The traditional approaches to symptomatic cholesterol granuloma (CG), the most common benign pathologic lesion of the petrous apex, have historically been transotic, including middle fossa, translabyrinthine, retrocochlear, or infra- or retrolabyrinthine approaches. These approaches were often fraught with risk to the vestibular or cochlear apparatus, the need for brain retraction, or lack of a natural drainage pathway after marsupialization of the granuloma. This article reviews the literature on the transnasal approach to petrous apex CGs, including medial, medial with carotid medialization, and transpterygoid approaches. Of the 19 reported CGs treated with endoscopic drainage, only one recurrence was noted.
- 1.
Cholesterol granuloma is the most common benign petrous apex lesion
- 2.
Cholesterol granulomas have a characteristic appearance on magnetic resonance imaging (MRI) that is hyperintense on both T1-weighted and T2-weighted sequences, owing to the fat and fluid content of these lesions
- 3.
The endonasal, endoscopic approach is safe, providing a low rate of recurrence and durable marsupialization of these lesions
- 4.
The medial endonasal approach is the most frequently used approach described in the literature.
Epidemiology of cholesterol granuloma
| EBM Question | Level of Evidence | Grade of Recommendation |
|---|---|---|
| What is rate of clinical and radiographic recurrence following endoscopic transnasal drainage of cholesterol granuloma? | 4 | C |
Clinical
Cholesterol granulomas are typically symptomatic or present with subtle symptoms such as headache. Larger lesions may present with hearing loss and cranial neuropathies.
Investigation
Computed tomography (CT) and MRI are typically used to characterize cholesterol granuloma. CT will typically show an expansive lesion of the petrous apex with bony erosion and scalloping. Thinning of the posterior wall of the sphenoid and petrous carotid artery dehiscence may also be seen.
Management of cholesterol granuloma
The traditional approaches to cholesterol granulomas have historically been transotic, including middle fossa, translabyrinthine, retrocochlear, or infralabyrinthine or retrolabyrinthine approaches. These approaches were often fraught with risk to the vestibular or cochlear apparatus, the need for brain retraction, or the lack of a natural drainage pathway after marsupialization of the granuloma. Since the early 1990s endoscopic transsphenoidal drainage and marsupialization of petrous apex cholesterol granulomas has been described. This approach avoids potential risk to the auditory and vestibular organs, and provides a natural drainage pathway into the sinuses. In this article the authors review the literature on the transnasal approach to petrous apex cholesterol granulomas and the described approaches, including medial, medial with carotid medialization, and transpterygoid approaches.
Prognosis and Natural History
Slow-growing cholesterol granulomas or small lesions may remain asymptomatic for many years. Larger lesions left untreated may continue to expand and cause increasing signs and symptoms with progressive hearing loss, vertigo, facial weakness, and lower cranial nerve neuropathies. The endoscopic transnasal transsphenoid approaches offer a low recurrence rate, with low risk to inner ear structures and a natural drainage pathway into the sinonasal cavity.
Anatomy and Epidemiology
The petrous apex is a pyramidal structure comprising the most medial portion of the temporal bone. The base of the pyramid is the bony labyrinth including the semicircular canals and cochlea. Anteriorly, the base of the pyramid is partly formed by the tensor tympani and internal carotid artery. The superior petrous apex makes up a large portion of the middle cranial fossa floor, extending from the superior semicircular canal and arcuate eminence to the ascending portion of the internal carotid artery and the Gasserian ganglion situated in Meckel’s cave. The superior surface extends from the superior petrosal sinus posteriorly to the petrosphenoid suture line anteriorly. The posterior surface of the petrous apex forms the anterolateral wall of the posterior cranial fossa, and extends medially from the posterior semicircular canal and the endolymphatic sac to the petroclinoid ligament and Dorello’s canal containing the abducens nerve. This posterior surface then extends from the petro-occipital suture line inferiorly to the superior petrosal sinus superiorly. Inferiorly, the petrous pyramid is bounded by the jugular bulb and the inferior petrosal sinus. The inferior surface also has a foramen for the entry of the internal carotid artery. Medial to the jugular fossa is a depression that is associated with the cochlear aqueduct (perilymphatic duct). The petrous bone articulates with the greater wing of the sphenoid anteriorly. The foramen lacerum is found between the petrous apex and the sphenoid bone and contains, but does not transmit, the internal carotid artery.
The posteroinferior margin of the temporal bone articulates with the occipital bone. Laterally, the petrous temporal bone fuses with the squamous portion of the temporal bone at the petrosquamosal fissure. The transverse portion of the internal carotid artery and the internal auditory canal traverse the petrous pyramid. The petrous apex can be divided into anterior and posterior areas by a vertical plane through the modiolus of the cochlea and the internal auditory canal. The posterior petrous apex, located between the internal auditory canal and the vestibular apparatus, is usually composed of compact bone and is rarely involved by disease processes. The petrous bone may be pneumatized, diploic (marrow-filled), or sclerotic. About 10% of the population has pneumatization of the petrous apex. Primary petrous apex lesions are those that arise from the central petrous apex or the anatomic boundaries of the region. Secondary lesions are those that impinge on the petrous apex from an outside source, possibly from invasion from a bordering region or from a metastatic lesion. Primary lesions account for approximately 40% of petrous apex lesions.
Management of cholesterol granuloma
The traditional approaches to cholesterol granulomas have historically been transotic, including middle fossa, translabyrinthine, retrocochlear, or infralabyrinthine or retrolabyrinthine approaches. These approaches were often fraught with risk to the vestibular or cochlear apparatus, the need for brain retraction, or the lack of a natural drainage pathway after marsupialization of the granuloma. Since the early 1990s endoscopic transsphenoidal drainage and marsupialization of petrous apex cholesterol granulomas has been described. This approach avoids potential risk to the auditory and vestibular organs, and provides a natural drainage pathway into the sinuses. In this article the authors review the literature on the transnasal approach to petrous apex cholesterol granulomas and the described approaches, including medial, medial with carotid medialization, and transpterygoid approaches.
Prognosis and Natural History
Slow-growing cholesterol granulomas or small lesions may remain asymptomatic for many years. Larger lesions left untreated may continue to expand and cause increasing signs and symptoms with progressive hearing loss, vertigo, facial weakness, and lower cranial nerve neuropathies. The endoscopic transnasal transsphenoid approaches offer a low recurrence rate, with low risk to inner ear structures and a natural drainage pathway into the sinonasal cavity.
Anatomy and Epidemiology
The petrous apex is a pyramidal structure comprising the most medial portion of the temporal bone. The base of the pyramid is the bony labyrinth including the semicircular canals and cochlea. Anteriorly, the base of the pyramid is partly formed by the tensor tympani and internal carotid artery. The superior petrous apex makes up a large portion of the middle cranial fossa floor, extending from the superior semicircular canal and arcuate eminence to the ascending portion of the internal carotid artery and the Gasserian ganglion situated in Meckel’s cave. The superior surface extends from the superior petrosal sinus posteriorly to the petrosphenoid suture line anteriorly. The posterior surface of the petrous apex forms the anterolateral wall of the posterior cranial fossa, and extends medially from the posterior semicircular canal and the endolymphatic sac to the petroclinoid ligament and Dorello’s canal containing the abducens nerve. This posterior surface then extends from the petro-occipital suture line inferiorly to the superior petrosal sinus superiorly. Inferiorly, the petrous pyramid is bounded by the jugular bulb and the inferior petrosal sinus. The inferior surface also has a foramen for the entry of the internal carotid artery. Medial to the jugular fossa is a depression that is associated with the cochlear aqueduct (perilymphatic duct). The petrous bone articulates with the greater wing of the sphenoid anteriorly. The foramen lacerum is found between the petrous apex and the sphenoid bone and contains, but does not transmit, the internal carotid artery.
The posteroinferior margin of the temporal bone articulates with the occipital bone. Laterally, the petrous temporal bone fuses with the squamous portion of the temporal bone at the petrosquamosal fissure. The transverse portion of the internal carotid artery and the internal auditory canal traverse the petrous pyramid. The petrous apex can be divided into anterior and posterior areas by a vertical plane through the modiolus of the cochlea and the internal auditory canal. The posterior petrous apex, located between the internal auditory canal and the vestibular apparatus, is usually composed of compact bone and is rarely involved by disease processes. The petrous bone may be pneumatized, diploic (marrow-filled), or sclerotic. About 10% of the population has pneumatization of the petrous apex. Primary petrous apex lesions are those that arise from the central petrous apex or the anatomic boundaries of the region. Secondary lesions are those that impinge on the petrous apex from an outside source, possibly from invasion from a bordering region or from a metastatic lesion. Primary lesions account for approximately 40% of petrous apex lesions.
Pathophysiology of cholesterol granuloma
Cholesterol granuloma of the petrous apex forms as a result of foreign-body giant-cell reaction to cholesterol crystals. The poor ventilation, interference with drainage, and hemorrhage in a usually pneumatized space are predisposing factors leading to the formation of the cyst. Negative pressure from air resorption leads to the degradation of the blood and formation of cholesterol crystals. These crystals initiate a foreign-body reaction that results in granuloma or cyst formation. The cyst wall is made up of fibrous connective tissue and lacks the keratinizing squamous epithelium seen in cholesteatoma. Foreign-body giant cells, hemosiderin-laden macrophages, and cholesterol crystals are seen on pathologic evaluation.
Clinical
Cholesterol granulomas are often asymptomatic and may be discovered incidentally on imaging for other reasons. When symptoms are present they may be subtle, such as headache or hearing loss. Cholesterol granulomas of the petrous apex can be dangerous, due to their proximity to the inner and middle ear structures and their proximity to the seventh, eighth, and lower cranial nerves, and more overt signs and symptoms such as permanent hearing loss, cranial neuropathies, and osseous destruction can occur if the cholesterol granuloma is left untreated and continues its expansile growth.
Investigation
Both CT and MRI are valuable in the workup of petrous apex cholesterol granuloma. A characteristic finding on CT of a cholesterol granuloma is a sharply marginated expansile lesion with bony erosion ( Fig. 1 ). The lesions are avascular and therefore do not enhance with contrast, and are isodense with brain tissue. A thin, peripheral calcified rim may be noted as well as pneumatization of the contralateral petrous apex. The MRI findings of cholesterol granuloma are unique. The lesions demonstrate high signal intensity on both T1-weighted and T2-weighted images ( Figs. 2 and 3 ). The unique increased signal intensity on the T1-weighted image may be due to the combination of cholesterol crystals, chronic hemorrhage, and proteinaceous crystals. Occasionally it may be difficult to distinguish a small cholesterol granuloma from normal marrow because both may demonstrate increased signal intensity on T1-weighted images; however, they may be differentiated by examining the T2-weighted images. Marrow fat will exhibit a progressive decrease in signal intensity with increasing T2 weighting (fat saturation). Cholesterol granulomas maintain their high signal intensity on T2-weighted images.
Specialized MRI pulse sequences may sometimes be needed to evaluate these lesions. Partial saturation gradient recalled echo (GRE) sequences may be employed instead of the usual spin-echo sequences. GRE imaging demonstrates an enlarging peripheral ring of decreased signal intensity as the echo time is lengthened. This ring indicates a peripheral magnetic susceptibility effect, suggesting that hemosiderin-laden macrophages are present on the wall of the lesion. Whereas this pattern may be possible in a thrombosed aneurysm, it is unlikely in vestibular schwannomas, cholesteatomas, or mucoceles. Protein chemical shift imaging may also be used to evaluate these lesions. The mixed population of aliphatic (CH2) and water protons produces a cyclic pattern of signal intensity in the center of the lesion. A cholesteatoma may produce this pattern but typically a thrombosed aneurysm, schwannoma, or mucocele would not. Because of this unique ability to evaluate cholesterol granulomas, MRI is now used both for preoperative evaluation and to follow the patient postoperatively for recurrence.
Management of cholesterol granulomas
Surgical Indications and Approaches
Treatment of cholesterol granulomas is based on presentation and progression of symptoms. The usual presentation is unilateral headaches; however, cranial nerve symptoms such as diplopia, hearing loss, or facial anesthesia can also occur. CT and MRI will reveal either an expanded or nonexpanded petrous apex. For nonexpanded lesions, it is at times difficult to know how much of the patient’s symptoms are a result of the petrous apex disease. In this situation, observation and serial imaging can be helpful in ensuring optimal selection of medically recalcitrant patients for surgical treatment.
The key to successful petrous apex cholesterol granuloma surgery is surgical drainage and aeration. Complete surgical removal is rarely performed because the lesion lacks an epithelial lining. Multiple surgical approaches have been described for treating these lesions, including infralabyrinthine, transcanal infracochlear, transsphenoidal, middle cranial fossa, and retrosigmoid approaches. The approach taken depends on the patient’s hearing and the site and extent of the lesion.
Traditional approaches to the petrous apex include transtemporal and middle fossa approaches. For the drainage of cystic lesions such as cholesterol granulomas, approaches to the petrous apex include infralabyrinthine, infracochlear, middle cranial fossa, and transsphenoidal approaches for ears with serviceable hearing. In patients with minimal residual hearing without the need for hearing preservation, translabyrinthine approaches can be used. In poorly pneumatized temporal bones transtemporal approaches to the petrous apex may not be feasible, and continuous drainage and aeration of the petrous apex may be difficult to maintain. Transtemporal approaches pose a potential risk to hearing and balance function as well as to facial nerve function. The transcranial middle fossa approach is technically difficult, and suffers from the lack of a permanent drainage pathway. Some degree of brain retraction is necessary, which carries the risk of brain injury.
The retrosigmoid and middle cranial fossa approaches are designed for hearing preservation but do not provide for permanent drainage or aeration. Chemical meningitis from subarachnoid space contamination by the contents of the cholesterol granuloma is also a risk. Drainage, aeration, and hearing preservation are the goals of the infralabyrinthine, transcanal infracochlear, and transsphenoidal approaches. Detailed analysis of the preoperative CT scan to define the jugular bulb, facial nerve, sigmoid sinus, bony labyrinth, and posterior wall of the sphenoid is mandatory before surgery.
The infralabyrinthine approach shares the same goals as the other transtemporal approaches. Thedinger and colleagues reported on 3 patients who underwent this approach for cholesterol granuloma. Two required surgery for recurrence. By contrast, Goldofsky and colleagues reported on 9 patients followed from 1 to 10 years who underwent such a procedure, with only one recurrence. Hearing results from 14 patients showed 7 improved with 7 remaining the same. The infralabyrinthine approach involves a simple mastoidectomy then removal of the air cells in Trautmann’s triangle. The sigmoid sinus is followed until the jugular bulb is identified: this represents the inferior margin of the approach. The semicircular canals are skeletonized and the infralabyrinthine air cell tract is developed anteriorly. Once the lesion is identified, it is evacuated and the opening is enlarged. A stent may be placed to prevent stenosis of the opening. This approach is limited if the patient has a high jugular bulb that limits access to the infralabyrinthine air cell tract.
The transcanal infracochlear approach involves a postauricular incision and reflection of the ear anteriorly. Typanomeatal flaps are developed, and the external auditory canal is enlarged anteriorly and inferiorly to expose the hypotympanum. The chorda tympani is followed inferoposteriorly to define the extent of posterior dissection possible without injury to the facial nerve. The air cell tract below the cochlea is developed in the hypotympanum to expose the course of the carotid artery and the jugular bulb. The round window provides the superior line of dissection, and Jacobson’s nerve leads to the “crutch” of the carotid and jugular bulb. If the plane of dissection remains below the round window, the internal auditory canal structures will not be at risk. Once the lesion is entered, it is drained and a catheter may be placed. A high jugular bulb does not block access via this route as it does with the infralabyrinthine approach, and provides more dependent drainage for the cyst.
The retrosigmoid approach is performed through a craniotomy posterior to the sigmoid sinus. The cerebellum is retracted posteromedially, allowing for access to the cerebellopontine angle. This procedure has been of limited usefulness because of the location of most petrous apex cholesteatomas anterior of the internal auditory canal. The interposition of the cerebellum, as well as brainstem and cranial nerves, makes total extirpation of these tumors extremely difficult. This approach may be useful for removal of petrous apex cholesteatomas with significant intracranial extension.
If the lesion does make a significant impression on the sphenoid, a transsphenoidal approach may be used. The approach has excellent panoramic endoscopic exposure but risks the optic nerve and internal carotid artery. The transsphenoidal approach is useful when the cholesterol granuloma forms a large surface area against the posterior/lateral wall of the sphenoid sinus. Multiple approaches may be used toward the sphenoid, including external ethmoidectomy sphenoidotomy, intranasal sphenoethmoidectomy, intranasal sphenoidotomy, transseptal sphenoidotomy, or transpalatal approaches. The lateral and superior walls of the sphenoid sinus must be closely examined to locate indentations of the pituitary gland, optic nerve, maxillary nerve, carotid artery, and cavernous sinus. Once the wall of the cyst is identified, it can be opened and drained. A large opening should be created to decrease the chance of postoperative stenosis.
Literature Review of Endoscopic Approaches
Endoscopic endonasal approaches to the ventral skull base have been used increasingly over the past 10 to 15 years, and are generally categorized based on their orientation in coronal and sagittal planes. For all of these approaches, the sphenoid sinus is the starting point and provides orientation to important vascular and neural structures.
In 1994 Fucci and colleagues described endoscopic drainage of a 2.5-cm giant cholesterol granuloma of the petrous apex. Their patient was a 36-year-old woman with a chief complaint of headache, with a CT scan showing a 2.5-cm lytic lesion in the left petrous apex, the anteromedial extent of which abutted the posterior wall of the sphenoid sinus. Their procedure took down the anterior wall of the sphenoid sinus, and exposed the posterior wall of the sphenoid sinus and the crest of bone along the posterolateral wall of the sphenoid sinus, identifying the carotid artery. The posterior wall of the sphenoid sinus was removed and a Hardy pituitary dissector was used to remove the bone of the posterior wall of the sphenoid sinus over the cholesterol granuloma. The cyst was exposed and opened from medial to lateral, the fluid was evacuated, and the anterior wall was marsupialized. A T-shaped silicone stent was placed into the cyst to maintain a tract between the cyst and the nasal cavity. Follow-up nasal endoscopy showed the stent to be open and in place at 3 months after surgery.
Michaelson and colleagues reported endoscopic management of a petrous apex cholesterol granuloma in a 13-year-old African American girl presenting with a 2-month history of intermittent right-sided, retro-orbital headaches and diplopia. MRI scan showed a large petrous apex mass abutting the sphenoid sinus. Endoscopic drainage of the granuloma was performed using a Messerklinger approach with a right sphenoidotomy. The inferior posterolateral wall of the sphenoid sinus was removed revealing a large, dark, fluid-filled cyst in the petrous apex (see Fig. 3 ; Fig. 4 ). The cyst was opened, drained, and widely marsupialized. Headaches were immediately relieved and sixth nerve function returned within 1 week. Six months after surgery, she remained asymptomatic with a widely patent posterior sphenoid sinus drainage port on office endoscopy.
