The Neuro-Ophthalmology of Pituitary Tumors




Introduction


The main purpose of this chapter is to provide a practical review of the visual and oculomotor manifestations of pituitary tumors. The close anatomical relationship between the pituitary gland with the visual pathways and the cavernous sinuses explains the rather frequent occurrence of decreased vision, loss of peripheral visual field, diplopia, and a myriad of ocular complaints, which may announce the presence of a pituitary tumor. We will begin this chapter with relevant anatomical considerations, followed by description of common symptoms and signs associated with pituitary tumors. A detailed examination protocol, with review of different patterns of visual loss, visual field defects, and oculomotor abnormalities, will be provided. Correlation between neuro-ophthalmologic and imaging findings, discussion of the differential diagnosis, and visual outcome associated with different treatment modalities will be the concluding section.




Neuro-Anatomical Considerations


The optic chiasm is formed by the converging intracranial optic nerves and lies within the suprasellar cistern. When viewed in the sagittal plane, it is tilted forward at an angle of 45 degrees ( Figure 5-1 ). It measures 12 mm in width, 8 mm in the A-P diameter, and 4 mm in thickness. It lies about 10 mm above the pituitary gland and is separated from it by the diaphragma sellas, a reflection of the dura mater, which is penetrated only by the pituitary stalk. The position of the optic chiasm may vary in relation with the sella. In 15% of patients, it is located just above the tuberculum sellas (prefixed) and in 5% of patients it is located above the dorsum sellas (postfixed).




Figure 5-1


Sagittal 1 mm MRI of the sellar region. The optic chiasm is tilted 45 degrees within the suprasellar cistern ( arrows ). The most frequent anatomic position of the midchiasm is above the tuberculum sella as shown in this figure. An anterior or posterior position to this structure constitutes a prefixed or postfixed chiasm. Compressive lesions will therefore cause different field defects as a function of this anatomical variant.


At the chiasm, axons originating from retinal ganglion cells located in the nasal retina will decussate and adjoin uncrossed axons from the fellow eye to form the optic tract. The ratio of crossed-uncrossed fibers is 53% to 47%. For nearly one century, it was assumed that axons originating from inferonasal retina ganglion cells took a backward detour into the contralateral optic nerve (Wilbrand knee). In recent elegant experiments, Horton demonstrated that the knee described by Wilbrand was the result of the technique used to demonstrate the anatomy of the decussating fibers rather than a normal anatomical structure. The method used in the early 1900s involved monocular enucleation, which was performed to allow better definition of the “crossing fibers.” Using the same technique used by Wilbrand, Horton demonstrated in enucleated monkeys how crossing axons from the normal eye compressed the atrophic axons from the enucleated eye and formed a “loop,” which is not present in nonenucleated monkeys ( Figure 5-2 ).






Figure 5-2


Left panel showing a darkfield view of a normal primate optic chiasm with an absent Wilbrand knee. The images represent a radiotracer (L-2,3,4,5- 3 H) proline, administered by intravitreal injection in the left eye only. It stains myelin dark in contrast to the dark unstained right optic nerve. Right panel showing a darkfield view of the optic chiasm using the same labeling technique. The monkey had an enucleation of the right eye 4 weeks earlier with secondary wallerian degeneration. Note the normal fibers of the left optic nerve entering the proximal optic nerve.

(Reproduced with permission by Horton JC: Wilbrand’s knee of the primate optic chiasm is an artefact of monocular enucleation. Trans Am Ophthalmo Soc 1997; 95:579-609.)


The cavernous sinuses, which contain cranial nerves III, IV, and VI; V1 and V2; the internal carotid arteries; and the surrounding sympathetic fibers, form the lateral wall of the sella. The pituitary gland lies in between both cavernous sinuses.


The normal anatomy of the sellar region can be visualized with precision using multiplanar T1-weighted MRI scans ( Figure 5-3 ). In the coronal plane the chiasm has a dumbbell shape and lies within the suprasellar cistern. The intracranial optic nerves can be visualized anterior to the chiasm. The posterior chiasm is flanked superiorly by the third ventricle and inferiorly by the pituitary stalk. The pituitary gland lies flat in the coronal plane. In the sagittal plane, the tilted chiasm can be easily identified above the pituitary. The normal signal from the anterior pituitary is isointense to the brain and the posterior pituitary is hyperintense. In the axial plane, the converging optic nerves, chiasm, and optic tract can be identified. Following contrast administration, the pituitary gland, pituitary stalk, and cavernous sinuses enhance.




Figure 5-3


Coronal 1 mm, unenhanced T1 MRI sections of the chiasm. In the left panel the optic nerves are emerging from the optic canal to begin the decussation ( arrows ). Middle panel shows the midchiasm and right panel shows the optic tracts.


The relationship of the chiasm with the circle of Willis is of significant importance when considering ocular signs of pituitary tumors. The supraclinoid carotid arteries ascend lateral to the optic chiasm. The anterior communicating and anterior cerebral arteries are anterior and superior, and the posterior communicating and posterior cerebral arteries lie posterior and below. The principal arterial supply to the chiasm originates from an inferior anastomotic circle formed by the hypophyseal artery (branch of the internal carotid artery), and branches from the posterior cerebral and posterior communicating arteries, which supply the main body of the chiasm and a superior anastomotic group that originates from the anterior cerebral arteries, which supply both the main body and lateral chiasm. Venous drainage is formed primarily by the pituitary portal system, which drains into the cavernous and petrosal sinuses.


Symptoms and Signs


The main symptoms and signs of pituitary tumors may be classified as endocrine, visual, oculomotor, and headache. This chapter will deal only with the neuro-ophthalmological manifestations and headache. Broadly speaking, the symptoms observed are the result of the relationship of the tumor with the displaced neighboring structures. Pituitary microadenomas cause primarily endocrine symptoms. Macroadenomas frequently cause compression of the chiasm and less commonly of the optic nerves or the optic tracts. The mass effect exerted by the tumor affects the axoplasmic flow and impairs vascular supply. If compression is exerted over a long period of time, functional recovery after decompression is significant. Abrupt compression, on the other hand, has a greater risk of suboptimal visual recovery.



  • 1.

    Visual symptoms: Slowly progressive loss of vision affecting one or both eyes is a frequent complaint. At times visual loss is acute in nature. Not infrequently, patients become aware of poor vision in one eye only when closing the fellow eye, and are unable to date the precise onset of symptoms. Monocular or binocular decreased brightness and poor color perception may be present as well. Loss of peripheral visual field may be reported. Given the slow growing rate of pituitary adenomas, it is not infrequent to find “silent visual deficits” among patients with macroadenomas; therefore, it is important to perform a careful neuro-ophthalmological examination in all patients with pituitary tumors.


  • 2.

    Oculomotor symptoms: Patients with lateral extension of pituitary tumors report diplopia and ptosis. These symptoms may be isolated or concurrent with visual loss and are typically slowly progressive as well.


  • 3.

    Headache: Large pituitary tumors exert a mass effect on the diaphragma sellas and the cavernous sinus. They lead to dural stretching in the first instance and ophthalmic nerve compression in the second. However, headache may also occur in the absence of overt compression of these structures and is probably the result of endocrine abnormalities interacting with an underlying migraine background. Patients with pituitary apoplexy may describe explosive headache and neck pain. We will discuss this syndrome in greater detail in this chapter.



Signs of Pituitary Adenomas: Visual Signs




  • 1.

    Visual acuity : Patients with pituitary adenomas may report poor vision. Decreased visual acuity in one or both eyes is frequently found on examination. When binocular visual loss is present, it is often asymmetric.


  • 2.

    Color vision : Poor monocular or binocular discrimination of red-green color, using the Ishihara or pseudoisochromatic plates, is often found among patients with pituitary adenomas. Bedside visual field testing, using a confrontation method with two red targets, may detect temporal-field red color desaturation. This is a valuable and specific sign, pointing to a chiasmal localization.


  • 3.

    Pupils : Examination of the pupils is of great value in patients with pituitary tumors. Anisocoria may occur in association with sympathetic and parasympathetic failure. The reaction to light may be impaired in one or both eyes. An afferent pupillary defect (APD) is relatively uncommon, despite the fact that visual acuity or field loss is frequently asymmetric. The reaction to light may be impaired as a result of partial or complete third nerve palsy (loss of efferent, parasympathetic function). In this case, the size of the pupil is in the 4 to 5 mm range, contrasting with the 8 mm size pupils seen with lesions affecting the third nerve in other locations in the neuraxis.


  • 4.

    Visual fields : A temporal visual field defect is a highly specific and sensitive sign of chiasmal localization. This observation also applies to monocular defects. Visual fields may be tested by confrontation. At times, bitemporal desaturation of red color may be found. Automated perimetry is of great value in patients with pituitary tumors. Since it is time-consuming, a normal attention span is required. This is generally the case in patients with pituitary tumors. Goldman, kinetic perimetry may be more suitable in uncooperative, ill patients.


    Computerized visual field (CVF) testing is of great value in patients with pituitary tumors. Over the last 2 decades, CVF testing has emerged as the preferred modality of testing the peripheral visual field. The Humphrey and the Octopus units are the best known at this time. Both of them provide a standardized, reproducible, and sensitive test of peripheral vision.


    A simplified explanation of CVF involves testing of the subject’s response to targets in different visual field locations while fixating at a central target. The main principle in static automated perimetry involves an evaluation of visual perception of stationary objects of constant size and changing intensity in different locations that are displayed at “threshold intensity” values at a given location tested. A threshold is defined as the stimulus intensity that has a 50% probability of being seen.


    The computer algorithm used in CVF measures the differential light sensitivity between the stimuli used against a constantly illuminated background. The programs allow display of the patient’s response in numeric and graytone format plots of the raw threshold data ( Figure 5-4 ). It provides a reliability index of fixation losses, and false-positive and false-negative responses. CVF has several programs, among them a program known as Statpac provides additional critical information. Statpac shows a comparison of the patient’s response with age-matched normal values at each location (total deviation) and mean deviation , which is a location-weighted mean of the values found in the total deviation plot. More localized abnormalities are also plotted in the pattern deviation plot. We recommend the use of automated perimetry whenever possible in patients with pituitary tumors.


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Jun 10, 2019 | Posted by in NEUROLOGY | Comments Off on The Neuro-Ophthalmology of Pituitary Tumors

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