H. influenzae is a fastidious, gram-negative, pleomorphic coccobacillus that requires factor X (hematin) and factor V (phosphopyridine nucleotide) for growth. Some H. influenzae isolates are surrounded by a polysaccharide capsule and can be serotyped into 6 antigenically and biochemically distinct types designated by letters  a–f.

Meningitis:

In the prevaccine era, meningitis accounted for more than half of invasive H influenzae disease. Clinically, meningitis caused by H. influenzae type b cannot be differentiated from Neisseria meningitidis or Streptococcus pneumoniae.

It may be complicated by other foci of infection such as the lungs, joints, bones, or pericardium.

Antimicrobial therapy should be administered intravenously for 7–14 days for uncomplicated cases. Cefotaxime, ceftriaxone, and ampicillin cross the blood-brain barrier during acute inflammation in adequate concentrations to treat H. influenzae meningitis. Intramuscular therapy with ceftriaxone is an alternative in patients with normal organ perfusion.

The prognosis of H. influenzae type b meningitis depends on the age at presentation, duration of illness before appropriate antimicrobial therapy, cerebrospinal fluid (CSF) capsular polysaccharide concentration, and rapidity with which it is cleared from CSF, blood, and urine. Clinically manifested inappropriate secretion of antidiuretic hormone and evidence of focal neurologic deficits at presentation are poor prognostic features. About 6% of patients with H. influenzae type b meningitis are left with some hearing impairment, probably because of inflammation of the cochlea and the labyrinth. Dexamethasone (0.6 mg/kg/day divided every 6 hr for 2 days), particularly when given shortly before or concurrent with the initiation of antimicrobial therapy, decreases the incidence of hearing loss.

Major neurologic sequelae of H. influenzae type b meningitis include behavior problems, language disorders, delayed development of language, impaired vision, mental retardation, motor abnormalities, ataxia, seizures, and hydrocephalus.

Cellulitis:

Children with H. influenzae cellulitis often have an antecedent upper respiratory tract infection. They usually have no prior history of trauma, and the infection is thought to represent seeding of the organism to the involved soft tissues during bacteremia. The head and neck, particularly the cheek and preseptal region, are the most common sites of involvement. The involved region generally has indistinct margins and is tender and indurated. Buccal cellulitis is classically erythematous with a violaceous hue, although this sign may be absent. H. influenzae may often be recovered directly from an aspirate of the leading edge. The blood culture may also reveal the causative organism.

Supraglottitis or Acute Epiglottitis:

Epiglottitis is a cellulitis of the tissues comprising the laryngeal inlet . It has become exceedingly rare since the introduction of vaccine. Direct bacterial invasion of the involved tissues is probably the initiating pathophysiologic event. This dramatic, potentially lethal condition can occur at any age. Because of the risk of sudden, unpredictable airway obstruction, supraglottitis is a medical emergency. Other foci of infection, such as meningitis, are rare. Antimicrobial therapy directed against H. influenzae type b and other etiologic agents should be administered parenterally but only after the airway is secured, and therapy should be continued until patients are able to take fluids by mouth. The duration of antimicrobial therapy typically is 7 days.

Pneumonia:

The true incidence of H. influenzae pneumonia in children is unknown because invasive procedures are required to obtain cultures and are seldom performed.

Children <12 mo of age suspected of having H. influenzae pneumonia should receive parenteral antimicrobial therapy initially because of their increased risk for bacteremia and its complications. Older children who do not appear severely ill may be managed with an orally administered antimicrobial. Therapy is continued for 7–10 days.

Suppurative Arthritis:

Large joints, such as the knee, hip, ankle, and elbow, are affected most commonly . Other foci of infection may be present concomitantly. Although single joint involvement is the rule, multiple joint involvement occurs in about 6% of cases. The signs and symptoms of septic arthritis caused by H. influenzae are indistinguishable from those of arthritis caused by other bacteria.

Miscellaneous Infections:

Urinary tract infection, epididymo-orchitis, cervical adenitis, acute glossitis, infected thyroglossal duct cysts, uvulitis, endocarditis, endophthalmitis, primary peritonitis, osteomyelitis, and periappendiceal abscess are rarely caused by H. influenzae

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