This page will diverge from the look and feel of the pages that have, thus far, dealt with the topic of hyperadrenocorticism . The reason is that surgery is a separate and much more complicated treatment method than any of the medical approaches. ..particularly surgery of the pituitary gland ( the hypophysectomy) which lies at the base of the brain. Neurosurgery is a topic in itself, and may be touched upon during the separate discussion of general neuroanatomy. The surgical treatment of ADH can also pose a challenge, especially when the right adrenal gland lies in tight juxtaposition to the caudal vena cava, a major and critical vessel which, if accidentally "nicked" during adrenalectomy, can prove fatal!
a PDH in which the mass is either producing neurological signs and/or is observed to be changing in a way that is potentially life-threatening (e.g. sequential CT scans or MRIs show a growing mass)
medical treatment for Cushing's Disease is unsuccessful and clinical signs are negatively affecting the animal's quality of life.
Surgical Considerations
the approach to the base of the brain is, itself, quite the challenge, as it requires considerable skill and dexterity,accessing the pituitary gland through the "roof" of the mouth, then drilling into the bones structure above
As the exact location of the diseases pituitary gland can not be assuredly pinpointed without imaging techniques, though general landmarks are identified, there is no room for error so dynamic CT imaging is performedpre-operatively in order to obtain a precise three dimensional orientation and location of the pituitary gland.
A thorough knowledge of the surgical anatomy is essential as there are vital vessles, nerves and vascular sinuses in the area and damaging these other structures during the procedure can result in permanent damage or even death to the animal
Since the pituitary gland lies outside of the blood-brain barrier (see another discussion of the blood-brain-barrier), radiographic contrast media can be injected at the time of CT imaging to enhance the identification of the pituitary relative to the rest of the brain
Then, once the approach is outlined, one must deal with important vessels, nerves and the boney protection, which encases the brain
The Brain: The development, general structure and organization of the brain has been described in some detail elsewhere. For the current discussion however, it is not necessary to refer to that description. The brain is surrounded by bone on all surfaces.
The Pituitary:
The pituitary gland (aka the hypophysis) in the dog contains three functional divisions: the anterior lobe (pars distalis adenohypophysis, pars infindibularis adenohypophysis), an intermediate portion (pars intermedia) and a posterior lobe (par proximalis neurohypophysis and par distalis neurohypophyis). The defect with PDH lies in the anterior lobe or intermediate portion. It is not necessary to elaborate on these areas for the purpose of this discussion.
(The pituitary sits at the "underside" (ventral aspect) of the brain within a shallow boney depression ( of the sphenoid bone called the sella turcica or pituitary fossa ). The sphenoid bone lies dorsal to (above) the roof , deep to the opening of the the mouth...within the portion of that cavity known as the pharynx. Pictured another way, the brain sits within the largest portion of the head ( the calvarium)...the eyes in the front-most aspect of the brain. The pituitary lies further back, but ventral, below the bulk of the brain, suspended by a stalk (the infundibulum of the neurohypophysis) from the hypothalamus above it, and cradled by the sellica below...which lies above the roof of the pharynx. The gland is surrounded by sinuses with major blood vessels and some cranial nerves. A portion of the third ventricle of the brain is present as an invagination of pituitary. . A diagramatic representation of some of these is shown here.
Pharynx: The sellica turca lies dorsal to (above) the deeper aspects of the pharynx (oral and nasal stuctures share this area), Traditionally, landmarks important for this surgery are the ....
soft palate: separates the nasal and oral portions of the shared cavity (the pharynx). The soft palate obstructs the approach to the roof of the pharynx and must be incised to allow visualization
sphenoid and related boney structures: are approached after dissection of the soft tissues overlying these; the pituitary is accessed by burring through the sphenoid . Useful landmarks for ascertaining the precise location of the pituitary of this area are the ptergoidhamular processes (palpable protuberances), and the intersphenoid suture...representing a fine line uniting the left and right portions of the sphenoid; it has been claimed that the latter can be used as a reference point for locating the appropriate burr site below the pituitary, however that is not always a reliable landmark. Likewise, the position of a vein in the
area (the emissary vein) is no longer considered a reliable anatomical landmark for locating the pituitary. There is a ridge of bone on the midline known as the presphenoid that flattens (becoming the basisphenoid) going from front to back. This protuberance is helpful for absolute identification of the midline.
Imaging: Successive contrast-enhanced CT imaging is the most reliable method for ascertaining the position of the pituitary gland and the shape of the bone proximate to the pituitary fossa relative to any or all of the aforementioned landmarks...e.g. the hamular processes.
Surgical Procedure:
Positioning and Preparation: Adequate exposure of structures in the pharynx requires adequate positioning of the dog so as to afford complete visualization. A special rod support system for accessing the pituitary through the mouth is utilized and illustrated in the figure below.
Surgical procedure: A visual representation of the procedure is presented in the image sequence below. A few details of this are provided below this image...if you are still interested.
Slide Show ...
After cleaning the mouth with a dilute povidone iodine solution, the soft palate is incised with an electrosurgical scalpel (or laser) and the two sides are retracted laterally (to the side) for visualization and identification of the ptergoid hamular processes. (the position of the pituitary relative to landmarks has already been ascertained via dynamic contrast CT scans).
The soft tissue overlying the sphenoid is likewise incised and retracted, exposing the presphenoid and basisphenoid ridges.
A burr slot into the sphenoid in created in the bone immediately and precisely overlying the pituitary fossa. Burring stops when the pituitary is seen through the paper-thin corvering of cortical bone overlying this area.
A ball-tipped bone hook is used to chip away flakes of the remaing bone; the hole is enlarged with bone punches, then the dura mater (firm tissue covering the outer layer of the brain) is incised, the pituitary is teased from the fossa using an angled ball hook and fine neurosurgical grasping hook. Some cerebral spinal fluid (CSF) may now leak from the surgical site.
Completeness of the excision is ascertained visually via identifiying the hypothalamus, the third ventricle, and verifying the absence of pituitary remnants in the sellae.
The fossa is filled with thrombin-containing gel to minimize hemorrhage and oozing of CSF; the bone slot is filled with bone way, and the soft tissue is closed over the filled bone.
Post-Operative Considerations
The pituitary is responsible for several endocrine (and non-endocrine) gland controls (see Thyroid basics, Regulation of Steroids, Cushing's Disease) as well as the storage and secretion of hormones synthesized by the hypothalamus (e.g. antidiuretic hormone ("ADH"..which is a little confusing..[this is NOT the same as ADH describing Adrenal-Dependent Hyperadrenocorticism], in the regulation of thirst). It is necessary to consider the loss of these controls following hypophysectomy.
In some dogs, antidiuretic hormone deficiency resultedin a condition of excess thirst and water imbalance due to lack of water conservation known as diabetes insipidus. In most dogs, however, the hypothalamus continued to release antidiuretic hormone into the circulation (rather than to the pituitary) and these dogs maintained normal thirst and water balance.
Interruption of thyroid hormone and adrenal hormone regulation are the most prominent complications of a hypophysectomy .
Supplementation with thyroid hormone, cortisone and ADH are begun immediately following surgery. Then at a couple of weeks, the the ADH is stopped and water consumption is observed; ADH supplementation is resumed, if necessary. The dose of cortisone is reduced to a maintainence level by 4 weeks post surgery.
Periodic monitoring of endocrine function is performed every 4-6 months.
Overall Success: was comparable to those animals with PDH that were treated with mitotane...except for the 8% of animals who died post-operatively.
Complications:
Treatment Failure: either no resolution of the hyperadrenocorticism or recurrence of hyperadrenocorticism
believed to be due to regrowth or excessive activity of remnants of the pituitary adenoma because a few (microscopic) cells were left behind. Recurrence occured in 5 of 43 (11.6%) dogs between 5 months and 20 months of surgery, in one study
even in healthy dogs undergoing a hypophysectomy (a very distasteful practice, in this author's opinion!!) microscopic "nests" of pituitary cells were found in the sella turcica post-operatively
Diabetes Insipidus: this has been already discussed, represents excessive thirst and water imbalance and secondary electrolyte imbalances (hypernatremia) due to loss of ADH . Only occurs in about 10% of surgeries; in the majority, it appears that ADH is released directly into the circulation by the hypothalamus.
Cranial Nerve Deficit: either related to the surgical procedure or to the neuropathologenic effects of thyroid hormone insufficiency on the lacrimal (tear production) system. These were both extremely rare occurences.
Death: Causes of death (8% of dogs in one study) were related to severe electrolyte imbalances (hypernatremia, hyponatremia) or bronchopneumonia
Kidney:The adrenal gland lie at the head-end (i.e. "cranial" end) of each kidney, and slightly displaced towards the midline;
Vertebrae: The left gland is at about the level of the second lumbar vertebrae while the right gland is positioned slightly more cranially, at the level of the 13th thoracic vertebrae
The Caudal Vena Cava (CVC): The CVC lies in very close association with the right adrenal gland. This major vessel must be protected!
The phrenicoabdominal blood vessels: These overlay the adrenal gland; the phrenicoabdominal vein lays over the lower (i.e. "ventral") aspect of each adrenal on it's way to the CVC.
Surgical Procedure:
Midline Abdominal Approach:
An incision from the end of the sternum (xyphoid) to the begining of the pelvis (pubis) is made in order to maximize exposure for manipulation of organs and tissues
The adrenal gland is identified after inspecting all major organs and tissues for evidence of malignancy.
If it is the right adrenal gland, the CVC is examined for possible invasion by the adrenal tumor (making this an extremely difficult surgery!).
Other vessels in close proximity are carefully identified; in particular, those going to the kidney are noted ( to be carefully avoided during the procedure)
The ureter is identified. (also to be avoided during the procedure)
Systematic retraction of other organs away from the adrenal gland allows optimal visualization of the surgical field.
The phrenicoabdominal vein is double ligated...and divided between ligatures.
The adrenal gland is carefully "teased" and dissected from the surrounding tissues paying attention to hemostasis (prevention of hemorrhage) as numerous small vessels are likely to be encountered.
Care is taken to avoid damaging the adrenal capsule..and potentially releasing potent neurotransmitters (epinephrine, norepineprine) from the core (medulla) of the gland.
If there is a thrombus (clot) in the CVC:
A (temporary) vascular clamp is made of umbilical tape and rubber tubing (a "Rumel Tourniquest") and the CVC is occluded on either side of the thrombus.
A longitudinal incision is made in the CVC, the thrombus is removed and the the CVC is closed (simple continuous suture pattern, vascular suture material).
The vascular clamp is removed.
If neoplastic tissue has invaded the CVC, it is unlikely that surgical dubulking will prevent metastasis (spreading of tumor tissue to other portions of the body); however, if invasion of the CVC is affecting blood flow and comproming circulation, then debulking (as described above for CVC thrombus removal, above) may provide temporary improvement.
Paralumbar Approach:
This approach differs from the previous in that the initial incision is created just caudal (behind) the last rib, as the patient is positioned on it's side, (relevant side up).
Other portions of the procedure are comparable to the technique described for the midline abdominal approach
1.Hypophysectomy for Cushing's Disease, Bjorn P. Meij, in: TheVeterinaryClinicsof North America, Small Animal Practice, September, 2001, pp1015
2. Surgery of the Endocrine System, Fossum, Theresa Welch, etal, in: SmallAnimalSurgery, Mosby-Year Book, Inc 1997, pp 401
2. Surgical Treatment of Canine Hyperadrenocorticism,Axlund,TW, Behrend EN and Winkler, JT: , Compendium May 2003 pp334
