University Exam Answers

Question 5 (5×2 marks)

5a. Why does cavernous sinus thrombosis cause paralysis of extrinsic muscles of the eye?

• CN VI palsy occurs first (it lies freely within the sinus and is most vulnerable)
• CN III and IV are also affected as thrombosis extends
• Total unilateral ophthalmoplegia results in complete paralysis of all extrinsic eye muscles

5b. Why does chronic middle ear infection (chronic otitis media) cause paralysis of facial muscles?

1. After entering the internal acoustic meatus, the facial nerve passes through the facial canal (Fallopian canal) in the petrous part of the temporal bone.
2. It runs along the medial wall of the middle ear (tympanic cavity), where it forms the genu (bend) and passes just above the oval window.
3. It then turns downward and exits through the stylomastoid foramen.

• The chronic infection and associated erosion of bone destroys the thin bony covering (dehiscence) of the facial canal.
• Cholesteatoma (keratinized epithelium) directly erodes the bony canal wall.
• Inflammatory mediators, pus, and granulation tissue compress and invade the exposed nerve.
• This leads to lower motor neuron (LMN) facial palsy - paralysis of all ipsilateral facial muscles (both upper and lower face), including inability to close the eye, drooping of the corner of the mouth, loss of nasolabial fold, etc.

Question 3 - Short Answer Questions (Applied) (5×2 marks)

Q3a. Why is parotid swelling very painful?

• When the gland swells (due to infection - parotitis, mumps, abscess, calculus, etc.), it cannot expand outward because the capsule cannot stretch.
• The swelling therefore builds up pressure within the confined space, compressing the parotid tissue, blood vessels, and nerve endings intensely.
• The auriculotemporal nerve and other sensory fibers within and around the gland are compressed, generating severe pain.
• The pain is aggravated by chewing because mastication (jaw movements) further squeezes and mechanically stimulates the already-tense gland.

Q3b. Infection of which paranasal air sinus is most difficult to treat, and why?

1. Position of the ostium (drainage opening): The natural ostium of the maxillary sinus opens into the middle meatus of the nasal cavity at its upper medial wall - near the roof of the sinus, not the floor. Since gravity cannot assist drainage (pus collects at the floor but drains from the roof), secretions accumulate.
2. Dependent drainage problem: The floor of the maxillary sinus is lower than its ostium, so mucopurulent secretions pool at the bottom and cannot drain naturally by gravity.
3. Small, narrow ostium: The natural ostium is small (~3mm) and easily becomes blocked by mucosal edema during infection, further impairing drainage.
4. Access difficulty: Surgical access requires either a Caldwell-Luc operation (via canine fossa) or functional endoscopic sinus surgery (FESS), both more involved than for other sinuses.
5. Dental root proximity: The roots of upper molar and premolar teeth are closely related to the floor - dental infections easily spread to this sinus (dental sinusitis), and vice versa.

Q3c. Bleeding into which layer of the scalp causes "black eye," and why?

• S - Skin
• C - Connective tissue (dense)
• A - Aponeurosis (epicranial/galea aponeurotica)
• L - Loose areolar tissue
• P - Pericranium

1. The loose areolar layer (Layer 4) is also called the "dangerous layer of the scalp" - it contains emissary veins and loose connective tissue with no fibrous septa to limit spread.
2. Blood and fluid in this layer can spread freely in all directions without any barrier.
3. Anteriorly, this layer is continuous with the loose connective tissue of the eyelids and orbital region (because the galea aponeurotica is attached anteriorly to the frontalis muscle which has no bony attachment anteriorly, allowing the plane to extend forward over the forehead and orbital margins).
4. Blood tracking forward through this plane gravitates into the periorbital region, producing the classic bilateral black eye (periorbital ecchymosis/raccoon eyes).

Q3d. Which area of the face is known as the "danger area of the face," and why?

• Apex: at the root of the nose (between the two eyebrows)
• Base: at the upper lip (along the line connecting the two corners of the mouth)

1. Angular vein → Ophthalmic vein (superior and inferior) → Cavernous sinus
2. Deep facial vein → Pterygoid plexus → Cavernous sinus

• Squeezing a boil/furuncle or superficial infection in this area can drive infected thrombi into the cavernous sinus via retrograde flow.
• This causes cavernous sinus thrombosis - a life-threatening condition with features of:
  • High fever, septicemia
  • Proptosis, chemosis, ophthalmoplegia
  • Meningism, and risk of death

Q3e. A patient with carcinoma of the tongue presents with pain in the ear. Explain its anatomical basis.

• The anterior 2/3 of the tongue receives general somatic sensation from the lingual nerve (branch of CN V3 - mandibular division of trigeminal).
• The posterior 1/3 of the tongue receives sensation from CN IX (glossopharyngeal).

• The external acoustic meatus and auricle receive sensory supply from the auriculotemporal nerve (also a branch of CN V3).
• CN IX also supplies the inner aspect of the tympanic membrane (Jacobson's nerve).

1. In carcinoma of the anterior 2/3 of the tongue, the tumor invades and irritates the lingual nerve (CN V3).
2. Since the lingual nerve and the auriculotemporal nerve are both branches of the same parent nerve (CN V3 - mandibular), their central pain fibers converge on the same neurons in the trigeminal nucleus (nucleus of the spinal tract of V).
3. The brain incorrectly interprets the pain signal as coming from the ear (auriculotemporal distribution) when it actually originates from the tongue.
4. This is classic referred otalgia (referred ear pain).

Summary Table for Quick Revision:

Blood-Brain Barrier (BBB)

Definition

Historical Note

Structural Components

Key structural features:

1. Tight junctions (zonula occludens) between adjacent endothelial cells - these are complex, resembling epithelial junctions more than typical endothelial junctions. They eliminate intercellular gaps and prevent paracellular diffusion.
2. Minimal pinocytosis - very few vesicles in endothelial cells; transcytosis is severely restricted.
3. Continuous-type capillaries (no fenestrations) throughout the CNS.
4. Pericytes embedded in the basement membrane help regulate the barrier.

Functions

1. Protect the brain from blood-borne toxins, pathogens, and fluctuating levels of electrolytes, hormones, and metabolites.
2. Maintain homeostasis of the CNS microenvironment (optimal ionic composition for action potentials).
3. Meet metabolic demands of the brain by regulating transport of nutrients.

Permeability - What Crosses and What Doesn't

Freely crosses:

• O₂ and CO₂ (lipid-soluble gases) - by simple diffusion
• Lipid-soluble substances - anaesthetic agents, alcohol, steroids (freely enter)
• Water - moves freely by bulk flow

Crosses with help (carrier-mediated):

• Glucose - via facilitated transport (GLUT-1 transporter); glucose is the brain's sole energy source
• Amino acids - via specific transporters
• Certain drugs - via active transport or facilitated diffusion

Does NOT cross (or crosses poorly):

• Hydrophilic (polar) and large molecules (molecular weight >500 Da)
• Ionized substances (e.g., Na⁺ - equilibration half-life is 2-4 hours)
• Proteins and large polar molecules
• Mannitol - does not cross; used clinically to draw water out of the brain (osmotic diuresis)
• Most antibiotics and many chemotherapy drugs (a major clinical problem)

Development

Disruption of the BBB

• Severe hypertension
• Tumors (cerebral metastases, gliomas)
• Trauma (head injury)
• Stroke (ischemia - endothelial cell compromise)
• Infections (meningitis, encephalitis)
• Marked hypercapnia or hypoxia
• Sustained seizure activity

Circumventricular Organs - Areas WITHOUT a BBB

• Area postrema (chemoreceptor trigger zone - vomiting)
• Posterior pituitary (neurohypophysis)
• Pineal gland
• Median eminence
• Subfornical organ

Clinical Significance

Summary (Quick Recall)

BBB = Endothelial tight junctions + Basement membrane + Astrocyte foot processes

• Allows: O₂, CO₂, lipid-soluble molecules, glucose (GLUT-1), water
• Blocks: large molecules (>500 Da), polar/ionized substances, proteins, mannitol
• Broken by: hypertension, trauma, tumors, infection, ischemia, hypoxia
• Absent at: circumventricular organs