STRUCTURAL BASIS OF MEDICAL PRACTICE EXAMINATION 1, August 23, 2012: Part III Short Answer - Answer Guide

Fossa Ovalis - mdl19

1. An atrial septal defect allows errant blood flow between the left and right atria. Symptoms include decreased exercise tolerance, palpitations, and syncope (fainting). Provide a developmental account for the adult anatomy of the fossa ovalis. (6 pts)

Embryology & Formation into Adult Structure

Septation begins around day 26. The primordial atrium become septated by a combination of two septum: primum and secundum. Each of these septa are only partially complete allowing right to left shunting of blood during gestation. As the lungs have not developed in the fetus (and thus result in increased pressure in the right atria), this shunting allows for oxygenated blood to flow from the umbilicus to the fetus\x92s systemic circulation and bypass the lungs.

The steps involved with fetal atrial septation are as follows:

  1. Septum primum forms and grows to septate the atria. This septum primum grows from superior to inferior leaving a foramen at its base (foramen primum). Eventually the septum primum meets the endocardial cushions which are forming the atrioventricular septum and results in the elimination of the foramen primum. Simultaneoulsy, to allow for continuous atrial shunting, on the superior aspect of the septum primum, there is programmed cell death resulting in the foramen (ostium) secundum. Thusly, the inferior foramen primum \x93closes\x94 as the superior foramen secundum \x93opens\x94 within the septum primum.
  2. While the septum primum is elongating, the septum secundum develops from the ceiling of the right atrium. This septum is thicker, more muscular, and lateral to the septum primum. The septum secundum grows cranial-caudally and ventral-dorsally, but unlike the septum primum, the septum secundum halts its growth before merging with the atrioventricular septum, resulting in the formation of the foramen ovale near the inferior aspect of the right atrium and within the tissue of the septum secundum.
  3. Blood will now flow in an oblique fashion through the inferior foramen ovale (formed in septum secundum) in the right atria to the superior foramen secundum into the left atria and eventually into the fetus systemic circulation. Because the septum primum is relatively weak, backflow of blood from the left to right atria is prohibited by the resulting collapse of the weak septum primum against the relatively thick septum secundum. Thusly, the septum primum becomes a \x93valve\x94 for the foramen ovale within the septum secundum.
  4. Following birth and function of the lungs, the pressure decreases in the right atria due to the opening and function of pulmonary vasculature. This results in an increase in blood pressure in in the left atria allowing for a physiological collapse of the septum primum against the septum secundum (as discussed above). It may take up to two years for this occlusion to become anatomical. In some cases, it remains physiologically patent (increased right atrial pressure may cause this area to shunt as a fetus within the adult heart), which is the case for this patient. In this instance, deoxygenated blood will shunt from the adult right atria into the left atria through the physiologically patent fossa ovalis. This would result, eventually in increased deoxygenated blood flow into the systemic circulation.

Anatomy:

Based on the development of the fossa ovalis, it is by definition located in the medial wall of the right atria. It is the location at which the foramen secundum was sealed with the septum primum. It is surrounded by the limbus fossa ovalis, most pronounced superiorly and anteriorly and least pronounced inferiorly. The fossa ovalis is a depression superior and to the left of the orifice of the inferior vena cava.

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Superior Gluteal Nerve - ems5482

2. An intragluteal injection into the upper medial quadrant may damage the superior gluteal nerve. Discuss the anatomy of the superior gluteal nerve and the consequences of damage to this nerve. (6 pts)

Anatomy of superior gluteal nerve

  • Exits pelvic cavity via greater sciatic foramen to enter gluteal region
  • Superior to piriformis muscle
  • Courses laterally between gluteus medius and minimus
  • Innervates gluteus medius and minimus
  • Vascular supply via superior gluteal artery and vein

Actions

  • Abduction and medial rotation of thigh (lateral rotation as well)
  • Functional reversal of origin and insertion to stabilize pelvis

Effect of Intragluteal injection

  • Upper medial quadrant injection puts superior gluteal nerve at risk
  • Upper lateral quadrant is preferred for injection due to nerve ramification prior to this location

Gait Disruption

  • Both lower extremities involved
  • Paralysis of gluteus medius and minimus
    • Contralateral pelvic drop (opposite injury side)
    • Compensation by ipsilateral lean/tilt (injury side)
  • Trendelenberg\x92s Gait

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Head and Neck of Fibula - jai134

3. The head and neck of the fibula are subcutaneous. Structures that cross this region are easily damaged by compression and trauma. Discuss the relationships of the head and neck of the fibula (proximal fibula) and the consequences of injury to this area. (6 pts)

Relationships and Injury

  • Common fibular nerve.
    • Damage at common: Common fibular nerve passes posterolateral to the neck of the fibula. Lose of function of superficial and deep fibular nerve.
    • Superficial nerve damage: Foot appears inverted due to unopposed everter muscles in the lateral compartment. Cutaneous innervation to a portion of the lateral leg and the dorsum of the foot lost.
    • Deep fibular nerve damage: Lose of innervation to the extenders of the talocrural joint. Foot Drop. Cutaneous innervation between digits 1 and 2 of the foot is lost.
  • Genicular Anastomosis
    • Primarily fibular circumflex artery is at risk.
    • Additional inferior arteries of this anastomosis could be damaged (depending on degree of injury to the fibula) including the anterior and posterior tibial arteries, the inferior lateral genicular artery and potentially even the recurrent arteries (ant. And post. tibial recurrent arteries).
  • Muscles and Ligaments
    • Interosseous membrane along with any muscles having origin/insertion on the head/neck of the fibula can be damaged as well (biceps femoris\x92 insertion)
    • Fibular collateral ligament inserts on the head of the fibular and could be damaged (deep to BF insertion)

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Intercostal Space and Facial Layers - aet5121

4. A pleural tap is performed by passing a needle through the 9th intercostal space at the midaxillary line into the costodiaphragmatic recess. Discuss the anatomy of an intercostal space at the midaxillary line and the fascial layers penetrated by a pleural tap. (6 pts)

Anatomy

  • too far with the needle: folding of pleura = serous layer - parietal layer - endothoracic fascia - diaphragm - liver
  • VAN - found in the superior aspect of intercostal space protected by the intercostal groove (rib)
  • Posterior intercostal VAN:
    • artery from aorta
    • vein - drains into azygos vein on right and hemiazygous on left
    • nerve - spinal nerve (from symp chain)
  • Intercostal muscle orientation
  • subcostal muscle - can arise anywhere within thoracic cavity
  • intercostal muscle breathing:
    • external - quiet and forced inspiration
    • internal - forced expiration
  • false ribs

Fascial layers

  • won't damage lung (in non-pathological state)
  • Skin to the Recess:
  • Skin - tela subcutaneous - deep fascia - serratus anterior muscle - external intercostal muscle - internal intercostal muscle - Intercostal VAN - innermost intercostal muscle - endothoracic fascia - fibrous layer of costoparietal pleura - serous layer of costoparietal pleura - pleural cavity (specifically at costodiaphragmatic recess)
  • Borders of intercostal space:
  • Superior and Inferior - ribs (9 - 10)
  • too far with the needle: folding of pleura = serous layer - parietal layer - endothoracic fascia - diaphragm - liver
  • VAN - found in the superior aspect of intercostal space protected by the intercostal groove (rib)
  • Posterior intercostal VAN:
    • artery from aorta
    • vein - drains into azygos vein on right and hemiazygous on left
    • nerve - spinal nerve (from symp chain)
  • Intercostal muscle orientation
  • subcostal muscle - can arise anywhere within thoracic cavity
  • intercostal muscle breathing:
    • external - quiet and forced inspiration
    • internal - forced expiration
  • false ribs

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Lymphatic Drainage of Right Breast - okh101

5. The incidence for new cases of breast cancer in 2012 is estimated to be 230,000 with 40,000 cases causing death. Discuss the lymphatic drainage of the right breast. Provide an account for spread of disease to the left breast and to the superficial inguinal lymph nodes. (6 pts)

Lymphatic drainage

  • The lymphatic drainage of the breast is clinically important because of its role in cancer metastasis. When cancer cells spread to the lymphatic system, it ultimately drains into the venous system. This makes localized cancer a systemic concern. Lymph from the right breast drains from the nipple, areola, and lobules of the mammary gland to the subareolar lymphatic plexus. From here, most lymph (~75%) especially from the lateral region drains initially to the anterior or pectoral nodes of the axillary group of lymph nodes. Some lymph drains to other axillary nodes (subscapular, central, or apical). From the axillary nodes, lymph drains to the clavicular lymph nodes (infraclavicular and supraclavicular), then to the right lymphatic duct, which will drain into the right subclavian vein near its juncture with the internal jugular vein. Most of the remaining lymph (~25%) especially in the medial region drains to the parasternal lymph nodes, then to the bronchomediastinal trunk and into the right lymphatic duct before draining into the right subclavian vein.

Spread to other breast and superficial inguinal nodes

  • When the deep drainage pathways are occluded, superficial drainage predominates. When this happens, lymph from the medial breast may cross the midline in connecting superficial lymphatic vessels to reach the left breast. Lymph from the most inferior portion of the breast may drain to the inguinal region from the internal thoracic chain by way of the superior and inferior epigastric lymphatic routes. Lymph may also reach the superficial inguinal nodes via lateral thoracic vessels to thoracoepigastric vessels to superficial circumflex and superficial epigastric vessels.

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