Enterobacteriaceae and Other Gram Negative Bacilli & Beta-Lactam Resistance

Gram negative bacilli can be resistant to various beta lactams, ranging from penicillins to carbapenems. Such resistance may be constitutive or acquired. Constitutive or in-built resistance is typically due to restricted entry of antibiotics into cells, by "gating" of entry points called porins, which span the outer hydrophobic cell wall in gram negative bacilli. The more hostile an environment the bacterium lives in, the more impermeable will be the porins. Thus, porins in Acintobacter, which lives in liquid media such as irrigation solutions in hospitals, is 100 times less permeable than E.coli, which lives in the relatively friendly and protected environment of the gut with a host of other Enterobacteriaceae. Here, it is worth mentioning that Acintobacter and Pseudomonas, albeit being gram negative bacilli, are not from the same family as Enterobacteria. That is to say, unlike E.coli, Klebsiealla, Citrobacter, Providentia, Serratia, Enterobacter and Proteus species, Pseudomonas and Acintobacter do not live in the gut.

Although not important for beta-lactams, which act in the peri-plasmic space between the inner and outer membranes, antibiotic resistance in gram negative bacteria is also served by an efflux pump, which simply pumps out intracellular antibiotics to the exterior. This mechanism is obviously more important for antibiotics which act inside the cell, such as aminoglycosides, quinolones, tetracyclines and macrolides.

Resistance can also be acquired ecologically, from other successful bacteria, for example in the gut. The most important example of this is the so called "accessory genome" epitomised by plasmids.

Some plasmids are so successful that they can self transmit during bacterial reproduction. They are called constitutive plasmids. Plasmids have their own genetic code called replicons. Plasmids from the same group have similar replicons, and are incompatible with each other. When two similar plasmids are in the same cell, they will only multiply once the cell has divided and the competing plasmids have passed into separate daughter cells.

Constitutive plasmids cause the bacterium in which they reside to become "addicted" to them. Such "plasmid addiction" often involves a toxin-antitoxin system. The toxin tends to be stable while the anti-toxin is unstable and subserved by the plasmid. When the plasmid is lost, so is the anti-toxin, resulting in destruction of the bacterium by the intracellular toxin. In this way, the plasmid ensures that only those bacteria that carry it, survive.


The Ambler Classification for gram-negative resistance to beta-lactams is widely used and comprises four classes of beta-lactam resistance.

Class A- Comprises both broad spectrum beta-lactamases encoded by genes such as TEM and SHV and ESBLs, such as CTX-M-15 (Cefotaxime hydrolysing, first described in Munich). These group of bacteria are sensitive under lab and clinical conditions to inhibition by beta-lactamase inhibitors such as clavulanate, sulbactam, tazobactam and avibactam.

Class A also includes carbapenamases (CPEs) such as KPC (Klebsiella pneumoniae carbapenamase).

Class B- Comprises MBLs or bacteria producing metallo-beta-lactamases, most of which contain zinc as the "metal". Examples include imipenamase (IMP), Verona-integron encoded metallo beta lactamase (VIM), and the New Delhi Metallo-beta-lactamase (NDM). These group of bacteria will hydrolyse almost all beta lactams except aztreonam (a monobactam). They are relatively insensitive to beta lactamase inhibitors.

Class C- These group of bacteria are called Amp-C (or ampicillin hydrolysing). Amp-C bacteria produce cephalosporinases that hydrolyse 1st, 2nd and 3rd generation cephalosporins. Their ability to hydrolyse 2nd gen cephalosporins, called cefamycins, comprising cefoxitin and cefotetam, sets them apart from ESBLs, which cannot hydrolyse cefamycins. This latter property is used in the lab to differentiate Amp-C from ESBL. Amp-C are also insensitive to clavulanate.

Class D- called OXA or oxacillinases. These bacteria are not inhibited by clavulanate either. They hydrolyse 3rd gen cephalosporins but with a differential between cefotaxime and ceftazidime.

There are various ways of screening for beta-lactamases. Paricularly important is to detect ESBL or carbapenamase producing strains, as their spread will lead to widespread bacterial resistance. Isolation methods are often used for such patients.

The two most widely used are the Combined Disc test and the modified Hodge test.

The combined disc test is a variation of the Double Disc Synergy Test (DDST).

Here, two separate 30 ug discs containing the bacterial inoculum are placed 30 mm from each other (centre to centre) in agar. One disc contains cefotaxime, while the other contains amoxycillin-clavulanate. For ESBL producing bacteria (typically strains of E.coli, K.pneumoniae, K.oxytoca or Proteus species), there will be synergy between cefotaxime and clavulanate and the area of bacterial lysis in the cefotaxime disc will be greater where it apposes the clavulanate disc. (Remember, ESBLs as well as carbapenamases belong to Ambler Class A, which is the only class of beta-lactamases significantly inhibited by clavulanate). The appearance of the cefotaxime disc therefore takes on the shape of a keyhole when ESBL or carbapenamase is present.

The problem arises when there are other classes of beta lactamases that hydrolyse 3rd gen cephalosporins, thus masking the presence of ESBL. It's still important to pick up ESBL due to epidemiologic and clinical reasons. When ESBL is being co-produced with significant amounts of Amp-C (Ambler Class C), the classical synergy with clavulanate may not be seen, as Amp-C is not inhibited by clavulanate. This limitation can be overcome by bringing the discs closer than 30 mm, or by adding cloxacillin to the agar, as this inhibits Amp-C.

When ESBL is being co-produced with Ambler Class B, the metallo-beta-lactamases, it is possible to neutralise the latter with EDTA (chelates Zinc), thus bringing out the synergy with the clavulanate disc.

Unfortunately, no tests exist, except PCR, to tell apart the co-existence of ESBL and Ambler Class D- OXA.

Pulmonary Hypertension & Left Main Coronary Compression

Beware of attributing worsening chest pain resembling angina, decreasing exercise tolerance, malignant arrhythmias or new onset heart failure in subjects with known pulmonary hypertension (PH) to the pre-existing condition.

Occasionally, an enlarged pulmonary artery (PA) can compress the ostium of the left main coronary artery (LMCA) and thus lead to worsening angina or angina equivalent such as dyspnoea. Most cases have been described in subjects with longstanding secundum type ASD, where RV volume overload gradually transforms into pressure overload, but the phenomenon has been reported with other causes of PH leading to PA enlargement. The latter exists if the PA diameter exceeds 33 mm or the PA:Aortic diameter exceeds 1.0.

The LMCA arises from the left posterior sinus of Valsalva (the RCA arises from the anterior sinus, which is not paired). The LMCA is uniquely vulnerable to compression by the enlarging PA, particularly when its ostium is higher than usual.

The diagnosis can be made by CT (MDCT) of the heart, or on coronary angiography with the left anterior oblique view.

Unlike atherosclertic LMCA occlusion, where CABG is almost always favoured, the condition responds well to PCI and stenting.

See the latest issue of JAMA Cardiology for a case report.

Biotin, Avidin and False Positive Tests for Grave's Disease

Some thirty years ago, as 2nd year medical students, a couple of us were sat outside our ramshackle hostel in the gathering dusk. Biochemistry exams were in the offing and we were quizzing each other just to make sure that we were all keeping up. Somebody piped up about the importance of not eating raw eggs. The prevailing dogma, which we all agreed upon then, was that biotin in the egg yolk binds to an egg protein called avidin. Cooking the egg broke the bond, which made biotin available to the body, where it subserved carboxylation reactions as a co-enzyme to acetyl co-A carboxylase and pyruvate carboxylase.

As it happens, we were wrong. I succumbed to atavism and purchased the latest edition of Harper's Biochemistry a few years ago. Biotin deficiency is virtually unknown in medicine. You would have to be a habitual eater of raw egg whites, eschewing the yolk, to get even close to being Biotin deficient.

However, elsewhere in the world, some other people took note of the affinity between biotin and avidin and thought of an application that would change immuno-assays for generations to come. It thus came about that biotin is used as a tag. Biotin has a valeric acid side chain that can be modified to make it reactive and conjugate it to almost any protein or nucleic acid. By then adding avidin or an avidin like bacterial molecule called Streptavidin, produced by Streptomyces avidinii to the mixture, you can home in to the protein that has been bound to biotin. Of course, you need to find where the avidin goes. So you label the avidin (or streptavidin) with a "dye" (or a fluorophore, to be more technical), or an enzyme, which will change colour once its substrate is added. And presto, by taking advantage of one of the strongest known non-covalent affinities in nature, every biotin molecule, attached to the protein that you seek to find, will be picked out like a guided missile by the avidin.

The biotin-streptavidin immunossay has become the standard in most labs for procedures such as ELISA, immunohistochemistry etc.

You can do the same thing to find RNA or DNA, known as Northern blotting or Southern blotting. Here, avidin is tagged to a nucleic acid probe that hybridises with the DNA or RNA bound by biotin. However, these blotting procedures have largely been supplanted by PCR.

But that's not the end of the story. Over the recent years, biotin has become quite a popular supplement for people with mitochondrial disorders and in general as a "pick up" as a component of multivitamins. It definitively found its way into mainstream medicine when trials showed that mega-doses of biotin had salutary effects on the functioning of subjects with multiple sclerosis. It is now increasingly prescribed for MS.

Couple of years ago, reports started filtering in of bizarre lab results, particularly TFTs, but also other endocrine tests such as testosterone, FSH and LH and vitamin B12 among others, in subjects who were on biotin supplements. Remarkably the free T3 and T4 would be very high, the TSH suppressed, and levels of TRAb would be high- an exact biochemical mimic of Grave's disease....except that these subjects were completely euthyroid.

It is now appreciated that pharmacologic doses of biotin interferes with the biotin-streptavidin assay. In competitive immunoassays, the substances being measured such as free T3, or free T4, will be falsely high, whereas substances measured in "sandwich immunoassays", such as TSH, PTH and PSA, will be very low.

There have been a number of instances where subjects on biotin supplements have been erroneously treated for Graves Disease, based on such lab results. There are two excellent letters in the latest edition of New England Journal of Medicine that expand on these. They are free to access. One can avoid such misrepresentation of lab results by stopping the biotin supplements 72 hours before running TFTs or other blood tests.

I shared this as it took me back three decades to that long forgotten, dilapidated hostel where we grew up as wide-eyed medical students, completely unaware of how things were about to explode from bench to bedside.

References:

1.Kummer S, Hermsen D, Distelmaier F. Biotin treatment mimicking Graves’ disease. N Engl J Med 2016;375:704-706
2. Trambas CM, Sikaris KA, Lu ZX. A caution regarding high-dose biotin therapy: misdiagnosis of hyperthyroidism in euthyroid patients. Med J Aust 2016;205:192-192

SLE & Complement Deficiency- Where Do We Stand?

The effect of complement deficiency is hierarchical in SLE, with 90% of those with deficiency of C1q, 80% with C4 deficiency and 30% with C2 deficiency developing the disease. While those with C1q deficiency develop classical SLE, with a high incidence of lupus nephritis, deficiency of C4 or C2 lead to unusual clinical and immunological phenotypes. Patients with deficiency of either of these two early complement components present with rash, but no nephritis, have low titre ANA, often in a speckled pattern, do not have antibodies to dsDNA, and have a high prevalence- more than 50%- of anti-Ro antibodies.

C4 and C2 are both in linkage disequilibrium with the MHC antigens. They are positioned on the short arm of chromosome 6 between HLA Class B and DR antigens and are part of HLA Class III molecules along with factor B. Homozygosity for the null allele of C2 is present in 1 in 20,000 Caucasions, and as stated above, around 30% of these subjects develop SLE.

The inheritance of C4 is unusual. Most people have not one, but two alleles of C4 on 6p, designated as C4A and C4B. The two C4 alleles are part of a 4-gene cassette called RCCX which is subject to duplication or deletion, leading to Copy Number Variation, such that individuals may have 1-8 C4 alleles across the two chromosomes. Most Caucasians have 2-4 functioning alleles. Thus, total deficiency of C4 is exceedingly uncommon.

Deletion of C4B has not been associated with lupus, but when both copies of C4A are deleted, there is a higher prevalence of lupus compared with those whose C4A copies are intact. Deletion of C4A is seen as part of extended haplotype HLA B8, SC01, DR3. (S stands for S allele of Factor B, C for C-allele of C2, 0 for null-allele of C4A and 1 is an allele of C4B). Thus, subjects who are homozygous for this extended haplotype are at greater risk of SLE.

The extended haplotype HLA B8, SC01, DR3 is one of the commonest extended haplotypes found in Caucasians, with a haplotype frequency of around 9%. Thus the likelihood of heterozygosity in the Caucasian population using the Hardy Weinberg equilibrium is just under 17%, and the likelihood of homozygosity is 0.8%.

In practice, C3 and C4 levels are measured by most Rheumatologists. C4 levels are normally much lower than C3. A low C4 may be inherited or due to active SLE with complement consumption. An isolated low C4 may thus be difficult to interpret. Total haemolytic complement, or CH50, which is an index of all complement components in the classical pathway and is often measured, will therefore also be low, and will not help in such cases. (CH50 is the reciprocal of the dilution of the patient's plasma which, when added to sheep erythrocytes coated with rabbit anti-sheep antibodies, leads to 50% lysis of the sheep RBC. CH100, which is measured in other labs, is self explanatory). However, complement activation products such as C4d will be raised in complement consumption and low in inherited deficiency without SLE. Complement activation products in plasma are heat labile and technically difficult to measure. Cell bound complement activation products hold a lot of promise.

The HLA B8,DR3 extended haplotype is also associated with Type 1 Diabetes, Coeliac Disease and Adrenal insufficiency as part of polyglandular autoimmune syndrome type II.

Interestingly, just as C4A deficiency is part of an extended haplotype, so is deficiency of the product encoded by another member of HLA Class III, 21-beta hydroxylase. CYP21B is often deleted alongwith C4B as part of two other extended haplotypes. Homozygosity for these haplotypes result in Congenital Adrenal Hyperplasia.

C2 deficiency also occurs as part of an extended haplotype- HLA B18, DR2. Around 1% of Caucasians are heterozygotes. C2 is the only complement component in the classical pathway where heterozygosity is associated with reduced C50. With the others, C50 is only low with a homozygous state.

Pulvinar Hyperintensity is a Feature of Fabry Disease

The pulvinus is a part of the thalamus. Isolated pulvinar hyperintensity on T1 weighted MR imaging, thought to be due to calcification, usually bilateral, is thought to be a reliable feature of Fabry disease.



Reference

Moore DF1, Ye F, Schiffmann R, Butman JA. Increased signal intensity in the pulvinar on T1-weighted images: a pathognomonic MR imaging sign of Fabry disease. Am J Neuroradiol. 2003;24:1096-101

Circulating Inhibitors, Lupus Anticoagulant and Heparin Contamination: How to Interpret Abnormal Coagulation Tests

Consider this scenario: inpatient with bruising has tests for coagulation. The APTT is prolonged at 40 s (normal <35). Is this abnormal? Is it due to the prophylactic clexane he is receiving? How do you tell? Heparin contamination of inpatient samples is common. However, the causes of a prolonged APTT with normal PT are varied, and must be considered in an (usually) elderly patient with ecchymoses and bruises. Aetiologies include heparin, LMWH, low dose dabigatran, inherited deficiencies of factor VIII, IX and XI or VWF in young persons(with bleeding) or factor XII, HMWK or prekallikrein (without bleeding) and ciculating inhibitors to Factor VIII (commonest), seen typically in postpartum women, in autoimmune disease such as SLE and RA, and in cancer, or inhibitors to factor IX or XI.

If heparin contamination is suspected in a repeatedly prolonged APTT (always repeat the test), the best test is a combination of thrombin time and reptilase time. Thrombin time (TT), performed by using the patient's plasma with human or bovine thrombin, will be prolonged in heparin contamination, while reptilase time (RT), performed by mixing the patient's plasma with a venom obtained from Bothrops snake remains normal. The venom, unlike thrombin, cannot be inhibited by heparin or LMWH or by direct thrombin inhibitors.

Alternatively, use an inhibitor of heparin such as protamine or a heparin binding resin such as Heparsorb and repeat the test. APTT will normalise if it was prolonged due to heparin.

In cases of hypofibrinogenemia or dysfibrinogenemia, both TT and RT will be prolonged.

A circulating inhibitor is often suspected in adults with ecchymoses or bruising, occasionally with associated joint swelling due to haemarthrosis, prolonged APTT, normal PT, and no alternative causes such as liver failure, DIC (usually pronlongs both PT & APTT) or concurrent anticoagulant adminstration. The commonest circulating inhibitor is to factor VIII. While this is typically seen in haemophiliacs, it can also be seen in postpartum women, usually primigravidas, and as stated above, with autoimmune disease or cancer.

After ruling out heparin contamination as above, the patient's plasma is incubated 1:1 with normal plasma for 1 to 2 hours at 37 degrees celsius (to allow stable binding of IgG antibodies, which work best at body temperature) . APTT will correct with factor VIII, IX, or XI deficiencies or with vWD. If the APTT does not correct, it's either a circulating inhibitor, usually to Factor VIII, or lupus anticoagulant. The latter presents with thrombosis rather than bleeding and can be easily ruled out by one of two tests. First, adding phospholipid (usually cephalin from brain tissue) normalises the APTT in subjects with lupus anticoagulant. Here, one must be careful in performing a hexagonal phase assay rather than a lamellar assay. Hexagonal phase assay includes adding phosphatidylethanolamine rather than phospatidylcholine, as is found in lamellar (bilipid layer) assays. Although the latter is found in physiological membranes, circulating lupus anticoagulant is only inhibited by phospholipids present in the hexagonal form, comprising packed phospatidylethanolamine in a six-cylinder configuration. Using lipids in a lamellar form will lead to a false negative test.

The second test is to simply repeat the clotting with the dilute Russell Viper Venom Time (dRVVT). This bypasses all factors upstream of Factor X. Since Factor X still needs phospholipids to activate thrombin, it is abnormal with antiphospholipid syndrome, but not with circulating Factor VIII inhibitor.

It is highly likely that a person with prolonged APTT that does not correct with normal plasma, and with a normal dRVVT has a circulating inhibitor to Factor VIII. The next step is the Bethesda assay. Here, increasing dilutions of the patient's plasma is added to normal plasma, and assay is performed for Factor VIII activity, as in haemophilia. The reciprocal of the dilution that produces 50% of the normal Factor VIII activity is expressed in Bethesda Units (BU). More than 5 BU is considered abnormal.

Therefore, I'd suggest the following algorithm when one comes across an unexplained rise in APTT with normal PT.

1. Repeat test.

2. If APTT still prolonged, rule out heparin contamination by checking Thrombin Time and Reptilase Time. If TT is prolonged and RT is normal, heparin is present. If both TT and RT are prolonged, proceed to the next step.

3. Dilute the patient's plasma 1:1 with normal plasma and incubate for 60 minutes at 37 degrees Celsius. If APTT normalises, the patient has deficiency of one of the following factors: Factor VIII, IX, XI, VWF, Factor XII, HMWK, or prekallekrein. Deficiency of one of the last three prolongs the APTT, but does not lead to bleeding.

4. If the APTT does not correct with addition of normal plasma, the patient either has lupus anticoagulant or a circulating inhibitor to one of Factor VIII (commonest), or Factor IX or XI. Lupus anticoagulant leads to clotting rather than bleeding in vivo.

5. Test for lupus anticoagulant by doing one of these two tests: Do the dilute Russell Viper Venom Time. If dRVVT is prolonged, patient has Lupus anticoagulant.

Alternatively, add phospholipid to the mixture of normal and patient plasma and repeat APTT. If the latter returns to normal, lupus anticoagulant is present.

6. If the dRVVT is normal or if APTT is not corrected by adding phospholipid, a circulating inhibitor to Factor VIII, IX or XI is present. Inhibitors to Factor VIII are by far the commonest, and should particularly be suspected in postpartum women, usually after their first child, or older subjects. In such cases, proceed directly to the Bethesda assay. Presence of more than 5 BU is consistent with presence of Factor VIII inhibitor.

Graft Versus Host Disease Associated With Blood Transfusion

GVHD typically occurs after HSCT as the donor lymphocytes attack antigen presenting cells with a different HLA haplotype in host tissues, typically in the skin, liver and gastrointestinal tract. But can you get GVHD in non-transplant subjects?

Normally, blood transfusions would not be expected to cause GVHD, as the recipient's immune system would destroy lymphocytes in the donor blood.

However, GVHD has been demonstrated after blood transfusion in subjects who have not had a transplant. This typically occurs in two settings.

Firstly, the subject is immunosuppressed, either because of an underlying T-cell disorder, as in children, or because of chemotherapy or radiotherapy for haematologic or solid cancer. Hence the donor lymphocytes are free to run amok.

The second setting is unique and occurs in immunocompetent subjects. This typically happens when a subject who is heterozygous for a HLA antigen receives a blood transfusion from a donor who is homozygous for that HLA antigen. Thus, the host's lymphocytes do not recognise the blood donor's lymphocytes as foreign, but the donor lymphocytes recognise the "mismatched" HLA antigen in the host tissues as alien. This kind of mishap is likely to occur when there is limited genetic variability in the population, as in Japan, or when the blood donor is a family member, particularly if there is a high rate of consanguinity, thus causing shared HLA antigens, such as in rural Turkey or on the subcontinent.

Unlike transplantion associated GVHD, transfusion associated GVHD is almost always fatal. This is because apart from attacking the skin, liver and GI tract, lymphocytes in the donor blood also attack the host's bone marrow, causing aplasia. Remember, that in HSCT, the marrow has been ablated, and replaced by the donor marrow, which will therefore not be attacked by the graft lymphocytes.

Transfusion associated GVHD starts 4-30 days after transfusion, is usually mild and insidious in onset and is therefore often not recognised, and is usually attributed to the underlying disease. It starts with fever and rash, which can become erythrodermatous in severe cases, and proceeds to vomiting, severe diarrhoea, right upper quadrant pain, deranged LFTs, and pancytopenia.

Transfusion associated GVHD can be caused by transfusing whole blood, packed RBC, platelets, leucocytes and fresh unfrozen plasma. It does not occur when transfusing deglycerolated, frozen RBC or cryoprecipitate or FFP.

Diagnosis can be made by a skin biopsy and by demonstrating that the circulating lymphocytes are the donor's rather than the host's.

This devastating complication can be prevented by gamma-irradiating donor blood in the following 4 instances- in immunosuppressed subjects, in subjects who have had HCT (although GVHD here would not affect the marrow), in instances where the blood donor is a relative and in HLA-matched platelet transfusions.

Diagnosing Renal Artery Stenosis- Look for Cardio-renal Syndrome and Flash Pulmonary Oedema

We all know renovascular hypertension exists. But how does it present clinically? Are their any distinguishing features that set it apart from other causes of hypertension and from subjects with essential hypertension?

Renovascular hypertension has two principal aetiologies. By far the commonest aetiology is atherosclerotic narrowing of the proximal renal artery. This can be predominantly unilateral or bilateral.

A much less common cause is fibromuscular dysplasia, affecting young females often in the 30-50 year range. This typically affects the distal renal artery, and for some reason, tends to be predominantly right sided.

So how does renovascular disease present? There are two terms often used to describe this condition, without realising that this is in fact the underlying aetiology. The first is cardio-renal syndrome and the second is flash pulmonary oedema.

Probably the most characteristic presentation of atherosclerotic renal artery stenosis is rapidly worsening hypertension, worsening renal impairment and incremental heart failure. This is often erroneously interpreted as cardio-renal syndrome, particularly in subjects with pre-existing heart failure, but the key finding is the hypertension. Subjects with longstanding heart failure, the cohort who develop cardio-renal syndrome, often have low or low-normal BP. They should not show a trend for a recent increase in BP.

This classical presentation has been somewhat tempered in the last decade or two by the widespread use of ACE inhibitors and direct angiotensin blockers for treating both hypertension and heart failure, but when present, is strongly suggestive of severe renal artery stenosis.

Some physicians measure serum renin and aldosterone levels to make a suggestive diagnosis of renovascular hypertension, before considering imaging studies such as CT angiogram. However, this could be misleading. While renin and aldosterone levels would be expected to be high in predominantly unilateral renal artery stenosis, these hormones are normal in bilateral renal stenosis and in subjects who have renal artery stenosis in a single surviving kidney.


The other, albeit, less common presentation is flash pulmonary oedema. Far too often, clinicians invoke the rare diagnosis of phaeochromocytoma in such subjects, when the blame lies with a much more common condition. Renal artery stenosis should always be suspected in subjects who have one or more episodes of "flash" pulmonary oedema.

Trautmann's Triangle & Otogenic Brain Abscess

A 64 year old woman presents with fever, headache and a discharging right ear.

A CT scan shows appearances consistent with a large right cerebellar abscess. This is later confirmed on a MR scan. You start antibiotics to cover S. aureus, gram negatives and anaerobes and given the discharging ear, decide to send off an ENT consult.

What putative diagnosis would you put on the consult request? Acute otitis media? Chronic suppurative otitis media?

Surprising though it may seem, in a case series of 40 otogenic abscesses, acute otitis media was not implicated even once. In all but one case, the predisposing factor was a cholesteatoma.

With otogenic brain abscesses affecting the temporal lobe, spread occurs through the tegmen tympani.

For abscesses affecting the cerebellum, spread from the affected ear occurs through the Trautmann's triangle, an area bounded by the sigmoid sinus posteriorly, bony labyrinth anteriorly, superior petrosal sinus superiorly and the internal jugular vein inferiorly.

On CT, an abscess may be confused with a necrotic tumour. The best arbiter is a diffusion weighted MR scan.

In the above case, a combined neurosurgical and ENT approach would be favoured. Most experts would drain the cerebellar abscess first, followed by a radical mastoidectomy.

Reference

Mandalà M, Muzzi E, Trabalzini F. Giant Cerebellar Lesion in a Patient With Purulent Ear Drainage. JAMA Otolaryngol Head Neck Surg 2016. doi:10.1001/jamaoto.2016.0014

Postural BP- You Are Looking At The Wrong Metric

How common is this? An elderly man is admitted to an inpatient facility following a fall at home. During his consultation, you find out that he felt "giddy" before falling. It's happened a few times in the past as well.

Quite reasonably, you instruct the nurse to measure supine and standing blood pressure. You specifically request the nurse to measure the BP after the patient has been lying or standing for a full three minutes.

The nurse informs you that the BP fell by 10/5 mm Hg when the gentleman stood. You are reassured and leave it at that.

But have you missed the diagnosis here?

The normal response to being stood is for the systolic BP to fall slightly and for the diastolic BP to rise slightly. Therefore, the pulse pressure narrows.

More importantly though, the pulse rate speeds up slightly. Apart from the vasoconstriction involved in raising the standing BP, the heart beats slightly faster to maintain the systolic BP at a nearly constant level.

In autonomic failure, be it peripheral, i.e. diabetes, amyloid, or central, as in Parkinsons and Parkinkons plus syndromes and much less commonly in primary autonomic failure, both systolic and diastolic BP fall. More significantly, the pulse rate, instead of rising, remains invariant. It neither rises nor falls. Most patients with autonomic failure also have postprandial hypotension.

Vasovagal syncope is more common than autonomic failure, with two principal types- cardioinhibitory- due to excessive parasympathetic activity, and vasodepressor- due to interruption of sympathetic outflow.

In cardioinhibitory syncope, the pulse rate actually falls on standing. The BP falls too. Some patients may develop asystole.

In vasopdepressor syncope, there is sinus tachycardia. The BP falls.

In postural orthostatic tachycardia sydrome (POTS), the patient's pulse rate increases by >30 beats per minute upon standing. The BP does not fall.

Therefore, the following is a guide to using the blood pressure & pulse rate to differentiate between various types of syncope- standing BP and pulse compared with supine.

Old subject- BP falls by >20/10, pulse rate remains unchanged- autonomic failure

Old or young subject- BP falls, pulse rate falls or becomes asystolic (ECG may show 1st degree block in other cases)- cardioinhibitory syncope

Young subject- BP falls, becomes tachycardic- vasodepressor syncope

Young subject- BP does not fall, pulse rate increases by >30/minute- POTS

Any age- BP does not fall, pulse rate increases by <30/minute- normal response.