs. FPCA contributions were stratified as 1) provided no added information, 2) complemented spinal DSA findings, 3) assisted therapy planning, and 4) contributed principal diagnostic findings.

RESULTS

No complications were observed. Diagnoses included vascular malformations (44%), stroke (9%), venous anomalies (10%), other (9%), and unremarkable (28%). Mean entrance skin doses were of 419 mGy for FPCA, 161 mGy for spinal DSA with venous phase, and 309 mGy for cerebral FPCAs. FPCA contributed the principal diagnostic finding in 16 cases (25.4%), assisted therapy planning in 13 cases (20.6%), complemented spinal DSA findings in 12 cases (19.1%), and provided no additional information in 20 cases (31.7%). In 8 of these 20 cases, FPCA documented a spinal venous anatomy that was poorly visualized or not visualized on spinal DSA.

CONCLUSIONS

Spinal FPCA is safe, with a moderate increase in radiation dose, compared with spinal DSA with venous phase or cerebral FPCA. It proved particularly valuable for therapy planning and the diagnosis of venous abnormalities. This study suggests that FPCA has an important role to play in the evaluation of the spinal venous system.

Angiographic detection and characterization of "cryptic venous anomalies" associated with spinal cord cavernous malformations using flat-panel catheter angiotomography

BACKGROUND

Spinal cord cavernous malformations (CMs) are associated with 2 types of angiographically occult "cryptic venous anomalies," which differ in location with respect to the spinal cord. The anatomic distinction between superficial and intramedullary is important in that the latter heighten the risks of CM resection.

OBJECTIVE

To report the observations of both types of cryptic venous anomalies documented during spinal digital subtraction angiography enhanced with flat-panel catheter angiotomography (FPCA).

METHODS

Spinal digital subtraction angiography enhanced with FPCA was performed in 2 adult patients with magnetic resonance imaging--documented intramedullary spinal cord CMs and prominent, nonspecific flow voids at the same levels. FPCA was obtained by selective injection of left T4 (case 1) and left T9 (case 2) with 5F Cobra 2 catheters (Terumo, Japan) during a 20-second rotational acquisition. Thirty milliliters of a 75% saline and 25% contrast solution (Omnipaque 300; GE) was administered. The rotational data set was reconstructed on a dedicated workstation (Leonardo; Siemens, Erlangen, Germany) through the use of regular and high-resolution matrixes, 0.4- and 0.1-mm voxel size, respectively.

RESULTS

Spinal digital subtraction angiography was unremarkable in both cases. In case 1, FPCA findings indicated an atypical network of prominent posterior perimedullary veins. In case 2, FPCA identified radially oriented channels forming a caput medusae pattern collecting into an enlarged intramedullary vein.

CONCLUSION

The unique ability of FPCA to image the spinal venous system enables the angiographic detection and characterization of abnormal spinal veins associated with CMs. Differentiating between the types of associated cryptic venous malformations may aid in surgical planning because the intramedullary type is associated with a higher risk of surgical complication.

Vein of Galen malformations: epidemiology, clinical presentations, management

The vein of Galen aneurysmal malformation is a congenital vascular malformation that comprises 30% of the pediatric vascular and 1% of all pediatric congenital anomalies. Treatment is dependent on the timing of presentation and clinical manifestations. With the development of endovascular techniques, treatment paradigms have changed and clinical outcomes have significantly improved. In this article, the developmental embryology, clinical features and pathophysiology, diagnostic workup, and management strategies are reviewed.

Classification schemes of cranial dural arteriovenous fistulas

The clinical presentation of dural arteriovenous fistulas (DAVFs), in particular the associated risk of intracranial hemorrhage, shows a strong correlation with their pattern of venous drainage. The two most commonly used and clinically accepted DAVF classifications are the Merland-Cognard classification and the Borden classification, both based on the morphology of the venous drainage. A revised classification that grades DAVFs through a combination of angiographic and clinical features has also been proposed. This article offers a review of these various classification schemes, and discusses their application to treatment decision making.

Unilateral hypoplasia of the rostral end of the superior sagittal sinus

BACKGROUND AND PURPOSE

Hypoplasia of the rostral third of the SSS is a well-known variant and constitutes the most frequent variation of the SSS after preferential drainage to one of the transverse sinuses. Our aim was to describe unilateral hypoplasia of the rostral end of the SSS.

MATERIALS AND METHODS

CTA performed in 100 consecutive patients studied for conditions other than dural sinus thrombosis was reviewed for the presence of a unilateral or bilateral hypoplastic rostral SSS. Associated dural venous sinus anomalies were recorded as well. The angiographic anatomy of unilateral hypoplastic rostral SSS was illustrated by 2 cases further imaged with DSA.

RESULTS

Unilateral hypoplastic rostral SSS was found in 7 patients (7%). In all cases, compensatory drainage occurred through a large superior frontal vein that joined the SSS in the region of the coronal suture. Three of the 7 patients with a unilateral hypoplastic rostral SSS had at least another dural venous sinus anomaly. Complete or bilateral hypoplastic rostral SSS was noted in 3 patients (3%).

CONCLUSIONS

Unilateral hypoplastic rostral SSS is more than twice as frequent as bilateral hypoplastic rostral SSS. It is the most frequently encountered variation of the SSS. Knowledge of this anatomic variation is important to avoid diagnostic pitfalls and to avoid erroneously mistaking it for a thrombosis. Four types of variations of the rostral SSS may be identified: 1) classic anatomy with a fully developed rostral SSS; 2) duplication of the rostral SSS; 3) complete or bilateral hypoplastic rostral SSS; 4) unilateral hypoplastic rostral SSS. The 4 types of rostral SSS variations can be explained by studying the embryologic development of the SSS.