Acute Osteomyelitis

This case concerns a ten year old boy presenting with pain and stiffness in
his left knee. Previously well, he did suffer from 'septic spots' that usually disappeared after prescription of amoxycillin. On Friday 22 March he became ill with fever (39oC), a raised pulse (120 beats per minute) and muscle aches and pains. His General Practitioner diagnosed influenza, and prescribed paracetamol for the boy.

The patient's condition deteriorated during the next 24 hours and by 6 p.m. on Saturday 23 March, he had a temperature of 40.2oC and a pulse of 140 beats per minute. In addition to general aches and pains, he complained of pain in his left leg, just below the knee joint. Flexing the left knee caused severe pain.

His mother was unhappy with the diagnosis of influenza and rang the doctor's deputising service, who reassured her that the infection was self-limiting, and that it might be wise to persist with the paracetamol.

During the next 24 hours he showed no improvement, and the symptoms were aggravated by nausea and vomiting. The boy became flushed and delirious. His temperature was 41.0oC by Sunday afternoon. The child was taken to the casualty department of St. James' University Hospital. He was seen at 7 p.m. and was diagnosed as suffering from meningitis because of his toxaemia, headache, and reduced level of consciousness. He was given intravenous cefotaxime. A lumbar puncture was performed.

Two hours later, the CSF was reported as normal, but his blood sample showed 3.5x 109 leukocytes per litre, of which 92% were polymorphs. The diagnosis was changed to pyogenic sepsis of unknown aetiology. An emergency brain scan was performed to exclude a cerebral abscess. At midnight this was reported as normal.

The Consultant noted that despite a general restlessness, the patient did not move his left leg spontaneously. Careful examination of the upper part of the left tibia revealed an area where any local pressure caused extreme pain. The limb was swollen and red, and the mother said it had been like this for the past three days.

A diagnosis of acute osteomyelitis was now made, and the patient was referred to the orthopaedic department. Two boreholes were drilled in the upper part of the left tibia where inflammation was most marked. Each aspirate yielded 5 ml of bloodstained pus. The left knee joint was aspirated and its fluid was cloudy. The following results were reported by the laboratory:

Both bone aspirates yielded a pure growth of Staphylococcus aureus resistant to penicillin, ampicillin and amoxycillin, but sensitive to erythromycin, fusidic acid, flucloxacillin and gentamicin. The knee aspirate contained 300 polymorphs/ but was sterile.

The osteomyelitis had not invaded the knee joint - the effusion was sympathetic. The pus aspirate confirmed the diagnosis of osteomyelitis. The patient was treated with flucloxacillin and fusidic acid, begun after surgery was complete.

Case Comments:

The patient was probably a nasal carrier of Staphylococcus aureus, the source of the bacterium causing osteomyelitis.

The probable delay in diagnosis and treatment was because:

a) The diagnosis was not considered because the condition is rare.
b) Cefotaxime was considered to be the correct therapy.

The use of two antibiotics initially in undiagnosed osteomyelitis is reasonable because the probable causative bacterium, Staphylococcus aureus, has an unpredictable sensitivity and because flucloxacillin and fusidic acid are synergistic against this bacterium.

In Short

In suspected cases of osteomyelitis, why are holes drilled into the bone?
To relieve the pain, to debride the wound, and to obtain a sample for laboratory analysis.

How long does it take for the laboratory to be fairly confident that Staphylococcus aureus is present?
Given the aetiology of the condition, about ten minutes - a Gram stain will show Gram-positive cocci in the pus.

How long does it take for the laboratory to be certain that Staphylococcus aureus is present?
Up to 48 hours, if the slide test for clumping factor is negative, and a DNase test and tube coagulase must be carried out.

If Staphylococcus aureus resists penicillin, why is it also resistant to ampicillin and amoxycillin, but sensitive to flucloxacillin?
Flucloxacillin is an anti-staphylococcal penicillin that can resist the action of staphylococcal penicillinase. The other penicillins listed are sensitive to staphylococcal penicillinase.

How would you have managed this case had the patient been allergic to penicillins?
Because of the patient's allergy to penicillins, an alternative antimicrobial regime must be sought. Due to the poor penetration of many antibiotics into bone, the choice of therapy in such cases is not easy. Clindamycin as a single agent is a reasonable choice, reserving vancomycin as a first-line agent for the treatment of resistant staphylococcal infections.