Drowning: Wet, Dry, Diatom Test and Vitreous Markers

In wet drowning, the immersed victim makes active breathing efforts against a medium that cannot support gas exchange. Water enters the trachea, bronchi, and alveoli. The sequence of physiological events is: aspiration of water, hypoxic stimulation producing further inspiratory effort, progressive flooding of the alveolar surface, disruption of surfactant, atelectasis of unflodded segments, and ultimately cardiac arrest from hypoxia or vagal inhibition.

Pulmonary changes. The hallmark autopsy finding is over-distended, pale, voluminous, waterlogged lungs that fill the thoracic cavity, overlap the pericardium, and leave a rib impression on the lateral surface. On cut section the lung exudes frothy, blood-tinged fluid from the alveoli and bronchioles. The cut surface has a pale, boggy, wet appearance distinct from the congested, dark appearance of other asphyxial deaths and from the air-trapping hyperinflation of emphysema.

Frothy exudate. Froth at the mouth, nostrils, and tracheal opening on external examination is the classic gross finding. The froth is white or slightly blood-tinged and is produced by the agitation of mucus and surfactant with water and air during the breathing struggle. It differs from the clear froth of pulmonary oedema in cardiac failure by its persistence and by the presence of water on the glottis and tracheal mucosa.

Washerwoman hands (skin maceration). In prolonged immersion, the skin of the hands and feet undergoes maceration (whitening, wrinkling, and softening from water absorption). This finding does not indicate ante-mortem versus post-mortem immersion; it reflects duration of immersion, not vital state during immersion. Severe maceration (wrinkling extending beyond the fingers into the forearm) indicates prolonged immersion of many hours.

Diatoms in the lung. In wet drowning, inhaled diatoms from the water are present in the alveolar contents and in the peribronchial tissue. The lung is therefore the reference specimen against which diatom species in bone marrow or brain are compared (if the same species are found in bone marrow, systemic circulation is confirmed).

In India, the AIIMS and CFSL drowning protocols require that, at autopsy, the lung surface, tracheal contents, and frothy fluid be examined grossly and then preserved separately for diatom analysis. In the UK, the RCPath autopsy guidelines (post-Pollard 1998) specify that the right lobe of the lung be sampled separately from the bone marrow specimen to allow species comparison between the aspiration evidence and the systemic-circulation evidence. In the US, the NAME does not mandate the diatom test but acknowledges it as a valid adjunct in cases where ante-mortem submersion is disputed.

Dry drowning accounts for approximately 10-15% of drowning deaths in autopsy series (Spitz and Fisher, 2020). The mechanism is laryngospasm: the sudden inrush of water stimulates the laryngeal mucosa to produce powerful reflex adductor spasm of the glottis, sealing the airway and preventing both water entry and air exchange. The victim asphyxiates on the trapped air; little or no water enters the lungs.

Autopsy findings in dry drowning. Because there is no pulmonary water aspiration, the lungs are not over-distended and there is no frothy exudate. The lungs show only the non-specific congestion and petechiae of asphyxia. The larynx may show mucosal hyperaemia and mild oedema from the spasm stimulus. This autopsy picture is essentially indistinguishable from non-drowning asphyxia such as manual strangulation or smothering, or in a decomposed body, from nothing at all.

The diagnostic problem. A body recovered from water with no pulmonary water, no froth, and no diatoms in the marrow presents a genuine diagnostic challenge. Dry drowning cannot be positively diagnosed by autopsy alone; the diagnosis is by exclusion of other causes and by scene evidence consistent with submersion. This is an acknowledged limitation in both the Saukko and Knight (2015) and Spitz and Fisher (2020) frameworks.

Post-mortem body flotation. Whether a body sinks or floats after drowning depends on the balance of body density and pulmonary gas content. In wet drowning, the waterlogged lungs increase density and the body sinks initially, then rises as putrefaction generates gas in the thorax and abdomen. In dry drowning, the trapped air may initially keep the body afloat. This matters for scene interpretation in suspected drownings where the body is found floating.

In the UK Coroner system, a body found in water with no other explanation for death and consistent maceration pattern is typically certified as drowning on the balance of probabilities even without diatom or chemical confirmation, a pragmatic approach sanctioned by the Coroners and Justice Act 2009 guidance. German practice (BKA; Madea 2014) tends to require laboratory corroboration before certifying drowning as cause of death; the diatom test or vitreous chemistry is therefore performed more routinely in German medicolegal autopsies.

The diatom test exploits the biology of diatoms (unicellular photosynthetic algae, class Bacillariophyta) and the physics of their siliceous frustule (cell wall). Diatoms are present in virtually all freshwater and marine environments in species compositions that are specific to location, season, and water chemistry. Their frustules survive strong acid digestion intact. If a victim was alive and circulating during submersion, aspirated diatoms pass from the alveolar water through the damaged capillary bed into the pulmonary veins and then via the systemic circulation to the liver, kidney, spleen, and bone marrow. Post-mortem bodies in water also accumulate diatoms via passive diffusion through the nose, trachea, and gut, but this passive contamination is limited to upper airway surfaces and is not expected to produce diatoms in intact bone marrow from a sealed femur.

The Pollard 1998 protocol. The standard modern diatom test protocol is based on the acid-digestion method published by A.M. Pollard in the Journal of Forensic Science (1998 series), which standardised the digestion steps and the reporting criteria. The protocol:

1. Recover a sample of femur shaft cortical bone, mid-diaphysis (20-30 g), from which the periosteum and endosteum are scraped away to minimise surface contamination.
2. Separately, recover a 20-30 g sample of lung tissue (right lobe, deep parenchyma, away from bronchial surface).
3. Digest each sample in concentrated nitric acid (HNO3) with 30% hydrogen peroxide (H2O2) at 80-100°C until fully dissolved.
4. Centrifuge, wash the pellet three times in distilled water, mount the pellet on a glass slide in a mounting medium (Naphrax or similar), and examine under phase-contrast or brightfield microscopy at 400-1000x.
5. Count and identify diatom species in the bone marrow specimen.
6. Compare species identified in bone marrow with species identified in the lung sample AND with species in a water sample from the alleged drowning scene (collected during the scene investigation).

A positive test requires: (a) diatoms present in the bone marrow specimen; (b) the species in bone marrow match species in both the lung sample and the scene water (ruling out environmental contamination); (c) the species are absent or present only in trace amounts in control samples from the autopsy room, acid reagents, and distilled water. The number of diatoms sufficient to call a positive result is a subject of continuing debate; Pollard (1998) suggested that five or more diatoms of matching species per high-power field in at least two separate fields constitutes a positive finding.

Limitations and contested evidence. The diatom test's evidentiary status varies across jurisdictions. In Germany, the BKA accepts a positive diatom test as strong evidence of ante-mortem submersion, subject to the species-match requirement. In the UK, the diatom test has been used in drowning cases since the mid-twentieth century and has been used since, but several subsequent UK cases raised the contamination objection (diatoms in laboratory reagents, autopsy tables, and water supplies, the "blank contamination" problem). India's CFSL laboratories perform the test using the Pollard protocol but note that freshwater species vary significantly between Indian river systems, requiring local reference water databases. In the US, DiMaio (2001) describes the test as useful but flags the lack of standardised blank controls as a weakness that limits its use as primary evidence. A negative diatom test does not exclude drowning (dry drowning, post-mortem submersion of a body, or advanced decomposition can all produce false negatives).

Vitreous humour chemistry is a supplementary biochemical test for drowning attribution, most useful when decomposition prevents diatom testing or when lung architecture is too disrupted for species identification.

The physiological basis. In a drowning victim who aspirated significant freshwater, the osmotically dilute water is absorbed rapidly from the pulmonary circulation into the systemic circulation. This dilutes serum sodium (hyponatraemia) and potassium rapidly. Conversely, saltwater aspiration produces the reverse: the hypertonic seawater draws fluid out of the pulmonary circulation, producing hypernatraemia. These electrolyte shifts occur while the victim is still alive, and they modify the fluid composition of the vitreous humour if the victim survives long enough for vitreous equilibration.

Post-mortem stability. The vitreous chamber is isolated by the fibrous sclera and maintains electrolyte concentrations for longer than serum post-mortem. This stability makes vitreous chemistry the standard ante-mortem biochemical reference fluid for time-since-death estimation (vitreous K+ rise, the Madea-Henssge formula) and for biochemical attribution of cause of death.

Drowning interpretation: sodium. In freshwater drowning, vitreous sodium may be reduced below reference range (normal approximately 134-155 mmol/L) as a result of systemic haemodilution. Values below 120 mmol/L in the absence of ante-mortem hyponatraemia from other causes have been associated with freshwater drowning in multiple case series (Saukko and Knight, 2015). In saltwater drowning, vitreous sodium is elevated. A normal vitreous sodium does not exclude drowning.

Left-right vitreous asymmetry. In ante-mortem drowning, both vitreous chambers should show similar (and deviated) sodium values, reflecting systemic electrolyte change. Post-mortem immersion (body placed in water after death) does not change vitreous sodium because the victim did not aspirate water and never had the systemic haemodilution. Left-right asymmetry may indicate post-mortem leakage rather than bilateral drowning-chemistry change.

The Madea regression formula for time since death estimation (vitreous K+) is discussed separately in the Post-Mortem Changes module; in drowning cases it is used in conjunction with the sodium test to estimate the post-mortem interval and to assess whether the potassium elevation is consistent with the suspected time since death.

In India, the CFSL Hyderabad toxicology handbook includes vitreous sodium testing as a recommended adjunct in drowning cases, particularly where diatom testing produces ambiguous results. In the UK, the current RCPath practice guidelines (2015 version) list vitreous chemistry as a standard sample in all medico-legal autopsies on bodies recovered from water. In the US, the NAME-affiliated medical examiners routinely submit vitreous samples for electrolyte analysis in suspected drowning deaths.

Secondary drowning (also termed delayed drowning or near-drowning pulmonary oedema) is a distinct clinical and medico-legal entity: the victim is resuscitated from an immersion incident or survives initial aspiration, remains apparently stable for hours, then deteriorates and dies from respiratory failure.

Mechanism. Aspirated water, even if not fatal acutely, disrupts the surfactant lining of the alveoli. Over hours, the alveolar surface collapses (atelectasis), and the inflammatory response to the water (particularly to seawater's hypertonic chemical components) produces non-cardiogenic pulmonary oedema. This delayed-onset respiratory failure can progress to adult respiratory distress syndrome (ARDS) and cardiovascular collapse within 24-72 hours of the submersion event.

Medico-legal relevance. When a child or adult is resuscitated at the scene, transferred to hospital, and dies 12-48 hours later, the medico-legal question is whether the drowning was the legal cause of death. In all jurisdictions (India BNSS § 3 definition of cause of death, US case law in drowning-related personal-injury cases, UK Coroner practice under the Coroners and Justice Act 2009), a death causally traceable to the drowning event (even with an intervening period of survival) is certified as drowning with a specific note that the mechanism was delayed pulmonary oedema. The criminal law question of whether the drowning was homicidal, accidental, or suicidal is unaffected by the delay.

The hospital post-mortem findings. In a secondary drowning death after hospital treatment, the lungs show diffuse alveolar damage on histology, hyaline membrane formation, and alveolar macrophage infiltration consistent with the timing from the immersion event. Water-aspirated diatoms may have been partially cleared by medical intervention. The diatom test may yield ambiguous results; vitreous chemistry taken at the time of death can still show the electrolyte shift if death is within 72 hours of the immersion event.

In India, the 2011 Vizag beach tragedy in Andhra Pradesh, in which several children who were revived on shore died in hospital with delayed pulmonary oedema, was examined by the AIIMS forensic medicine review as a teaching case for secondary drowning certification. In the UK, the Hampshire and Isle of Wight Constabulary's 2016 review of near-drowning deaths noted that secondary drowning was the most common mechanism in drowning deaths where the victim had initial Glasgow Coma Scale scores above 8 at scene rescue.