Unit 5 Part 4: Laboratory Investigation in Ocular Disease PDF

MSc in Clinical Optometry: Principles of Therapeutics Unit 5: Part 4 LABORATORY INVESTIGATION IN OCULAR DISEASE Description: This final part of Unit 5 will review the common laboratory investigations used in ocular disease. It will outline methods of specimen collection, laboratory processing and in...

MSc in Clinical Optometry: Principles of Therapeutics Unit 5: Part 4 LABORATORY INVESTIGATION IN OCULAR DISEASE Description: This final part of Unit 5 will review the common laboratory investigations used in ocular disease. It will outline methods of specimen collection, laboratory processing and interpretation of results. Hours: Eight Learning outcomes Following successful completion of this third part of Unit 5, you should be able to:  Identify common laboratory tests that are used in the diagnosis and management of ocular diseases and their systemic associations  Understand the indications for each test and be able to interpret the significance of the results Introduction Any systemic or ocular disease is initially assessed by a thorough medical history and clinical examination. In many cases medical laboratory tests provide the clinician with further objective information and aid in differential diagnosis. The tests may also help in the assessment of the stage of the disease or its severity. Investigations carried out in the pathology laboratory can be classified as:  Anatomical pathology  Haematology  Chemical pathology  Immunopathology  Genetics  Microbiology  Special tests For each investigation, a specimen is collected and transported, along with a pathology request form, to the laboratory. Analysis of the specimen is carried out and a report of the results produced, often with normative data or reference values. The request form level, and the time of collection and drug dosage should be recorded on the request form. In addition, blood lipid The request form that accompanies each specimen studies to look for hyperlipidaemia in ocular vascular provides patient identification details, a clinical history, examination findings and relevant medications. In the disease should be completed whilst the patient is case of microbiology specimens, the antibiotics fasting, and again this should be noted on the request form. currently prescribed should be noted. This information enables the pathologist to select the most appropriate Specimen handling and transport means of testing and aids interpretation of the results. In general, all specimens are placed in a sterile The timing of specimen collection is important. For container, which is labelled immediately. Care is taken example, a blood sample for monitoring drug levels is to prevent contamination of the specimen and ensure usually taken prior to the next dose; this is a ‘trough’ the safety of staff handling. The specimen is transported Laboratory investigation in ocular disease to the laboratory in biohazard warning plastic bags, removed. Correct handling of the tissue is vital in order accompanied by the request form, with minimal delay. If to obtain accurate results. there is a delay, most specimens can be refrigerated, except for blood requiring electrolyte studies as Tissue specimens refrigeration will invalidate the results. Certain If light microscopy (LM) is required, specimens are organisms such as N. gonorrhoeae are particularly commonly fixed in 10% formalin. Infrequently, fresh fragile and may also be affected by refrigeration. specimens are used, such as for a frozen section, Moh’s micrographic surgery or flow cytometry studies. Artefactual results may occur if the correct procedures Fresh specimens can be transported in tissue culture for specimen collection, handling and transport are not media or normal saline. followed. If there is uncertainty as to how to collect, handle or transport a specimen the pathology laboratory Frozen sections are used to ensure complete excision, is consulted. of typically a malignant lesion, with adequate margins. During surgery, freshly excised specimens are sent to INVESTIGATION OF OCULAR DISEASE: EYELIDS the laboratory, rapidly frozen to form hard tissue blocks AND ORBIT for thin sectioning, stained and examined with LM. An INVESTIGATION OF LID LUMPS AND TUMOURS experienced pathologist is required for processing and interpretation. If the lesion extends to the margin of the  Biopsy section, the surgeon is informed and the process  Cytology repeated until an adequate, clear margin is obtained. Benign lid tumours include papillomas, seborrhoeic keratosis (Figure 1), keratoacanthoma, naevi and Moh’s micrographic surgery provides a more rigorous vascular lesions. Premalignant tumours include actinic control of margins during excision than frozen section keratosis. The most common malignant lid lesion is a and is therefore recommended for canthal cancers, basal cell carcinoma. Much rarer, is squamous cell which have a high recurrence rate. Moh’s surgery carcinoma and sebaceous gland carcinoma. involves use of histological drawings and markings to create a three-dimensional view of the lesion. Moh’s Chalazia are the most common of all lid lumps (see Unit surgery is time-consuming and requires a specialised 5, Part 1). Small chalazia often resolve over time technician and surgeon. Peri-orbital basal cell, although persistent or symptomatic lesions can be squamous cell and sebaceous cell carcinoma are often treated surgically. It is important to note that any excised using frozen sections or Moh’s surgery. recurrence of a lesion should be treated as suspicious. However, surgeons often prefer to await definitive A biopsy should be taken in order to obtain a histology to determine if a lesion has been adequately histological report. excised. A biopsy can be incisional, in which only a section of a If electron microscopy (EM) is required, the lesion is taken or excisional, where the whole lesion is pathologist is contacted and arrangements made to deliver the tissue as soon as possible. Specimens are transported fresh to the laboratory or rapidly fixed in glutaraldehyde to reduce the risk of artefact. EM allows specimens to be viewed at very high magnification, which is useful, for example, in tumours that are so undifferentiated that a diagnosis cannot be made with LM. For immunohistological investigations, tissue collected by biopsy is fixed in formalin. However, some tests are best performed on frozen sections of fresh tissue that are transported in media, such as tissue culture media. A wide range of antigens, such as hormones, receptors and infectious agents, can be detected in paraffin-embedded specimens using tagged Figure 1: Seborrhoeic keratosis of the lid monoclonal antibodies. 2 Laboratory investigation in ocular disease INVESTIGATION OF ORBITAL CONDITIONS  Cytology  Ophthalmic radiology  Ophthalmic ultrasonography Cytology is often used to investigate inflammatory disease or infectious disease when standard microbiological investigations have been negative. Cytology allows the collection of a specimen with minimal interference to the tissues, but is not as accurate as tissue biopsy since the histological pattern cannot be evaluated. Cytology specimens can be Figure 2: Orbital cellulitis obtained by fine needle aspiration (FNAB), impression cytology, conjunctival scrapings and intraocular biopsy. children as the orbital septum is not fully developed and FNAB is an invasive procedure that has proved so there is a high risk of progression. Infective successful in the diagnosis of orbital disorders. In organisms in orbital cellulitis include Streptococcus FNAB, a sampling needle is directed into a deeply pneumoniae, Staphylococcus aureus, Streptococcus located lesion and aspirated to obtain a specimen, pyogense and Haemophilus influenza. Infective sometimes under imaging guidance, usually using a organisms in pre-septal cellulitis include Staphylococci computerised tomography (CT) scan or ultrasound. and Streptococci sp. INVESTIGATION OF ORBITAL AND PRE-SEPTAL Investigation of suspect orbital cellulitis includes: CELLULITIS determination of body temperature, full blood count  Haematology (FBC), blood culture and connective tissue (CT) scan of the orbit sinuses and brain. Investigation of pre-septal  Culture cellulitis is not usually required, unless there is concern  CT imaging over possible orbital or sinus involvement. Pre-septal cellulitis and orbital cellulitis (Figure 2) are Full blood count the most significant infections of the ocular adnexa and orbital tissues. They can be differentiated on the basis The full blood count measures red blood cell (RBC) of soft tissue location—specifically whether the infection count, haematocrit, haemoglobin, mean cell lies in front or behind the septum orbitale (see Unit 1, haemoglobulin concentration, mean cell haemoglobin, Part 1). It is important to differentiate life-threatening white blood cell (WBC) count with differential and orbital cellulitis from the much more limited pre-septal platelet count using an automated analyser (Table 1). cellulitis. Pre-septal cellulitis is life-threatening in FULL BLOOD COUNT Normal Values Male Female 12 12 Red blood cells (RBC) 4.6–6.2 x 10 /L 4.2–5.4 x 10 /L Haematocrit (Hct) 40–54% 38–47% Haemoglobin (Hgb) 14–18 g/dL 12–16 g/dL Mean cell volume (MCV) 82–98 fL Mean cell haemoglobin (MCH) 27–31 pg Mean cell haemoglobin concentration (MCHC) 32–36g/dL 9 White blood cells (WBC) 4.5–11.0 x 10 /L 9 Platelets 140–400 x 10 /L Table 1: Table to show the composition of a full blood count 3 Laboratory investigation in ocular disease INVESTIGATION OF THYROID DISEASE disease. Antimicrosomal antibodies are elevated in Hashimoto’s disease (hypothyroidism caused by  Chemical pathology—hormones antibodies to thyroid cell components). Chemical pathology involves the detection of a large variety of substances in blood or body fluids including Thyroid function tests are normal in 10–20% of patients electrolytes, hormones, lipids and vitamins, as well as with presumed thyroid eye disease (euthyroid); tumour markers, poisons and therapeutic or illicit drugs. diagnosis is then made clinically. In this situation, the TSH receptor antibody (thyroid-stimulating Thyroid function tests are used to diagnose and monitor immunoglobulin) may be useful for diagnosis and the treatment of thyroid dysfunction, in conjunction with monitoring. a thorough clinical assessment. Eyelid retraction is the most consistent clinical feature, but may be difficult to INVESTIGATION OF GIANT CELL (TEMPORAL) recognise. Colour vision, visual fields and connective ARTERITIS tissue (CT) scanning may also be indicated. Thyroid function tests measure thyroxine (T4), triiodothyronine  Erythrocyte sedimentation rate (ESR) (T3), thyroid hormone binding ratio (THBR) and thyroid-  C-Reactive protein stimulating hormone (TSH).  Biopsy Total T4 and total T3 are measured using Giant cell arteritis (GCA) is inflammation and damage to radioimmunoassay, but are affected by changes in blood vessels that supply the head area, particularly the serum binding proteins. Free T4 and free T3 are large or medium arteries that branch from the neck. calculated by determining the THBR. T4 levels are used Symptoms include excessive sweating, fever, general ill to diagnose and monitor thyroid dysfunction (Table 2). feeling, jaw pain (intermittent or when chewing), loss of appetite, muscle aches, throbbing headache on one side of the head or the back of the head, scalp Hypo- Hyper- sensitivity, tenderness when touching the scalp and Test Euthyroid thyroidism thyroidism vision difficulties. Blood tests are key in the diagnosis of GCA: Total T4 ↓ ↑ Normal Erythrocyte sedimentation rate THBR ↓ ↑ Normal The erythrocyte sedimentation rate (ESR) is a non- Free T4 ↓ ↑ Normal specific marker of inflammatory or neoplastic disease. Blood is placed in a vertical tube and the length (in millimetres) of the column of plasma above sedimented Table 2: The different tests used to investigate red cells at one hour is the ESR. The maximum normal thyroid eye disease ESR depends on the patient’s age and can be T3 levels are requested when T4 levels are normal and calculated in men as the (age in years)/2 and in women, there is clinical suspicion of hyperthyroidism. ‘T3 (age in years +10)/2. Inflammation causes the RBCs to toxicosis’ occurs in around 10% of Graves’ disease clump together and settle out-of-solution faster. The patients with normal T4 and may be associated with ESR is elevated in infections, neoplasms, vasculitis and thyroid ophthalmopathy. most common tissue disorders and is markedly elevated in GCA. TSH is the best screening test for thyroid dysfunction in C-reactive protein the ambulatory patient. TSH is measured by a rapid and highly sensitive immunometric assay and remains C-reactive protein (CRP) is an acute phase reactant unaffected by changes in serum binding proteins. The protein that rapidly increases during infection and assay detects subclinical disease earlier than inflammation—reducing as the condition subsides. measurement of T3 or T4 levels can, but is not as useful CRP is reported to be more sensitive than ESR and has in ill patients or those on medication, as TSH can be been used with increasing frequency. However, much suppressed. like ESR, it simply indicates the presence of an abnormality not its diagnosis. The normal value of Antithyroid antibodies are quantitatively measured in