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Clinical case: Optic neuropathy due to optic nerve compression - want to learn more about it?

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Clinical case: Optic neuropathy due to optic nerve compression

In this article, we describe a case of a woman who presented with progressive deterioration of visual function in her left eye. Her diagnosis was left optic neuropathy, but what was the cause of her pathology? Stick around to find out the answer, and more details about her examinations, investigations, as well as the management strategy, and evolution. You will also learn about the anatomical knowledge pertinent to this clinical case.

After reviewing this case you should be able to describe the following:

  • The functional anatomy and pathway of the optic nerve.
  • The clinical relationship between tumors of the sella turcica and the optic nerve.
  • What is meant by a pterional craniotomy; the clinical importance of the pterion.
Key facts
Optic nerve pathway Ganglion cells of retina (optic head) -> orbital part -> optic canal -> middle cranial fossa (cranial part) -> optic chiasm -> optic tract -> lateral geniculate body of thalamus -> optic radiation -> visual cortex
Vision dysfunctions

Impingement distal to the optic chiasm -> vision defects only in ipsilateral eye

Pressure on the optic chiasm -> bitemporal defects

Impingement proximal to the optic chiasm -> vision defects in both eyes

Pterional craniotomy Neurosurgical approach to the middle cranial fossa, anterior cranial fossa, suprasellar and parasellar structures, and the Circle of Willis via the pterion.

This article is based on a case report published in the Journal "Case Reports in Surgery" in 2015, by Caroline C. Jadlowiec, Beata E. Lobel, Namita Akolkar, Michael D. Bourque, Thomas J. Devers, and David W. McFadden.

Case Description

History & Examination

Figure 1. Results of left visual field examination showing visual perception only in superonasal quadrant. 

The patient was a 50-year-old woman with no pertinent prior disease; she was referred to the Neurosurgery Department for progressive deterioration of visual function in her left eye of 3-4 months duration. Serial visual field examination of the left eye showed a loss of three visual quadrants, with only the superonasal quadrant showing significant remaining visual function (Figure 1). The right visual field was normal, suggesting the patient had a left optic neuropathy.

Imaging

T1 contrast MRI revealed a sella turcica meningioma (tumor arising from the dura overlying sella turcica) measuring approximately 1.7 cm × 1.9 cm × 1.3 cm that was presumably compressing the left optic nerve (Figure 2).

Figure 2. Coronal T1 MRI depicting tumor and surrounding structures.

Management & Surgery

The patient underwent a left pterional craniotomy (Figure 3). The dura was opened and the Sylvian fissure was expanded. Upon elevating the left frontal lobe, the tumor became visible. Further examination showed that the tumor was compressing the nerve on its medial side whereas the initial part of the A1 segment of the anterior cerebral artery (ACA) was compressing it on its lateral side (Figure 4).

Figure 3. A. Lateral view of the skull with the individual skull bones in different colors. The approximate incision line for a pterional craniotomy is shown as is the skull region known as pterion. B. View of interior skull base showing the lesser wing of sphenoid and the superior orbital fissure, both of which are critical landmarks in the pterional craniotomy (see text).

The tumor was then internally decompressed, and the ACA separated from the nerve. Once the tumor was extirpated, the nerve became freely movable. The patient recovered well from the procedure well and was discharged one week postoperative.

Evolution

A complete neuro-ophthalmologic evaluation performed seven days after surgery revealed best-corrected visual acuity of 0.8 (mild visual loss) in both eyes with no relative afferent pupillary defect (no difference in the response of the eyes to bright light). Examination of the right eye showed a normal pink disc, whereas in the left fundus there was pallor (paleness) of the temporal part of the optic disc (indicating some remaining damage to the left optic nerve, which is consistent with the mild loss). The right visual field was normal, whereas in the left visual field there was still central scotoma (dark spot) and some temporal field depression.

Follow-up visual field examination performed ten months after surgery showed again a normal right visual field but a solitary paracentral scotoma in the left visual field, indicating that the left optic nerve had sustained some permanent damage. There was, however, no evidence of residual tumor on MRI performed six months after surgery.

Figure 4. A. High magnification photograph of the left orbit and sella turcica region in a cadaver; this view is similar to that would be available to the surgeons, albeit without the removal of the orbital roof. In A the tumor (white dashed circle) is drawn in and reduced in size so that the surrounding anatomy can be seen. The anterior cerebral artery (ACA), which has also been drawn in, is shown originating from the internal carotid artery (ICA) and crossing over optic nerve (lateral to medial) and as it does so, acting as a boundary to the nerve, preventing the nerve from moving in response to the tumor. The tumor is shown compressing the nerve on its medial side so that the nerve is thus being strangulated from both sides. In the surgical procedure described in this report, the tumor was removed and nerve freed from the artery. C is a cadaver dissection of the entire extent of the left optic nerve from the eyeball to the optic chiasm. The bony walls of the optic canal have been removed. C is courtesy of Dr. John Selhorst. TS, sella turcica. MC, middle cerebral artery.

Surgical and Anatomical Considerations

This case describes a patient with optic nerve strangulation by the A1 segment of the ACA in a patient because of the presence of a sella turcica tumor. Once the tumor was debulked, the optic nerve was seen to have been compressed between the tumor and the artery. The preoperative neuro-ophthalmic examination had shown a significant deficit in the left visual function, which dramatically improved immediately after surgery (but which still remained less than normal). Visual loss resulting from mechanical compression of the optic nerve by tumors, particularly by sella turcica meningiomas is well established, and vascular elements (e.g. ACA as in this case) may play a significant role in the mechanism of this compression.

The left ACA is one of a pair of arteries that supplies most midline portions of the frontal lobes of the brain and superior medial parietal lobes. Both of the anterior cerebral arteries arise from the respective internal carotid arteries and are part of the circle of Willis (Figure 5).

Figure 5. The large image shows a magnified cadaver photograph of the Circle of Willis. The smaller image shows where the Circle is located at the base of the brain.

The A1 segment of the ACA, which is the segment of interest for this case, originates from the internal carotid artery and extends to the anterior communicating artery, which connects the left and right arteries. The circle of Willis (circulus arteriosus cerebri) is a collection of arteries that are located at the base of the brain. The “circle” was named after Thomas Willis by his student, Richard Lower. Willis authored, Cerebri Anatome, which described and depicted this vascular ring of arteries. The circle of Willis encircles the stalk of the pituitary gland and provides a connection between the internal carotid and vertebrobasilar arterial brain supply systems. The circle of Willis is formed when each of the internal carotid arteries divides into the anterior cerebral artery and the middle cerebral artery. The anterior cerebral arteries are then united by an anterior communicating artery. Posteriorly, the basilar artery branches into a left and right posterior cerebral artery, forming the posterior circulation. The circle of Willis is completed by the posterior communicating arteries, which join the posterior and internal carotid arteries.

Figure 6. Arteries of the brain - inferior view


 

Explanations to Objectives

Objectives

  • The functional anatomy and pathway of the optic nerve.
  • The clinical relationship between tumors of the sella turcica and the optic nerve.
  • What is meant by a pterional craniotomy; the clinical importance of the pterion.

Anatomy & Pathway of the Optic Nerve

The optic nerve (Figure 4C) may be divided into four sections: (a) optic head, (b) orbital part, (c) intracanalicular part (in the optic canal), and (d) cranial part. The axons of the ganglion cells of the retina converge in the innermost layer of the retina toward the optic nerve head. At the optic nerve head, the nerve fibers acquire myelin sheaths and become an approximately 4mm thick cord forming the nerve. The optic nerve is enveloped by the three meninges and thus also by the subarachnoid and subdural spaces.

The second part of the optic nerve (part b) traverses from the optic head through the orbit to the optic canal (part c). The nerve exits the optic canal into the middle cranial fossa (cranial part; part d) and assumes a medial direction, fusing with the contralateral nerve to form the optic chiasm. From the chiasm, the axons of the optic nerve travel in a posterior (dorsal) direction as the optic tract. The optic tract extends to the lateral geniculate body of the thalamus. The majority of the fibers of the optic nerve (and tract) synapse with neurons of the lateral geniculate body. Some fibers, however, bend medially into the superior colliculus and the pretectal region, and are responsible for optic reflexes. The axons of the neurons of the lateral geniculate body form the optic radiation that continues to the visual cortex in the occipital lobe.

Figure 7. Optic tract.

At the optic chiasm, all of the axons that arise from the lateral (temporal) halves of the two retinae continue without crossing into the ipsilateral optic tract whereas fibers from the medial (nasal) halves of the retinae, decussate to the optic tract of the contralateral side. Thus, all the light that enters the eyes from the left visual field (and therefore impinges on the right halves of both retinae) will be transmitted in the axons of the right optic tract and reach to the visual cortex of the right brain.

The fibers within the optic nerve, tract and radiation maintain a topographic relationship in the brain as visual field quadrants. These quadrants are separated by imaginary horizontal and vertical lines that pass through the fovea, which is the region of highest visual acuity of the retina. Thus, any partial lesion of the optic nerve will affect vision in particular areas of the visual field (the alternative would be for the fibers from each part of the retina to be randomly distributed in the nerve). Adjoining the intracranial part of the optic nerve are the anterior cerebral and anterior communicating arteries (Figure 5). The nerve is also adjacent to the internal carotid artery at its division into the anterior cerebral and middle cerebral arteries, to the first part of the posterior cerebral artery and to the ophthalmic artery. As in this case, any pressure from these arteries on the nerve (often as an aneurysm) can cause slow but increasing loss of visual acuity.

Relationship Between Tumors of the Sella Turcica and the Optic Nerve

The length of the intracranial portion of the optic nerve is typically about 10mm. The exact length is important because of the position of the chiasm relative to any underlying tumors especially the pituitary gland within the sellae turcica. Pituitary tumors can cause vision dysfunction if the tumor impinges on the optic nerve and the exact relationship between the chiasm and the tumor determines the nature of the visual field loss (which is partially dependent on the length of this portion of the nerve). Similarly, the visual loss in the patient in this case (unilateral) was based on the topographic relations of the fibers within the nerve and where the artery and tumor applied pressure on the nerve.

Figure 8. Paris views showing normal vision and loss of the temporal visual fields. Courtesy of Wikipedia.

Pathologies distal to the chiasm cause vision defects only on the ipsilateral eye. Bitemporal defects (loss of the temporal or outer visual fields) are the most common presenting finding in patients with pituitary tumors that affect vision due to impingement of the chiasm (Figure 8). Pathologies of optical track proximal to chiasm generally result in vision defects in both eyes.

Pterional Craniotomy

The pterional craniotomy is a unique approach that facilitates wide access to the skull base (Figure 3). It is named for the point on the skull called pterion, which is at the junction point of the frontal, temporal, greater wing of sphenoid, and parietal bones. The pterional craniotomy is a standard neurosurgical approach to the middle cranial fossa, anterior cranial fossa, suprasellar and parasellar structures, and the Circle of Willis. The pterional craniotomy is the ideal approach for resection of lateral skull base tumors (meningiomas, schwannomas, epidermoids, dermoids, fibrous dysplasia, orbital tumors, arachnoid cysts and brain malignancies) and the clipping of cerebral aneurysms (both ruptured and unruptured). This craniotomy is centered over the pterion.

The incision for this procedure should begin approximately 1 cm anterior to the tragus, starting at the zygoma. The temporalis muscle is then typically mobilized and retracted with the skin in a single flap. A skull bone flap is then made and temporarily removed. Once the bone flap is removed, the lesser sphenoid wing is thinned out in a medial direction, until the lateral edge of the superior orbital fissure is reached (Figure 3). This is an important step that enlarges the space between the anterior and middle fossa, therefore broadening access to the basal skull structures and the Circle of Willis.

Figure 9. Cadaver photographs showing the relationship between the middle meningeal artery and pterion. The artery is shown in the dura.

Pterion itself is a weak point in the skull that is clinically important pertaining to an extradural hematoma. The branches of the middle meningeal artery traverse the endocranial surface of the area surrounding pterion and this artery can be ruptured by a skull fracture in this region (Figure 9). The classic example of this injury is when a hockey player is struck by a puck in this region. When the artery ruptures blood fills the extradural space and, if not treated, compresses and displaces the brain leading eventually to death (mass effect).

Clinical case: Optic neuropathy due to optic nerve compression - want to learn more about it?

Our engaging videos, interactive quizzes, in-depth articles and HD atlas are here to get you top results faster.

Sign up for your free Kenhub account today and join over 1,007,531 successful anatomy students.

“I would honestly say that Kenhub cut my study time in half.” – Read more. Kim Bengochea Kim Bengochea, Regis University, Denver

Show references

Reference:

  • Mizrahi CJ, Moscovici S, Dotan S., and Spektor S. Optic Nerve Vascular Compression in a Patient with a sella turcica Meningioma. Case Reports in Ophthalmological Medicine Volume 2015, Article ID 681632, 3 pages.
  • Modified by Joel A. Vilensky PhD, Carlos A. Suárez-Quian PhD, Aykut Üren, MD.

Authors:

  • Joel A. Vilensky 
  • Carlos A. Suárez-Quian
  • Aykut Üren

Layout:

  • Abdulmalek Albakkar
  • Adrian Rad
© Unless stated otherwise, all content, including illustrations are exclusive property of Kenhub GmbH, and are protected by German and international copyright laws. All rights reserved.

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