All India NEET January-2018

Mucormycosis

The Mucorales are ubiquitous environmental fungi to which humans are constantly exposed. These fungi cause infection primarily in patients with diabetes, defects in phagocytic function (e.g., neutropenia or glucocorticoid treatment), and/or elevated levels of free iron, which supports fungal growth in serum and tissues.

Most mucormycosis infections are life-threatening. Severe infection of the facial sinuses, which may extend into the brain, is the most common presentation. Mucormycosis is highly invasive and relentlessly progressive, resulting in higher rates of morbidity and mortality than many other infections.

Elevated levels of free iron support fungal growth in serum and tissues. Nevertheless, the majority of diabetic patients who present with mucormycosis are not acidotic. Patients with diabetic ketoacidosis are at high risk of developing rhinocerebral mucormycosis. The acidosis causes dissociation of iron from sequestering proteins, resulting in enhanced fungal survival and virulence. Hyperglycemia directly contributes to the risk of mucormycosis. Hyperglycation of iron-sequestering proteins disrupts normal iron sequestration.

• Harrison's Principles of Internal Medicine, 20e
  
  
  
  • Chapter 213: Mucormycosis
  
  
  
  

The third cranial nerve (the ocu­lomotor nerve) controls several movements of the globe, including upward and medial movements, through its control of the medial rectus, superior rectus, and inferior oblique muscles. Its inactivity leads to displacement of the eye down and out. Fourth‑nerve palsy leads to weakness of the supe­rior oblique muscle, with resultant difficulty looking down and medially; patients often complain of trouble walking down stairs. Sixth‑nerve palsy produces weakness of the lateral rectus muscle, causing horizontal diplopia. Fractures of the orbit can entrap individual muscles, but there is no history of this here. Thyroid ophthalmopathy or Graves disease, can produce diplopia, but there is usually proptosis or lid retraction. The inferior and medial recti are most frequently affected. Because this is caused by infiltration of the muscles, there is usually limitation of passive movement of the eyes (ie, forced ductions). Diabetes is a common cause of third‑nerve palsy (approximately 10% of cases). Usually, when diabetes is the cause, there is sparing of the pupillo motor parasympathetic fibers, which travel on the outside of the nerve. Diabetes causes third‑nerve palsy via nerve infarction, which affects the interior of the nerve but spares the external fibers. Compressive lesions, however, can injure the surface fibers, thereby causing pupillary dilation due to unopposed sympathetic activity.

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Niacin

Through multiple actions, niacin (but not nicotinamide) lowers serum LDL cholesterol and triglyceride concentrations and increases HDL cholesterol concentrations; therefore, niacin has wide clinical usefulness in the treatment of hypercholesterolemia, hypertriglyceridemia, and low levels of HDL cholesterol.

Cutaneous flushing is a common adverse effect of niacin. Pretreatment with aspirin or other NSAIDs reduces the intensity of this flushing. Tolerance to the flushing reaction usually develops within a few days. Dose-dependent nausea and abdominal discomfort often occur. Moderate elevations of liver enzymes and even severe hepatotoxicity may occur. Hyperuricemia occurs in about 20% of patients.

The use of niacin should be considered carefully in patients with insulin resistance. Carbohydrate tolerance may be moderately impaired. Some patients have moderate elevations of blood glucose during treatment; this is reversible except in those who have latent type 2 diabetes.

• Katzung & Trevor's Pharmacology: Examination & Board Review, 12e
  
  
  
  • Chapter 35: Agents Used in Dyslipidemia
  
  
  
  

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Third nerve palsy in diabetes

The third cranial nerve innervates the medial, inferior, and superior recti; inferior oblique; levator palpebrae superioris; and the iris sphincter. Total palsy of the oculomotor nerve causes ptosis, a dilated pupil, and leaves the eye “down and out” because of the unopposed action of the lateral rectus and superior oblique.

Pupillomotor fibers are in the superomedial portion of the third nerve. These nerve fibers may be either involved or spared, depending on the type of disease involving the nerve.

Painful isolated third nerve palsy with pupillary involvement suggests a compressive lesion (tumor or circle of Willis aneurysm.) Aneurysm usually arises from the junction of the internal carotid and posterior communicating arteries. Intracranial tumor cause third nerve palsy by direct damage to the nerve or due to mass effect. Pupillary dilation, initially unilateral and then bilateral, is an important sign of herniation of the medial temporal lobe through the tentorial hiatus (tentorial herniation) due to a rapidly expanding supratentorial mass. In painful isolated third nerve palsy with pupillary involvement, urgent neuroimaging should be obtained, along with a CT or MR angiogram.

Causes of isolated third nerve palsy without pupillary involvement include diabetes mellitus, hypertension, giant cell arteritis, and herpes zoster. Diabetes mellitus causes an ischemic (microvascular) palsy. A useful guide clinically is that in ischemic lesions pupil function is preserved, whereas in compression, including aneurysmal, pupil function is abnormal, with initially loss of reactivity and then also dilation.

• Adams and Victor's Principles of Neurology, 10e
  
  
  
  • Chapter 14. Disorders of Ocular Movement and Pupillary Function