Introduction
Hyrtl's fissure is a temporary anatomical feature that appears in a fetus's developing petrous temporal bone. Normally, it closes as the bone hardens around the 24th week of pregnancy. Sometimes, however, it remains open after birth and has been noted as a rare cause of cerebrospinal fluid (a clear, colorless body fluid found in the brain and spinal cord, which acts as a cushion and helps protect these structures) leak near the inner ear. Spontaneous cerebrospinal fluid (CSF) leakage from the ear is uncommon.
Proper diagnosis and treatment are crucial to preventing serious complications like meningitis. Diagnosing this condition relies on CT (computer scan) scans, so a thorough understanding of embryology and anatomy is essential to identify the various pathways of congenital fistulas (an abnormal connection or passageway between two organs or vessels that normally do not connect). These pathways include the facial canal, petromastoid canal, and tympanomeningeal (Hyrtl's) fissure, which can cause CSF leakage without abnormalities in the nearby membranous labyrinth. This article explains the significance of Hyrtl's fissure.
What Is Hyrtl's Fissure?
Hyrtl’s (tympanomeningeal) fissure is a notable feature in the developing fetal temporal bone that typically closes by the 24th week of gestation (nine-month periods of pregnancy) through normal bone formation. In a small number of people, it remains open as a variation of normal anatomy. This abnormality can sometimes cause a perilabyrinthine fistula or other clinically significant complications.
Hyrtl’s fissure and the cochlear aqueduct are located close to each other. If Hyrtl’s fissure is present, it may look like a series of small air cells or be blocked by arachnoid or fibrous tissue. The cochlear aqueduct starts wide at its funnel-shaped medial opening in the posterior cranial fossa, narrows significantly as it passes into the anterior part of the jugular foramen, and becomes very narrow in the otic capsule, where it opens into the basal turn of the cochlea.
How Does Hyrtl's Fissure Develop?
Hyrtl’s fissure developed in the fetus from the 16th to the 18th weeks of pregnancy; the outer edge of the round window is already turning into bone, but the inner edge is still cartilage, to which the round window membrane is attached. During this stage, the early cochlear aqueduct is straight and relatively wide. It lies next to the round window niche and contains the periotic duct, Hyrtl’s fissure at the back and side, and the inferior cochlear vein at the front and side. This creates a connection between the round window niche and the pars nervosa of the jugular fossa, where the cochlear aqueduct and Hyrtl’s fissure open near the superior ganglion of the glossopharyngeal nerve. Hyrtl’s fissure, which links the middle ear to the back of the skull, is also called the tympanomeningeal fissure or hiatus. Between the 20th and 24th weeks of pregnancy, the cartilaginous bar around the round window gradually turns into bone. This happens as the ossification centers of the cochlear and canalicular otic capsules merge and fuse.
By the 24th week, Hyrtl’s fissure closes, making the round window niche a part of the middle ear, no longer connected to the back of the skull. This closure also helps form the outer wall of the mature cochlear aqueduct. At the same time, the petrous apex and internal auditory canal begin to ossify, forming the inner wall of the cochlear aqueduct.
The petrous apex ossifies slightly before the canalicular otic capsule, and by around the 22nd week, the inferior cochlear vein is enclosed in a separate bony channel. In contrast, a study of the temporal bones of seven adults who had died from spontaneous subarachnoid hemorrhage found that blood had entered the inner ear structures through the internal auditory canal and, possibly, the cochlear aqueduct, depending on its size.
What Is the Clinical Significance of Hyrtl's Fissure?
1. First Documented Case: The study represents the first documented imaging case of Hyrtl’s fissure, highlighting its rarity and clinical importance.
2. Possible Route for Congenital Fistula: Hyrtl's fissure can be a pathway for perilabyrinthine congenital fistulas, allowing cerebrospinal fluid (CSF) leakage into the middle ear.
3. Historical Anatomy: Initially described by Hyrtl in 1936, this fissure was later renamed the "tympanomeningeal fissure or hiatus" by Anton and Bast.
4. Embryonic Development: During fetal development (16-18 weeks), Hyrtl’s fissure connects the fossula of the round window with the posterior cranial fossa, allowing direct communication between these structures.
5. Ossification Process: Normally, Hyrtl's fissure closes between 24 and 26 weeks of gestation as the cartilaginous bar is replaced by bone. However, incomplete ossification can result in a persistent tympano meningeal fissure.
6. Rare Persistence: Persistent Hyrtl's fissure is an anatomical variant that occurs in a tiny proportion of the population, with few documented cases in adults and children.
7. Associated Complications: In rare instances, the persistent fissure can lead to conditions like meningocele, as previously reported in the medical literature.
8. Diagnostic Challenges: The presence of a CSF fistula through Hyrtl’s fissure can be difficult to diagnose and may only be revealed through symptoms like recurrent meningitis, clear otorrhea, or rhinorrhea.
9. Imaging Techniques: Accurate diagnosis typically requires high-resolution CT scans and MR imaging to detect the presence of CSF in the middle ear and to assess the surrounding bone structures.
10. Surgical Treatment: Unlike acquired fistulas, congenital CSF leakage through Hyrtl’s fissure often necessitates surgical intervention to prevent complications such as meningitis. Various surgical techniques, including osteoconductive biomaterials, have been developed to address this issue effectively.
11. Research Implications: The studies have prompted further review and research into the developmental anatomy of the region and the potential complications arising from the patency of Hyrtl’s fissure.
Conclusion
This disorder is rare, so misdiagnosis or delayed diagnosis is common, which can delay suitable therapy. Better knowledge of possible sites and pathways of fistulas, even rare ones, is necessary. Radiologists and surgeons should clearly understand and carefully examine the different pathways of spontaneous CSF leakage. Hyrtl’s fissure is a possible, infrequent fistula route that can be misdiagnosed even with CT. Treatment for this congenital fistula involves filling the bone pathway with biomaterials.
